Intel 8086 Family Architecture
Instruction Clock Cycle Calculation
8088/8086 Effective Address (EA) Calculation
Task State Calculation
FLAGS - Intel 8086 Family Flags Register
MSW - Machine Status Word (286+ only)
8086/80186/80286/80386/80486 Instruction Set
AAA - Ascii Adjust for Addition
AAD - Ascii Adjust for Division
AAM - Ascii Adjust for Multiplication
AAS - Ascii Adjust for Subtraction
ADC - Add With Carry
ADD - Arithmetic Addition
AND - Logical And
ARPL - Adjusted Requested Privilege Level of Selector (286+ PM)
BOUND - Array Index Bound Check (80188+)
BSF - Bit Scan Forward (386)
BSR - Bit Scan Reverse (386+)
BSWAP - Byte Swap (486+)
BT - Bit Test (386+)
BTC - Bit Test with Compliment (386+)
BTR - Bit Test with Reset (386+)
BTS - Bit Test and Set (386+)
CALL - Procedure Call
CBW - Convert Byte to Word
CDQ - Convert Double to Quad (386+)
CLC - Clear Carry
CLD - Clear Direction Flag
CLI - Clear Interrupt Flag (disable)
CLTS - Clear Task Switched Flag (286+ privileged
CMC - Complement Carry Flag
CMP - Compare
CMPS - Compare String (Byte, Word or Doubleword)
CMPXCHG - Compare and Exchange
CWD - Convert Word to Doubleword
CWDE - Convert Word to Extended Doubleword (386+)
DAA - Decimal Adjust for Addition
DAS - Decimal Adjust for Subtraction
DEC - Decrement
DIV - Divide
ENTER - Make Stack Frame (80188+)
ESC - Escape
HLT - Halt CPU
IDIV - Signed Integer Division
IMUL - Signed Multiply
IN - Input Byte or Word From Port
INC - Increment
INS - Input String from Port (80188+)
INT - Interrupt
INTO - Interrupt on Overflow
INVD - Invalidate Cache (486+)
INVLPG - Invalidate Translation Look-Aside Buffer Entry (486+)
IRET/IRETD - Interrupt Return
Jxx - Jump Instructions Table
JCXZ/JECXZ - Jump if Register (E)CX is Zero
JMP - Unconditional Jump
LAHF - Load Register AH From Flags
LAR - Load Access Rights (286+ protected)
LDS - Load Pointer Using DS
LEA - Load Effective Address
LEAVE - Restore Stack for Procedure Exit(80188+)
LES - Load Pointer Using ES
LFS - Load Pointer Using FS (386+)
LGDT - Load Global Descriptor Table (286+ privileged)
LIDT - Load Interrupt Descriptor Table (286+ privileged)
LGS - Load Pointer Using GS (386+)
LLDT - Load Local Descriptor Table (286+ privileged)
LMSW - Load Machine Status Word (286+ privileged)
LOCK - Lock Bus
LODS - Load String (Byte, Word or Double)
LOOP - Decrement CX and Loop if CX Not Zero
LOOPE/LOOPZ - Loop While Equal / Loop While Zero
LOOPNZ/LOOPNE - Loop While Not Zero / Loop While Not Equal
LSL - Load Segment Limit (286+ protected)
LSS - Load Pointer Using SS (386+)
LTR - Load Task Register (286+ privileged)
MOV - Move Byte or Word
MOVS - Move String (Byte or Word)
MOVSX - Move with Sign Extend (386+)
MOVZX - Move with Zero Extend (386+)
MUL - Unsigned Multiply
NEG - Two's Complement Negation
NOP - No Operation (90h)
NOT - One's Compliment Negation (Logical NOT)
OR - Inclusive Logical OR
OUT - Output Data to Port
OUTS - Output String to Port (80188+)
POP - Pop Word off Stack
POPA/POPAD - Pop All Registers onto Stack (80188+)
POPF/POPFD - Pop Flags off Stack
PUSH - Push Word onto Stack
PUSHA/PUSHAD - Push All Registers onto Stack (80188+)
PUSHF/PUSHFD - Push Flags onto Stack
RCL - Rotate Through Carry Left
RCR - Rotate Through Carry Right
REP - Repeat String Operation
REPE/REPZ - Repeat Equal / Repeat Zero
REPNE/REPNZ - Repeat Not Equal / Repeat Not Zero
RET/RETF - Return From Procedure
ROL - Rotate Left
ROR - Rotate Right
SAHF - Store AH Register into FLAGS
SAL/SHL - Shift Arithmetic Left / Shift Logical Left
SAR - Shift Arithmetic Right
SBB - Subtract with Borrow/Carry
SCAS - Scan String (Byte, Word or Doubleword)
SETAE/SETNB - Set if Above or Equal / Set if Not Below (386+)
SETB/SETNAE - Set if Below / Set if Not Above or Equal (386+)
SETBE/SETNA - Set if Below or Equal / Set if Not Above (386+)
SETE/SETZ - Set if Equal / Set if Zero (386+)
SETNE/SETNZ - Set if Not Equal / Set if Not Zero (386+)
SETL/SETNGE - Set if Less / Set if Not Greater or Equal (386+)
SETGE/SETNL - Set if Greater or Equal / Set if Not Less (386+)
SETLE/SETNG - Set if Less or Equal/Set if Not greater or Equal (386+)
SETG/SETNLE - Set if Greater / Set if Not Less or Equal (386+)
SETS - Set if Signed (386+)
SETNS - Set if Not Signed (386+)
SETC - Set if Carry (386+)
SETNC - Set if Not Carry (386+)
SETO - Set if Overflow (386+)
SETNO - Set if Not Overflow (386+)
SETP/SETPE - Set if Parity / Set if Parity Even (386+)
SETNP/SETPO - Set if No Parity / Set if Parity Odd (386+)
SGDT - Store Global Descriptor Table (286+ privileged)
SIDT - Store Interrupt Descriptor Table (286+ privileged)
SHL - Shift Logical Left
SHR - Shift Logical Right
SHLD/SHRD - Double Precision Shift (386+)
SLDT - Store Local Descriptor Table (286+ privileged)
SMSW - Store Machine Status Word (286+ privileged)
STC - Set Carry
STD - Set Direction Flag
STI - Set Interrupt Flag (Enable Interrupts)
STOS - Store String (Byte, Word or Doubleword)
STR - Store Task Register (286+ privileged)
SUB - Subtract
TEST - Test For Bit Pattern
VERR - Verify Read (286+ protected)
VERW - Verify Write (286+ protected)
WAIT/FWAIT - Event Wait
WBINVD - Write-Back and Invalidate Cache (486+)
XCHG - Exchange
XLAT/XLATB - Translate
XOR - Exclusive OR
Intel 8086 Family Architecture
General Purpose Registers Segment Registers
AH/AL AX (EAX) Accumulator CS Code Segment
BH/BL BX (EBX) Base DS Data Segment
CH/CL CX (ECX) Counter SS Stack Segment
DH/DL DX (EDX) Data ES Extra Segment
(FS) 386 and newer
(Exx) indicates 386+ 32 bit register (GS) 386 and newer
Pointer Registers Stack Registers
SI (ESI) Source Index SP (ESP) Stack Pointer
DI (EDI) Destination Index BP (EBP) Base Pointer
IP Instruction Pointer
Status Registers
FLAGS Status Flags (see FLAGS)
Special Registers (386+ only)
CR0 Control Register 0 DR0 Debug Register 0
CR2 Control Register 2 DR1 Debug Register 1
CR3 Control Register 3 DR2 Debug Register 2
DR3 Debug Register 3
TR4 Test Register 4 DR6 Debug Register 6
TR5 Test Register 5 DR7 Debug Register 7
TR6 Test Register 6
TR7 Test Register 7
Register Default Segment Valid Overrides
BP SS DS, ES, CS
SI or DI DS ES, SS, CS
DI strings ES None
SI strings DS ES, SS, CS
- see CPU DETECTING Instruction Timing
Back to Top
Instruction Clock Cycle Calculation
Some instructions require additional clock cycles due to a "Next
Instruction Component" identified by a "+m" in the instruction
clock cycle listings. This is due to the prefetch queue being
purge on a control transfers. Below is the general rule for
calculating "m":
88/86 not applicable
286 "m" is the number of bytes in the next instruction
386 "m" is the number of components in the next instruction
(the instruction coding (each byte), plus the data and
the displacement are all considered components)
Back to Top
8088/8086 Effective Address (EA) Calculation
Description Clock Cycles
Displacement 6
Base or Index (BX,BP,SI,DI) 5
Displacement+(Base or Index) 9
Base+Index (BP+DI,BX+SI) 7
Base+Index (BP+SI,BX+DI) 8
Base+Index+Displacement (BP+DI,BX+SI) 11
Base+Index+Displacement (BP+SI+disp,BX+DI+disp) 12
- add 4 cycles for word operands at odd addresses
- add 2 cycles for segment override
- 80188/80186 timings differ from those of the 8088/8086/80286
Back to Top
Task State Calculation
"TS" is defined as switching from VM/486 or 80286 TSS to one of
the following:
Miscellaneous
- all timings are for best case and do not take into account wait
states, instruction alignment, the state of the prefetch queue,
DMA refresh cycles, cache hits/misses or exception processing.
- to convert clocks to nanoseconds divide one microsecond by the
processor speed in MegaHertz:
(1000MHz/(n MHz)) = X nanoseconds
- see 8086 Architecture
Back to Top
FLAGS - Intel 8086 Family Flags Register
- see PUSHF POPF STI CLI STD CLD
Back to Top
MSW - Machine Status Word (286+ only)
Bit 0 PE Protection Enable, switches processor between
protected and real mode
Bit 1 MP Math Present, controls function of the WAIT
instruction
Bit 2 EM Emulation, indicates whether coprocessor functions
are to be emulated
Bit 3 TS Task Switched, set and interrogated by coprocessor
on task switches and when interpretting coprocessor
instructions
Bit 4 ET Extension Type, indicates type of coprocessor in
system
Bits 5-30 Reserved
bit 31 PG Paging, indicates whether the processor uses page
tables to translate linear addresses to physical
addresses
- see SMSW LMSW
Back to Top
8086/80186/80286/80386/80486 Instruction Set
AAA - Ascii Adjust for Addition
Usage: AAA
Modifies flags: AF CF (OF,PF,SF,ZF undefined)
Changes contents of AL to valid unpacked decimal. The high order
nibble is zeroed.
Clocks Size
Operands 808x 286 386 486 Bytes
none 8 3 4 3 1
Back to Top
AAD - Ascii Adjust for Division
Usage: AAD
Modifies flags: SF ZF PF (AF,CF,OF undefined)
Used before dividing unpacked decimal numbers. Multiplies AH by
10 and the adds result into AL. Sets AH to zero. This instruction
is also known to have an undocumented behavior.
AL := 10*AH+AL
AH := 0
Clocks Size
Operands 808x 286 386 486 Bytes
none 60 14 19 14 2
Back to Top
AAM - Ascii Adjust for Multiplication
Usage: AAM
Modifies flags: PF SF ZF (AF,CF,OF undefined)
AH := AL / 10
AL := AL mod 10
Used after multiplication of two unpacked decimal numbers, this
instruction adjusts an unpacked decimal number. The high order
nibble of each byte must be zeroed before using this instruction.
This instruction is also known to have an undocumented behavior.
Clocks Size
Operands 808x 286 386 486 Bytes
none 83 16 17 15 2
Back to Top
AAS - Ascii Adjust for Subtraction
Usage: AAS
Modifies flags: AF CF (OF,PF,SF,ZF undefined)
Corrects result of a previous unpacked decimal subtraction in AL.
High order nibble is zeroed.
Clocks Size
Operands 808x 286 386 486 Bytes
none 8 3 4 3 1
Back to Top
ADC - Add With Carry
Usage: ADC dest,src
Modifies flags: AF CF OF SF PF ZF
Sums two binary operands placing the result in the destination.
If CF is set, a 1 is added to the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 7 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 6 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=23+EA)
accum,immed 4 3 2 1 2-3
Back to Top
ADD - Arithmetic Addition
Usage: ADD dest,src
Modifies flags: AF CF OF PF SF ZF
Adds "src" to "dest" and replacing the original contents of "dest".
Both operands are binary.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 7 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 6 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=23+EA)
accum,immed 4 3 2 1 2-3
Back to Top
AND - Logical And
Usage: AND dest,src
Modifies flags: CF OF PF SF ZF (AF undefined)
Performs a logical AND of the two operands replacing the
destination with the result.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 7 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 6 1 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=23+EA)
accum,immed 4 3 2 1 2-3
Back to Top
ARPL - Adjusted Requested Privilege Level of Selector (286+ PM)
Usage: ARPL dest,src
(286+ protected mode)
Modifies flags: ZF
Compares the RPL bits of "dest" against "src". If the RPL bits
of "dest" are less than "src", the destination RPL bits are set
equal to the source RPL bits and the Zero Flag is set. Otherwise
the Zero Flag is cleared.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - 10 20 9 2
mem,reg - 11 21 9 4
Back to Top
BOUND - Array Index Bound Check (80188+)
Usage: BOUND src,limit
Modifies flags: None
Array index in source register is checked against upper and lower
bounds in memory source. The first word located at "limit" is
the lower boundary and the word at "limit+2" is the upper array
bound. Interrupt 5 occurs if the source value is less than or
higher than the source.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,mem32 - nj=13 nj=10 7 2
reg32,mem64 - nj=13 nj=10 7 2
- nj = no jump taken
Back to Top
BSF - Bit Scan Forward (386+)
Usage: BSF dest,src
Modifies flags: ZF
Scans source operand for first bit set. Sets ZF if a bit is
found set and loads the destination with an index to first
set bit. Clears ZF is no bits are found set. BSF scans
forward across bit pattern(0-n) while BSR scans in reverse (n-0).
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - - 10+3n 6-42 3
reg,mem - - 10+3n 7-43 3-7
reg32,reg32 - - 10+3n 6-42 3-7
reg32,mem32 - - 10+3n 7-43 3-7
Back to Top
BSR - Bit Scan Reverse (386+)
Usage: BSR dest,src
Modifies flags: ZF
Scans source operand for first bit set. Sets ZF if a bit is
found set and loads the destination with an index to first
set bit. Clears ZF is no bits are found set. BSF scans forward
across bit pattern (0-n) while BSR scans in reverse (n-0).
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - - 10+3n 6-103 3
reg,mem - - 10+3n 7-104 3-7
reg32,reg32 - - 10+3n 6-103 3-7
reg32,mem32 - - 10+3n 7-104 3-7
Back to Top
BSWAP - Byte Swap (486+)
Usage: BSWAP reg32
Modifies flags: none
Changes the byte order of a 32 bit register from big endian to
little endian or vice versa. Result left in destination register
is undefined if the operand is a 16 bit register.
Clocks Size
Operands 808x 286 386 486 Bytes
reg32 - - - 1 2
Back to Top
BT - Bit Test (386+)
Usage: BT dest,src
Modifies flags: CF
The destination bit indexed by the source value is copied into the
Carry Flag.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,immed8 - - 3 3 4-8
mem16,immed8 - - 6 6 4-8
reg16,reg16 - - 3 3 3-7
mem16,reg16 - - 12 12 3-7
Back to Top
BTC - Bit Test with Compliment (386+)
Usage: BTC dest,src
Modifies flags: CF
The destination bit indexed by the source value is copied into the
Carry Flag after being complimented (inverted).
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,immed8 - - 6 6 4-8
mem16,immed8 - - 8 8 4-8
reg16,reg16 - - 6 6 3-7
mem16,reg16 - - 13 13 3-7
Back to Top
BTR - Bit Test with Reset (386+)
Usage: BTR dest,src
Modifies flags: CF
The destination bit indexed by the source value is copied into the
Carry Flag and then cleared in the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,immed8 - - 6 6 4-8
mem16,immed8 - - 8 8 4-8
reg16,reg16 - - 6 6 3-7
mem16,reg16 - - 13 13 3-7
Back to Top
BTS - Bit Test and Set (386+)
Usage: BTS dest,src
Modifies flags: CF
The destination bit indexed by the source value is copied into the
Carry Flag and then set in the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,immed8 - - 6 6 4-8
mem16,immed8 - - 8 8 4-8
reg16,reg16 - - 6 6 3-7
mem16,reg16 - - 13 13 3-7
Back to Top
CALL - Procedure Call
Usage: CALL destination
Modifies flags: None
Pushes Instruction Pointer (and Code Segment for far calls) onto
stack and loads Instruction Pointer with the address of proc-name.
Code continues with execution at CS:IP.
Clocks
Operands 808x 286 386 486
rel16 (near, IP relative) 19 7 7+m 3
rel32 (near, IP relative) - - 7+m 3
reg16 (near, register indirect) 16 7 7+m 5
reg32 (near, register indirect) - - 7+m 5
mem16 (near, memory indirect) - 21+EA 11 10+m 5
mem32 (near, memory indirect) - - 10+m 5
ptr16:16 (far, full ptr supplied) 28 13 17+m 18
ptr16:32 (far, full ptr supplied) - - 17+m 18
ptr16:16 (far, ptr supplied, prot. mode) - 26 34+m 20
ptr16:32 (far, ptr supplied, prot. mode) - - 34+m 20
m16:16 (far, indirect) 37+EA 16 22+m 17
m16:32 (far, indirect) - - 22+m 17
m16:16 (far, indirect, prot. mode) - 29 38+m 20
m16:32 (far, indirect, prot. mode) - - 38+m 20
ptr16:16 (task, via TSS or task gate) - 177 TS 37+TS
m16:16 (task, via TSS or task gate) - 180/185 5+TS 37+TS
m16:32 (task) - - TS 37+TS
m16:32 (task) - - 5+TS 37+TS
ptr16:16 (gate, same privilege) - 41 52+m 35
ptr16:32 (gate, same privilege) - - 52+m 35
m16:16 (gate, same privilege) - 44 56+m 35
m16:32 (gate, same privilege) - - 56+m 35
ptr16:16 (gate, more priv, no parm) - 82 86+m 69
ptr16:32 (gate, more priv, no parm) - - 86+m 69
m16:16 (gate, more priv, no parm) - 83 90+m 69
m16:32 (gate, more priv, no parm) - - 90+m 69
ptr16:16 (gate, more priv, x parms) - 86+4x 94+4x+m 77+4x
ptr16:32 (gate, more priv, x parms) - - 94+4x+m 77+4x
m16:16 (gate, more priv, x parms) - 90+4x 98+4x+m 77+4x
m16:32 (gate, more priv, x parms) - - 98+4x+m 77+4x
Back to Top
CBW - Convert Byte to Word
Usage: CBW
Modifies flags: None
Converts byte in AL to word Value in AX by extending sign of AL
throughout register AH.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 3 3 1
Back to Top
CDQ - Convert Double to Quad (386+)
Usage: CDQ
Modifies flags: None
Converts signed DWORD in EAX to a signed quad word in EDX:EAX by
extending the high order bit of EAX throughout EDX
Clocks Size
Operands 808x 286 386 486 Bytes
none - - 2 3 1
Back to Top
CLC - Clear Carry
Usage: CLC
Modifies flags: CF
Clears the Carry Flag.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 2 1
Back to Top
CLD - Clear Direction Flag
Usage: CLD
Modifies flags: DF
Clears the Direction Flag causing string instructions to increment
the SI and DI index registers.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 2 1
Back to Top
CLI - Clear Interrupt Flag (disable)
Usage: CLI
Modifies flags: IF
Disables the maskable hardware interrupts by clearing the Interrupt
flag. NMI's and software interrupts are not inhibited.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 3 5 1
Back to Top
CLTS - Clear Task Switched Flag (286+ privileged)
Usage: CLTS
Modifies flags: None
Clears the Task Switched Flag in the Machine Status Register. This
is a privileged operation and is generally used only by operating
system code.
Clocks Size
Operands 808x 286 386 486 Bytes
none - 2 5 7 2
Back to Top
CMC - Complement Carry Flag
Usage: CMC
Modifies flags: CF
Toggles (inverts) the Carry Flag
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 2 1
Back to Top
CMP - Compare
Usage: CMP dest,src
Modifies flags: AF CF OF PF SF ZF
Subtracts source from destination and updates the flags but does
not save result. Flags can subsequently be checked for conditions.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 9+EA 7 5 2 2-4 (W88=13+EA)
reg,mem 9+EA 6 6 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 10+EA 6 5 2 3-6 (W88=14+EA)
accum,immed 4 3 2 1 2-3
Back to Top
CMPS - Compare String (Byte, Word or Doubleword)
Usage: CMPS dest,src
CMPSB
CMPSW
CMPSD (386+)
Modifies flags: AF CF OF PF SF ZF
Subtracts destination value from source without saving results.
Updates flags based on the subtraction and the index registers
(E)SI and (E)DI are incremented or decremented depending on the
state of the Direction Flag. CMPSB inc/decrements the index
registers by 1, CMPSW inc/decrements by 2, while CMPSD increments
or decrements by 4. The REP prefixes can be used to process
entire data items.
Clocks Size
Operands 808x 286 386 486 Bytes
dest,src 22 8 10 8 1 (W88=30)
Back to Top
CMPXCHG - Compare and Exchange
Usage: CMPXCHG dest,src (486+)
Modifies flags: AF CF OF PF SF ZF
Compares the accumulator (8-32 bits) with "dest". If equal the
"dest" is loaded with "src", otherwise the accumulator is loaded
with "dest".
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - - - 6 2
mem,reg - - - 7 2
- add 3 clocks if the "mem,reg" comparison fails
Back to Top
CWD - Convert Word to Doubleword
Usage: CWD
Modifies flags: None
Extends sign of word in register AX throughout register DX forming
a doubleword quantity in DX:AX.
Clocks Size
Operands 808x 286 386 486 Bytes
none 5 2 2 3 1
Back to Top
CWDE - Convert Word to Extended Doubleword (386+)
Usage: CWDE
Modifies flags: None
Converts a signed word in AX to a signed doubleword in EAX by
extending the sign bit of AX throughout EAX.
Clocks Size
Operands 808x 286 386 486 Bytes
none - - 3 3 1
Back to Top
DAA - Decimal Adjust for Addition
Usage: DAA
Modifies flags: AF CF PF SF ZF (OF undefined)
Corrects result (in AL) of a previous BCD addition operation.
Contents of AL are changed to a pair of packed decimal digits.
Clocks Size
Operands 808x 286 386 486 Bytes
none 4 3 4 2 1
Back to Top
DAS - Decimal Adjust for Subtraction
Usage: DAS
Modifies flags: AF CF PF SF ZF (OF undefined)
Corrects result (in AL) of a previous BCD subtraction operation.
Contents of AL are changed to a pair of packed decimal digits.
Clocks Size
Operands 808x 286 386 486 Bytes
none 4 3 4 2 1
Back to Top
DEC - Decrement
Usage: DEC dest
Modifies flags: AF OF PF SF ZF
Unsigned binary subtraction of one from the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 3 2 2 1 2
mem 15+EA 7 6 3 2-4
reg16/32 3 2 2 1 1
Back to Top
DIV - Divide
Usage: DIV src
Modifies flags: (AF,CF,OF,PF,SF,ZF undefined)
Unsigned binary division of accumulator by source. If the source
divisor is a byte value then AX is divided by "src" and the
quotient is placed in AL and the remainder in AH. If source
operand is a word value, then DX:AX is divided by "src" and the
quotient is stored in AX and the remainder in DX.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 80-90 14 14 16 2
reg16 144-162 22 22 24 2
reg32 - - 38 40 2
mem8 (86-96)+EA 17 17 16 2-4
mem16 (150-168)+EA 25 25 24 2-4 (W88=158-176+EA)
mem32 - - 41 40 2-4
Back to Top
ENTER - Make Stack Frame (80188+)
Usage: ENTER locals,level
Modifies flags: None
Modifies stack for entry to procedure for high level language.
Operand "locals" specifies the amount of storage to be allocated
on the stack. "Level" specifies the nesting level of the routine.
Paired with the LEAVE instruction, this is an efficient method of
entry and exit to procedures.
Clocks Size
Operands 808x 286 386 486 Bytes
immed16,0 - 11 10 14 4
immed16,1 - 15 12 17 4
immed16,immed8 - 12+4(n-1) 15+4(n-1) 17+3n 4
Back to Top
ESC - Escape
Usage: ESC immed,src
Modifies flags: None
Provides access to the data bus for other resident processors.
The CPU treats it as a NOP but places memory operand on bus.
Clocks Size
Operands 808x 286 386 486 Bytes
immed,reg 2 9-20 ? 2
immed,mem 2 9-20 ? 2-4
Back to Top
HLT - Halt CPU
Usage: HLT
Modifies flags: None
Halts CPU until RESET line is activated, NMI or maskable interrupt
received. The CPU becomes dormant but retains the current CS:IP
for later restart.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 5 4 1
Back to Top
IDIV - Signed Integer Division
Usage: IDIV src
Modifies flags: (AF,CF,OF,PF,SF,ZF undefined)
Signed binary division of accumulator by source. If source is a
byte value, AX is divided by "src" and the quotient is stored in
AL and the remainder in AH. If source is a word value, DX:AX is
divided by "src", and the quotient is stored in AL and the
remainder in DX.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 101-112 17 19 19 2
reg16 165-184 25 27 27 2
reg32 - - 43 43 2
mem8 (107-118)+EA 20 22 20 2-4
mem16 (171-190)+EA 38 30 28 2-4 (W88=175-194)
mem32 - - 46 44 2-4
Back to Top
IMUL - Signed Multiply
Usage: IMUL src
IMUL src,immed (286+)
IMUL dest,src,immed8 (286+)
IMUL dest,src (386+)
Modifies flags: CF OF (AF,PF,SF,ZF undefined)
Signed multiplication of accumulator by "src" with result placed
in the accumulator. If the source operand is a byte value, it
is multiplied by AL and the result stored in AX. If the source
operand is a word value it is multiplied by AX and the result is
stored in DX:AX. Other variations of this instruction allow
specification of source and destination registers as well as a
third immediate factor.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 80-98 13 9-14 13-18 2
reg16 128-154 21 9-22 13-26 2
reg32 - - 9-38 12-42 2
mem8 86-104 16 12-17 13-18 2-4
mem16 134-160 24 12-25 13-26 2-4
mem32 - - 12-41 13-42 2-4
reg16,reg16 - - 9-22 13-26 3-5
reg32,reg32 - - 9-38 13-42 3-5
reg16,mem16 - - 12-25 13-26 3-5
reg32,mem32 - - 12-41 13-42 3-5
reg16,immed - 21 9-22 13-26 3
reg32,immed - 21 9-38 13-42 3-6
reg16,reg16,immed - 2 9-22 13-26 3-6
reg32,reg32,immed - 21 9-38 13-42 3-6
reg16,mem16,immed - 24 12-25 13-26 3-6
reg32,mem32,immed - 24 12-41 13-42 3-6
Back to Top
IN - Input Byte or Word From Port
Usage: IN accum,port
Modifies flags: None
A byte, word or dword is read from "port" and placed in AL, AX or
EAX respectively. If the port number is in the range of 0-255
it can be specified as an immediate, otherwise the port number
must be specified in DX. Valid port ranges on the PC are 0-1024,
though values through 65535 may be specified and recognized by
third party vendors and PS/2's.
Clocks Size
Operands 808x 286 386 486 Bytes
accum,immed8 10/14 5 12 14 2
accum,immed8 (PM) 6/26 8/28/27 2
accum,DX 8/12 5 13 14 1
accum,DX (PM) 7/27 8/28/27 1
- 386+ protected mode timings depend on privilege levels.
first number is the timing if: CPL � IOPL
second number is the timing if: CPL > IOPL or in VM 86 mode (386)
CPL � IOPL (486)
third number is the timing when: virtual mode on 486 processor
- 486 virtual mode always requires 27 cycles
Back to Top
INC - Increment
Usage: INC dest
Modifies flags: AF OF PF SF ZF
Adds one to destination unsigned binary operand.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 3 2 2 1 2
reg16 3 2 2 1 1
reg32 3 2 2 1 1
mem 15+EA 7 6 3 2-4 (W88=23+EA)
Back to Top
INS - Input String from Port (80188+)
Usage: INS dest,port
INSB
INSW
INSD (386+)
Modifies flags: None
Loads data from port to the destination ES:(E)DI (even if a
destination operand is supplied). (E)DI is adjusted by the size
of the operand and increased if the Direction Flag is cleared and
decreased if the Direction Flag is set. For INSB, INSW, INSD no
operands are allowed and the size is determined by the mnemonic.
Clocks Size
Operands 808x 286 386 486 Bytes
dest,port - 5 15 17 1
dest,port (PM) - 5 9/29 10/32/30 1
none - 5 15 17 1
none (PM) - 5 9/29 10/32/30 1
- 386+ protected mode timings depend on privilege levels.
first number is the timing if: CPL � IOPL
second number is the timing if: CPL > IOPL
third number is the timing if: virtual mode on 486 processor
Back to Top
INT - Interrupt
Usage: INT num
Modifies flags: TF IF
Initiates a software interrupt by pushing the flags, clearing the
Trap and Interrupt Flags, pushing CS followed by IP and loading
CS:IP with the value found in the interrupt vector table. Execution
then begins at the location addressed by the new CS:IP
Clocks Size
Operands 808x 286 386 486 Bytes
3 (constant) 52/72 23+m 33 26 2
3 (prot. mode, same priv.) - 40+m 59 44 2
3 (prot. mode, more priv.) - 78+m 99 71 2
3 (from VM86 to PL 0) - - 119 82 2
3 (prot. mode via task gate) - 167+m TS 37+TS 2
immed8 51/71 23+m 37 30 1
immed8 (prot. mode, same priv.) - 40+m 59 44 1
immed8 (prot. mode, more priv.) - 78+m 99 71 1
immed8 (from VM86 to PL 0) - - 119 86 1
immed8 (prot. mode, via task gate) - 167+m TS 37+TS 1
Back to Top
INTO - Interrupt on Overflow
Usage: INTO
Modifies flags: IF TF
If the Overflow Flag is set this instruction generates an INT 4
which causes the code addressed by 0000:0010 to be executed.
Clocks Size
Operands 808x 286 386 486 Bytes
none: jump 53/73 24+m 35 28 1
no jump 4 3 3 3
(prot. mode, same priv.) - - 59 46 1
(prot. mode, more priv.) - - 99 73 1
(from VM86 to PL 0) - - 119 84 1
(prot. mode, via task gate) - TS 39+TS 1
Back to Top
INVD - Invalidate Cache (486+)
Usage: INVD
Modifies flags: none
Flushes CPU internal cache. Issues special function bus cycle
which indicates to flush external caches. Data in write-back
external caches is lost.
Clocks Size
Operands 808x 286 386 486 Bytes
none - - - 4 2
Back to Top
INVLPG - Invalidate Translation Look-Aside Buffer Entry (486+)
Usage: INVLPG
Modifies flags: none
Invalidates a single page table entry in the Translation
Look-Aside Buffer. Intel warns that this instruction may be
implemented differently on future processors.
Clocks Size
Operands 808x 286 386 486 Bytes
none - - - 12 2
- timing is for TLB entry hit only.
Back to Top
IRET/IRETD - Interrupt Return
Usage: IRET
IRETD (386+)
Modifies flags: AF CF DF IF PF SF TF ZF
Returns control to point of interruption by popping IP, CS
and then the Flags from the stack and continues execution at
this location. CPU exception interrupts will return to the
instruction that cause the exception because the CS:IP placed
on the stack during the interrupt is the address of the offending
instruction.
Clocks Size
Operands 808x 286 386 486 Bytes
iret 32/44 17+m 22 15 1
iret (prot. mode) - 31+m 38 15 1
iret (to less privilege) - 55+m 82 36 1
iret (different task, NT=1) - 169+m TS TS+32 1
iretd - - 22/38 15 1
iretd (to less privilege) - - 82 36 1
iretd (to VM86 mode) - - 60 15 1
iretd (different task, NT=1) - - TS TS+32 1
- 386 timings are listed as real-mode/protected-mode
Back to Top
Jxx - Jump Instructions Table
Mnemonic Meaning Jump Condition
JA Jump if Above CF=0 and ZF=0
JAE Jump if Above or Equal CF=0
JB Jump if Below CF=1
JBE Jump if Below or Equal CF=1 or ZF=1
JC Jump if Carry CF=1
JCXZ Jump if CX Zero CX=0
JE Jump if Equal ZF=1
JG Jump if Greater (signed) ZF=0 and SF=OF
JGE Jump if Greater or Equal (signed) SF=OF
JL Jump if Less (signed) SF != OF
JLE Jump if Less or Equal (signed) ZF=1 or SF != OF
JMP Unconditional Jump unconditional
JNA Jump if Not Above CF=1 or ZF=1
JNAE Jump if Not Above or Equal CF=1
JNB Jump if Not Below CF=0
JNBE Jump if Not Below or Equal CF=0 and ZF=0
JNC Jump if Not Carry CF=0
JNE Jump if Not Equal ZF=0
JNG Jump if Not Greater (signed) ZF=1 or SF != OF
JNGE Jump if Not Greater or Equal (signed) SF != OF
JNL Jump if Not Less (signed) SF=OF
JNLE Jump if Not Less or Equal (signed) ZF=0 and SF=OF
JNO Jump if Not Overflow (signed) OF=0
JNP Jump if No Parity PF=0
JNS Jump if Not Signed (signed) SF=0
JNZ Jump if Not Zero ZF=0
JO Jump if Overflow (signed) OF=1
JP Jump if Parity PF=1
JPE Jump if Parity Even PF=1
JPO Jump if Parity Odd PF=0
JS Jump if Signed (signed) SF=1
JZ Jump if Zero ZF=1
Clocks Size
Operands 808x 286 386 486 Bytes
Jx: jump 16 7+m 7+m 3 2
no jump 4 3 3 1
Jx near-label - - 7+m 3 4
no jump - - 3 1
- It's a good programming practice to organize code so the
expected case is executed without a jump since the actual
jump takes longer to execute than falling through the test.
- see JCXZ and JMP for their respective timings
Back to Top
JCXZ/JECXZ - Jump if Register (E)CX is Zero
Usage: JCXZ label
JECXZ label (386+)
Modifies flags: None
Causes execution to branch to "label" if register CX is zero. Uses
unsigned comparision.
Clocks Size
Operands 808x 286 386 486 Bytes
label: jump 18 8+m 9+m 8 2
no jump 6 4 5 5
Back to Top
JMP - Unconditional Jump
Usage: JMP target
Modifies flags: None
Unconditionally transfers control to "label". Jumps by default
are within -32768 to 32767 bytes from the instruction following
the jump. NEAR and SHORT jumps cause the IP to be updated while FAR
jumps cause CS and IP to be updated.
Clocks
Operands 808x 286 386 486
rel8 (relative) 15 7+m 7+m 3
rel16 (relative) 15 7+m 7+m 3
rel32 (relative) - - 7+m 3
reg16 (near, register indirect) 11 7+m 7+m 5
reg32 (near, register indirect) - - 7+m 5
mem16 (near, mem indirect) 18+EA 11+m 10+m 5
mem32 (near, mem indirect) 24+EA 15+m 10+m 5
ptr16:16 (far, dword immed) - - 12+m 17
ptr16:16 (far, PM dword immed) - - 27+m 19
ptr16:16 (call gate, same priv.) - 38+m 45+m 32
ptr16:16 (via TSS) - 175+m TS 42+TS
ptr16:16 (via task gate) - 180+m TS 43+TS
mem16:16 (far, indirect) - - 43+m 13
mem16:16 (far, PM indirect) - - 31+m 18
mem16:16 (call gate, same priv.) - 41+m 49+m 31
mem16:16 (via TSS) - 178+m 5+TS 41+TS
mem16:16 (via task gate) - 183+m 5+TS 42+TS
ptr16:32 (far, 6 byte immed) - - 12+m 13
ptr16:32 (far, PM 6 byte immed) - - 27+m 18
ptr16:32 (call gate, same priv.) - - 45+m 31
ptr16:32 (via TSS) - - TS 42+TS
ptr16:32 (via task state) - - TS 43+TS
m16:32 (far, address at dword) - - 43+m 13
m16:32 (far, address at dword) - - 31+m 18
m16:32 (call gate, same priv.) - - 49+m 31
m16:32 (via TSS) - - 5+TS 41+TS
m16:32 (via task state) - - 5+TS 42+TS
Back to Top
LAHF - Load Register AH From Flags
Usage: LAHF
Modifies flags: None
Copies bits 0-7 of the flags register into AH. This includes flags
AF, CF, PF, SF and ZF other bits are undefined.
AH := SF ZF xx AF xx PF xx CF
Clocks Size
Operands 808x 286 386 486 Bytes
none 4 2 2 3 1
Back to Top
LAR - Load Access Rights (286+ protected)
Usage: LAR dest,src
Modifies flags: ZF
The high byte of the of the destination register is overwritten by
the value of the access rights byte and the low order byte is zeroed
depending on the selection in the source operand. The Zero Flag is
set if the load operation is successful.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,reg16 - 14 15 11 3
reg32,reg32 - - 15 11 3
reg16,mem16 - 16 16 11 3-7
reg32,mem32 - - 16 11 3-7
Back to Top
LDS - Load Pointer Using DS
Usage: LDS dest,src
Modifies flags: None
Loads 32-bit pointer from memory source to destination register
and DS. The offset is placed in the destination register and the
segment is placed in DS. To use this instruction the word at the
lower memory address must contain the offset and the word at the
higher address must contain the segment. This simplifies the loading
of far pointers from the stack and the interrupt vector table.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,mem32 16+EA 7 7 6 2-4
reg,mem (PM) - - 22 12 5-7
Back to Top
LEA - Load Effective Address
Usage: LEA dest,src
Modifies flags: None
Transfers offset address of "src" to the destination register.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,mem 2+EA 3 2 1 2-4
- the MOV instruction can often save clock cycles when used in
place of LEA on 8088 processors
Back to Top
LEAVE - Restore Stack for Procedure Exit (80188+)
Usage: LEAVE
Modifies flags: None
Releases the local variables created by the previous ENTER
instruction by restoring SP and BP to their condition before
the procedure stack frame was initialized.
Clocks Size
Operands 808x 286 386 486 Bytes
none - 5 4 5 1
Back to Top
LES - Load Pointer Using ES
Usage: LES dest,src
Modifies flags: None
Loads 32-bit pointer from memory source to destination register
and ES. The offset is placed in the destination register and the
segment is placed in ES. To use this instruction the word at the
lower memory address must contain the offset and the word at the
higher address must contain the segment. This simplifies the loading
of far pointers from the stack and the interrupt vector table.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,mem 16+EA 7 7 6 2-4 (W88=24+EA)
reg,mem (PM) - - 22 12 5-7
Back to Top
LFS - Load Pointer Using FS (386+)
Usage: LFS dest,src
Modifies flags: None
Loads 32-bit pointer from memory source to destination register
and FS. The offset is placed in the destination register and the
segment is placed in FS. To use this instruction the word at the
lower memory address must contain the offset and the word at the
higher address must contain the segment. This simplifies the loading
of far pointers from the stack and the interrupt vector table.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,mem - - 7 6 5-7
reg,mem (PM) - - 22 12 5-7
Back to Top
LGDT - Load Global Descriptor Table (286+ privileged)
Usage: LGDT src
Modifies flags: None
Loads a value from an operand into the Global Descriptor Table
(GDT) register.
Clocks Size
Operands 808x 286 386 486 Bytes
mem64 - 11 11 11 5
Back to Top
LIDT - Load Interrupt Descriptor Table (286+ privileged)
Usage: LIDT src
Modifies flags: None
Loads a value from an operand into the Interrupt Descriptor Table
(IDT) register.
Clocks Size
Operands 808x 286 386 486 Bytes
mem64 - 12 11 11 5
Back to Top
LGS - Load Pointer Using GS (386+)
Usage: LGS dest,src
Modifies flags: None
Loads 32-bit pointer from memory source to destination register
and GS. The offset is placed in the destination register and the
segment is placed in GS. To use this instruction the word at the
lower memory address must contain the offset and the word at the
higher address must contain the segment. This simplifies the loading
of far pointers from the stack and the interrupt vector table.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,mem - - 7 6 5-7
reg,mem (PM) - - 22 12 5-7
Back to Top
LLDT - Load Local Descriptor Table (286+ privileged)
Usage: LLDT src
Modifies flags: None
Loads a value from an operand into the Local Descriptor Table
Register (LDTR).
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 17 20 11 3
mem16 - 19 24 11 5
Back to Top
LMSW - Load Machine Status Word (286+ privileged)
Usage: LMSW src
Modifies flags: None
Loads the Machine Status Word (MSW) from data found at "src"
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 3 10 13 3
mem16 - 6 13 13 5
Back to Top
LOCK - Lock Bus
Usage: LOCK
LOCK: (386+ prefix)
Modifies flags: None
This instruction is a prefix that causes the CPU assert bus lock
signal during the execution of the next instruction. Used to
avoid two processors from updating the same data location. The
286 always asserts lock during an XCHG with memory operands. This
should only be used to lock the bus prior to XCHG, MOV, IN and
OUT instructions.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 0 0 1 1
Back to Top
LODS - Load String (Byte, Word or Double)
Usage: LODS src
LODSB
LODSW
LODSD (386+)
Modifies flags: None
Transfers string element addressed by DS:SI (even if an operand is
supplied) to the accumulator. SI is incremented based on the size
of the operand or based on the instruction used. If the Direction
Flag is set SI is decremented, if the Direction Flag is clear SI
is incremented. Use with REP prefixes.
Clocks Size
Operands 808x 286 386 486 Bytes
src 12/16 5 5 5 1
Back to Top
LOOP - Decrement CX and Loop if CX Not Zero
Usage: LOOP label
Modifies flags: None
Decrements CX by 1 and transfers control to "label" if CX is not
Zero. The "label" operand must be within -128 or 127 bytes of the
instruction following the loop instruction
Clocks Size
Operands 808x 286 386 486 Bytes
label: jump 18 8+m 11+m 6 2
no jump 5 4 ? 2
Back to Top
LOOPE/LOOPZ - Loop While Equal / Loop While Zero
Usage: LOOPE label
LOOPZ label
Modifies flags: None
Decrements CX by 1 (without modifying the flags) and transfers
control to "label" if CX != 0 and the Zero Flag is set. The
"label" operand must be within -128 or 127 bytes of the instruction
following the loop instruction.
Clocks Size
Operands 808x 286 386 486 Bytes
label: jump 18 8+m 11+m 9 2
no jump 5 4 ? 6
Back to Top
LOOPNZ/LOOPNE - Loop While Not Zero / Loop While Not Equal
Usage: LOOPNZ label
LOOPNE label
Modifies flags: None
Decrements CX by 1 (without modifying the flags) and transfers
control to "label" if CX != 0 and the Zero Flag is clear. The
"label" operand must be within -128 or 127 bytes of the instruction
following the loop instruction.
Clocks Size
Operands 808x 286 386 486 Bytes
label: jump 19 8+m 11+m 9 2
no jump 5 4 ? 6
Back to Top
LSL - Load Segment Limit (286+ protected)
Usage: LSL dest,src
Modifies flags: ZF
Loads the segment limit of a selector into the destination register
if the selector is valid and visible at the current privilege level.
If loading is successful the Zero Flag is set, otherwise it is
cleared.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,reg16 - 14 20/25 10 3
reg32,reg32 - - 20/25 10 3
reg16,mem16 - 16 21/26 10 5
reg32,mem32 - - 21/26 10 5
- 386 times are listed "byte granular" / "page granular"
Back to Top
LSS - Load Pointer Using SS (386+)
Usage: LSS dest,src
Modifies flags: None
Loads 32-bit pointer from memory source to destination register
and SS. The offset is placed in the destination register and the
segment is placed in SS. To use this instruction the word at the
lower memory address must contain the offset and the word at the
higher address must contain the segment. This simplifies the loading
of far pointers from the stack and the interrupt vector table.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,mem - - 7 6 5-7
reg,mem (PM) - - 22 12 5-7
Back to Top
LTR - Load Task Register (286+ privileged)
Usage: LTR src
Modifies flags: None
Loads the current task register with the value specified in "src".
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 17 23 20 3
mem16 - 19 27 20 5
Back to Top
MOV - Move Byte or Word
Usage: MOV dest,src
Modifies flags: None
Copies byte or word from the source operand to the destination
operand. If the destination is SS interrupts are disabled except
on early buggy 808x CPUs. Some CPUs disable interrupts if the
destination is any of the segment registers
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 2 2 2 1 2
mem,reg 9+EA 3 2 1 2-4 (W88=13+EA)
reg,mem 8+EA 5 4 1 2-4 (W88=12+EA)
mem,immed 10+EA 3 2 1 3-6 (W88=14+EA)
reg,immed 4 2 2 1 2-3
mem,accum 10 3 2 1 3 (W88=14)
accum,mem 10 5 4 1 3 (W88=14)
segreg,reg16 2 2 2 3 2
segreg,mem16 8+EA 5 5 9 2-4 (W88=12+EA)
reg16,segreg 2 2 2 3 2
mem16,segreg 9+EA 3 2 3 2-4 (W88=13+EA)
reg32,CR0/CR2/CR3 - - 6 4
CR0,reg32 - - 10 16
CR2,reg32 - - 4 4 3
CR3,reg32 - - 5 4 3
reg32,DR0/DR1/DR2/DR3 - 22 10 3
reg32,DR6/DR7 - - 22 10 3
DR0/DR1/DR2/DR3,reg32 - 22 11 3
DR6/DR7,reg32 - - 16 11 3
reg32,TR6/TR7 - - 12 4 3
TR6/TR7,reg32 - - 12 4 3
reg32,TR3 3
TR3,reg32 6
- when the 386 special registers are used all operands are 32 bits
Back to Top
MOVS - Move String (Byte or Word)
Usage: MOVS dest,src
MOVSB
MOVSW
MOVSD (386+)
Modifies flags: None
Copies data from addressed by DS:SI (even if operands are given) to
the location ES:DI destination and updates SI and DI based on the
size of the operand or instruction used. SI and DI are incremented
when the Direction Flag is cleared and decremented when the Direction
Flag is Set. Use with REP prefixes.
Clocks Size
Operands 808x 286 386 486 Bytes
dest,src 18 5 7 7 1 (W88=26)
Back to Top
MOVSX - Move with Sign Extend (386+)
Usage: MOVSX dest,src
Modifies flags: None
Copies the value of the source operand to the destination register
with the sign extended.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - - 3 3 3
reg,mem - - 6 3 3-7
Back to Top
MOVZX - Move with Zero Extend (386+)
Usage: MOVZX dest,src
Modifies flags: None
Copies the value of the source operand to the destination register
with the zeroes extended.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg - - 3 3 3
reg,mem - - 6 3 3-7
Back to Top
MUL - Unsigned Multiply
Usage: MUL src
Modifies flags: CF OF (AF,PF,SF,ZF undefined)
Unsigned multiply of the accumulator by the source. If "src" is
a byte value, then AL is used as the other multiplicand and the
result is placed in AX. If "src" is a word value, then AX is
multiplied by "src" and DX:AX receives the result. If "src" is
a double word value, then EAX is multiplied by "src" and EDX:EAX
receives the result. The 386+ uses an early out algorithm which
makes multiplying any size value in EAX as fast as in the 8 or 16
bit registers.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 70-77 13 9-14 13-18 2
reg16 118-113 21 9-22 13-26 2
reg32 - - 9-38 13-42 2-4
mem8 (76-83)+EA 16 12-17 13-18 2-4
mem16 (124-139)+EA 24 12-25 13-26 2-4
mem32 - - 12-21 13-42 2-4
Back to Top
NEG - Two's Complement Negation
Usage: NEG dest
Modifies flags: AF CF OF PF SF ZF
Subtracts the destination from 0 and saves the 2s complement of
"dest" back into "dest".
Clocks Size
Operands 808x 286 386 486 Bytes
reg 3 2 2 1 2
mem 16+EA 7 6 3 2-4 (W88=24+EA)
Back to Top
NOP - No Operation (90h)
Usage: NOP
Modifies flags: None
This is a do nothing instruction. It results in occupation of both
space and time and is most useful for patching code segments.
(This is the original XCHG AL,AL instruction)
Clocks Size
Operands 808x 286 386 486 Bytes
none 3 3 3 1 1
Back to Top
NOT - One's Compliment Negation (Logical NOT)
Usage: NOT dest
Modifies flags: None
Inverts the bits of the "dest" operand forming the 1s complement.
Clocks Size
Operands 808x 286 386 486 Bytes
reg 3 2 2 1 2
mem 16+EA 7 6 3 2-4 (W88=24+EA)
Back to Top
OR - Inclusive Logical OR
Usage: OR dest,src
Modifies flags: CF OF PF SF ZF (AF undefined)
Logical inclusive OR of the two operands returning the result in
the destination. Any bit set in either operand will be set in the
destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 7 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 6 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem8,immed8 17+EA 7 7 3 3-6
mem16,immed16 25+EA 7 7 3 3-6
accum,immed 4 3 2 1 2-3
Back to Top
OUT - Output Data to Port
Usage: OUT port,accum
Modifies flags: None
Transfers byte in AL,word in AX or dword in EAX to the specified
hardware port address. If the port number is in the range of 0-255
it can be specified as an immediate. If greater than 255 then the
port number must be specified in DX. Since the PC only decodes 10
bits of the port address, values over 1023 can only be decoded by
third party vendor equipment and also map to the port range 0-1023.
Clocks Size
Operands 808x 286 386 486 Bytes
immed8,accum 10/14 3 10 16 2
immed8,accum (PM) - - 4/24 11/31/29 2
DX,accum 8/12 3 11 16 1
DX,accum (PM) - - 5/25 10/30/29 1
- 386+ protected mode timings depend on privilege levels.
first number is the timing when: CPL � IOPL
second number is the timing when: CPL > IOPL
third number is the timing when: virtual mode on 486 processor
Back to Top
OUTS - Output String to Port (80188+)
Usage: OUTS port,src
OUTSB
OUTSW
OUTSD (386+)
Modifies flags: None
Transfers a byte, word or doubleword from "src" to the hardware
port specified in DX. For instructions with no operands the "src"
is located at DS:SI and SI is incremented or decremented by the
size of the operand or the size dictated by the instruction format.
When the Direction Flag is set SI is decremented, when clear, SI is
incremented. If the port number is in the range of 0-255 it can
be specified as an immediate. If greater than 255 then the port
number must be specified in DX. Since the PC only decodes 10 bits
of the port address, values over 1023 can only be decoded by third
party vendor equipment and also map to the port range 0-1023.
Clocks Size
Operands 808x 286 386 486 Bytes
port,src - 5 14 17 1
port,src (PM) - - 8/28 10/32/30 1
- 386+ protected mode timings depend on privilege levels.
first number is the timing when: CPL � IOPL
second number is the timing when: CPL > IOPL
third number is the timing when: virtual mode on 486 processor
Back to Top
POP - Pop Word off Stack
Usage: POP dest
Modifies flags: None
Transfers word at the current stack top (SS:SP) to the destination
then increments SP by two to point to the new stack top. CS is not
a valid destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 8 5 4 4 1
reg32 4 - - 4 1
segreg 8 5 7 3 1
mem16 17+EA 5 5 6 2-4
mem32 5 - - 6 2-4
Back to Top
POPA/POPAD - Pop All Registers onto Stack (80188+)
Usage: POPA
POPAD (386+)
Modifies flags: None
Pops the top 8 words off the stack into the 8 general purpose 16/32
bit registers. Registers are popped in the following order: (E)DI,
(E)SI, (E)BP, (E)SP, (E)DX, (E)CX and (E)AX. The (E)SP value popped
from the stack is actually discarded.
Clocks Size
Operands 808x 286 386 486 Bytes
none - 19 24 9 1
Back to Top
POPF/POPFD - Pop Flags off Stack
Usage: POPF
POPFD (386+)
Modifies flags: all flags
Pops word/doubleword from stack into the Flags Register and then
increments SP by 2 (for POPF) or 4 (for POPFD).
Clocks Size
Operands 808x 286 386 486 Bytes
none 8/12 5 5 9 1 (W88=12)
none (PM) - - 5 6 1
Back to Top
PUSH - Push Word onto Stack
Usage: PUSH src
PUSH immed (80188+ only)
Modifies flags: None
Decrements SP by the size of the operand (two or four, byte values
are sign extended) and transfers one word from source to the stack
top (SS:SP).
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 11/15 3 2 1 1
reg32 - - 2 1 1
mem16 16+EA 5 5 4 2-4 (W88=24+EA)
mem32 - - 5 4 2-4
segreg 10/14 3 2 3 1
immed - 3 2 1 2-3
Back to Top
PUSHA/PUSHAD - Push All Registers onto Stack (80188+)
Usage: PUSHA
PUSHAD (386+)
Modifies flags: None
Pushes all general purpose registers onto the stack in the following
order: (E)AX, (E)CX, (E)DX, (E)BX, (E)SP, (E)BP, (E)SI, (E)DI. The
value of SP is the value before the actual push of SP.
Clocks Size
Operands 808x 286 386 486 Bytes
none - 19 24 11 1
Back to Top
PUSHF/PUSHFD - Push Flags onto Stack
Usage: PUSHF
PUSHFD (386+)
Modifies flags: None
Transfers the Flags Register onto the stack. PUSHF saves a 16 bit
value while PUSHFD saves a 32 bit value.
Clocks Size
Operands 808x 286 386 486 Bytes
none 10/14 3 4 4 1
none (PM) - - 4 3 1
Back to Top
RCL - Rotate Through Carry Left
Usage: RCL dest,count
Modifies flags: CF OF
Rotates the bits in the destination to the left "count" times with
all data pushed out the left side re-entering on the right. The
Carry Flag holds the last bit rotated out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 9 3 2
mem,1 15+EA 7 10 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 9 8-30 2
mem,CL 20+EA+4n 8+n 10 9-31 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 9 8-30 3
mem,immed8 - 8+n 10 9-31 3-5
Back to Top
RCR - Rotate Through Carry Right
Usage: RCR dest,count
Modifies flags: CF OF
Rotates the bits in the destination to the right "count" times with
all data pushed out the right side re-entering on the left. The
Carry Flag holds the last bit rotated out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 9 3 2
mem,1 15+EA 7 10 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 9 8-30 2
mem,CL 20+EA+4n 8+n 10 9-31 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 9 8-30 3
mem,immed8 - 8+n 10 9-31 3-5
Back to Top
REP - Repeat String Operation
Usage: REP
Modifies flags: None
Repeats execution of string instructions while CX != 0. After
each string operation, CX is decremented and the Zero Flag is
tested. The combination of a repeat prefix and a segment override
on CPU's before the 386 may result in errors if an interrupt occurs
before CX=0. The following code shows code that is susceptible to
this and how to avoid it:
again: rep movs byte ptr ES:[DI],ES:[SI] ; vulnerable instr.
jcxz next ; continue if REP successful
loop again ; interrupt goofed count
next:
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 1
Back to Top
REPE/REPZ - Repeat Equal / Repeat Zero
Usage: REPE
REPZ
Modifies flags: None
Repeats execution of string instructions while CX != 0 and the Zero
Flag is set. CX is decremented and the Zero Flag tested after
each string operation. The combination of a repeat prefix and a
segment override on processors other than the 386 may result in
errors if an interrupt occurs before CX=0.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 1
Back to Top
REPNE/REPNZ - Repeat Not Equal / Repeat Not Zero
Usage: REPNE
REPNZ
Modifies flags: None
Repeats execution of string instructions while CX != 0 and the Zero
Flag is clear. CX is decremented and the Zero Flag tested after
each string operation. The combination of a repeat prefix and a
segment override on processors other than the 386 may result in
errors if an interrupt occurs before CX=0.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 1
Back to Top
RET/RETF - Return From Procedure
Usage: RET nBytes
RETF nBytes
RETN nBytes
Modifies flags: None
Transfers control from a procedure back to the instruction address
saved on the stack. "n bytes" is an optional number of bytes to
release. Far returns pop the IP followed by the CS, while near
returns pop only the IP register.
Clocks Size
Operands 808x 286 386 486 Bytes
retn 16/20 11+m 10+m 5 1
retn immed 20/24 11+m 10+m 5 3
retf 26/34 15+m 18+m 13 1
retf (PM, same priv.) - 32+m 18 1
retf (PM, lesser priv.) - 68 33 1
retf immed 25/33 15+m 18+m 14 3
retf immed (PM, same priv.) 32+m 17 1
retf immed (PM, lesser priv.) 68 33 1
Back to Top
ROL - Rotate Left
Usage: ROL dest,count
Modifies flags: CF OF
Rotates the bits in the destination to the left "count" times with
all data pushed out the left side re-entering on the right. The
Carry Flag will contain the value of the last bit rotated out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 3 3 2
mem,1 15+EA 7 7 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 3 3 2
mem,CL 20+EA+4n 8+n 7 4 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 3 2 3
mem,immed8 - 8+n 7 4 3-5
Back to Top
ROR - Rotate Right
Usage: ROR dest,count
Modifies flags: CF OF
Rotates the bits in the destination to the right "count" times with
all data pushed out the right side re-entering on the left. The
Carry Flag will contain the value of the last bit rotated out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 3 3 2
mem,1 15+EA 7 7 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 3 3 2
mem,CL 20+EA+4n 8+n 7 4 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 3 2 3
mem,immed8 - 8+n 7 4 3-5
Back to Top
SAHF - Store AH Register into FLAGS
Usage: SAHF
Modifies flags: AF CF PF SF ZF
Transfers bits 0-7 of AH into the Flags Register. This includes
AF, CF, PF, SF and ZF.
Clocks Size
Operands 808x 286 386 486 Bytes
none 4 2 3 2 1
Back to Top
SAL/SHL - Shift Arithmetic Left / Shift Logical Left
Usage: SAL dest,count
SHL dest,count
Modifies flags: CF OF PF SF ZF (AF undefined)
Shifts the destination left by "count" bits with zeroes shifted
in on right. The Carry Flag contains the last bit shifted out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 3 3 2
mem,1 15+EA 7 7 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 3 3 2
mem,CL 20+EA+4n 8+n 7 4 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 3 2 3
mem,immed8 - 8+n 7 4 3-5
Back to Top
SAR - Shift Arithmetic Right
Usage: SAR dest,count
Modifies flags: CF OF PF SF ZF (AF undefined)
Shifts the destination right by "count" bits with the current sign
bit replicated in the leftmost bit. The Carry Flag contains the
last bit shifted out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 3 3 2
mem,1 15+EA 7 7 4 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 3 3 2
mem,CL 20+EA+4n 8+n 7 4 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 3 2 3
mem,immed8 - 8+n 7 4 3-5
Back to Top
SBB - Subtract with Borrow/Carry
Usage: SBB dest,src
Modifies flags: AF CF OF PF SF ZF
Subtracts the source from the destination, and subtracts 1 extra if
the Carry Flag is set. Results are returned in "dest".
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 6 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 7 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=25+EA)
accum,immed 4 3 2 1 2-3
Back to Top
SCAS - Scan String (Byte, Word or Doubleword)
Usage: SCAS string
SCASB
SCASW
SCASD (386+)
Modifies flags: AF CF OF PF SF ZF
Compares value at ES:DI (even if operand is specified) from the
accumulator and sets the flags similar to a subtraction. DI is
incremented/decremented based on the instruction format (or
operand size) and the state of the Direction Flag. Use with REP
prefixes.
Clocks Size
Operands 808x 286 386 486 Bytes
string 15 7 7 6 1 (W88=19)
Back to Top
SETAE/SETNB - Set if Above or Equal / Set if Not Below (386+)
Usage: SETAE dest
SETNB dest
(unsigned, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Carry Flag is clear
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETB/SETNAE - Set if Below / Set if Not Above or Equal (386+)
Usage: SETB dest
SETNAE dest
(unsigned, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Carry Flag is set
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETBE/SETNA - Set if Below or Equal / Set if Not Above (386+)
Usage: SETBE dest
SETNA dest
(unsigned, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Carry Flag or the Zero
Flag is set, otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETE/SETZ - Set if Equal / Set if Zero (386+)
Usage: SETE dest
SETZ dest
Modifies flags: none
Sets the byte in the operand to 1 if the Zero Flag is set,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETNE/SETNZ - Set if Not Equal / Set if Not Zero (386+)
Usage: SETNE dest
SETNZ dest
Modifies flags: none
Sets the byte in the operand to 1 if the Zero Flag is clear,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETL/SETNGE - Set if Less / Set if Not Greater or Equal (386+)
Usage: SETL dest
SETNGE dest
(signed, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Sign Flag is not equal
to the Overflow Flag, otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETGE/SETNL - Set if Greater or Equal / Set if Not Less (386+)
Usage: SETGE dest
SETNL dest
(signed, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Sign Flag equals the
Overflow Flag, otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETLE/SETNG - Set if Less or Equal / Set if Not greater or Equal (386+)
Usage: SETLE dest
SETNG dest
(signed, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Zero Flag is set or the
Sign Flag is not equal to the Overflow Flag, otherwise sets the
operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETG/SETNLE - Set if Greater / Set if Not Less or Equal (386+)
Usage: SETG dest
SETNLE dest
(signed, 386+)
Modifies flags: none
Sets the byte in the operand to 1 if the Zero Flag is clear or the
Sign Flag equals to the Overflow Flag, otherwise sets the operand
to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETS - Set if Signed (386+)
Usage: SETS dest
Modifies flags: none
Sets the byte in the operand to 1 if the Sign Flag is set, otherwise
sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETNS - Set if Not Signed (386+)
Usage: SETNS dest
Modifies flags: none
Sets the byte in the operand to 1 if the Sign Flag is clear,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETC - Set if Carry (386+)
Usage: SETC dest
Modifies flags: none
Sets the byte in the operand to 1 if the Carry Flag is set,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETNC - Set if Not Carry (386+)
Usage: SETNC dest
Modifies flags: none
Sets the byte in the operand to 1 if the Carry Flag is clear,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETO - Set if Overflow (386+)
Usage: SETO dest
Modifies flags: none
Sets the byte in the operand to 1 if the Overflow Flag is set,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETNO - Set if Not Overflow (386+)
Usage: SETNO dest
Modifies flags: none
Sets the byte in the operand to 1 if the Overflow Flag is clear,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETP/SETPE - Set if Parity / Set if Parity Even (386+)
Usage: SETP dest
SETPE dest
Modifies flags: none
Sets the byte in the operand to 1 if the Parity Flag is set,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SETNP/SETPO - Set if No Parity / Set if Parity Odd (386+)
Usage: SETNP dest
SETPO dest
Modifies flags: none
Sets the byte in the operand to 1 if the Parity Flag is clear,
otherwise sets the operand to 0.
Clocks Size
Operands 808x 286 386 486 Bytes
reg8 - - 4 3 3
mem8 - - 5 4 3
Back to Top
SGDT - Store Global Descriptor Table (286+ privileged)
Usage: SGDT dest
Modifies flags: none
Stores the Global Descriptor Table (GDT) Register into the
specified operand.
Clocks Size
Operands 808x 286 386 486 Bytes
mem64 - 11 9 10 5
Back to Top
SIDT - Store Interrupt Descriptor Table (286+ privileged)
Usage: SIDT dest
Modifies flags: none
Stores the Interrupt Descriptor Table (IDT) Register into the
specified operand.
Clocks Size
Operands 808x 286 386 486 Bytes
mem64 - 12 9 10 5
Back to Top
SHL - Shift Logical Left
See: SAL
Back to Top
SHR - Shift Logical Right
Usage: SHR dest,count
Modifies flags: CF OF PF SF ZF (AF undefined)
Shifts the destination right by "count" bits with zeroes shifted
in on the left. The Carry Flag contains the last bit shifted out.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,1 2 2 3 2
mem,1 15+EA 7 7 2-4 (W88=23+EA)
reg,CL 8+4n 5+n 3 2
mem,CL 20+EA+4n 8+n 7 2-4 (W88=28+EA+4n)
reg,immed8 - 5+n 3 3
mem,immed8 - 8+n 7 3-5
Back to Top
SHLD/SHRD - Double Precision Shift (386+)
Usage: SHLD dest,src,count
SHRD dest,src,count
Modifies flags: CF PF SF ZF (OF,AF undefined)
SHLD shifts "dest" to the left "count" times and the bit positions
opened are filled with the most significant bits of "src". SHRD
shifts "dest" to the right "count" times and the bit positions
opened are filled with the least significant bits of the second
operand. Only the 5 lower bits of "count" are used.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16,reg16,immed8 - - 3 2 4
reg32,reg32,immed8 - - 3 2 4
mem16,reg16,immed8 - - 7 3 6
mem32,reg32,immed8 - - 7 3 6
reg16,reg16,CL - - 3 3 3
reg32,reg32,CL - - 3 3 3
mem16,reg16,CL - - 7 4 5
mem32,reg32,CL - - 7 4 5
Back to Top
SLDT - Store Local Descriptor Table (286+ privileged)
Usage: SLDT dest
Modifies flags: none
Stores the Local Descriptor Table (LDT) Register into the
specified operand.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 2 2 2 3
mem16 - 2 2 3 5
Back to Top
SMSW - Store Machine Status Word (286+ privileged)
Usage: SMSW dest
Modifies flags: none
Store Machine Status Word (MSW) into "dest".
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 2 10 2 3
mem16 - 3 3 3 5
Back to Top
STC - Set Carry
Usage: STC
Modifies flags: CF
Sets the Carry Flag to 1.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 2 1
Back to Top
STD - Set Direction Flag
Usage: STD
Modifies flags: DF
Sets the Direction Flag to 1 causing string instructions to
auto-decrement SI and DI instead of auto-increment.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 2 1
Back to Top
STI - Set Interrupt Flag (Enable Interrupts)
Usage: STI
Modifies flags: IF
Sets the Interrupt Flag to 1, which enables recognition of all
hardware interrupts. If an interrupt is generated by a hardware
device, an End of Interrupt (EOI) must also be issued to enable
other hardware interrupts of the same or lower priority.
Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 5 1
Back to Top
STOS - Store String (Byte, Word or Doubleword)
Usage: STOS dest
STOSB
STOSW
STOSD
Modifies flags: None
Stores value in accumulator to location at ES:(E)DI (even if operand
is given). (E)DI is incremented/decremented based on the size of
the operand (or instruction format) and the state of the Direction
Flag. Use with REP prefixes.
Clocks Size
Operands 808x 286 386 486 Bytes
dest 11 3 4 5 1 (W88=15)
Back to Top
STR - Store Task Register (286+ privileged)
Usage: STR dest
Modifies flags: None
Stores the current Task Register to the specified operand.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 2 2 2 3
mem16 - 3 2 3 5
Back to Top
SUB - Subtract
Usage: SUB dest,src
Modifies flags: AF CF OF PF SF ZF
The source is subtracted from the destination and the result is
stored in the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 6 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 7 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=25+EA)
accum,immed 4 3 2 1 2-3
Back to Top
TEST - Test For Bit Pattern
Usage: TEST dest,src
Modifies flags: CF OF PF SF ZF (AF undefined)
Performs a logical AND of the two operands updating the flags
register without saving the result.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 1 1 2
reg,mem 9+EA 6 5 1 2-4 (W88=13+EA)
mem,reg 9+EA 6 5 2 2-4 (W88=13+EA)
reg,immed 5 3 2 1 3-4
mem,immed 11+EA 6 5 2 3-6
accum,immed 4 3 2 1 2-3
Back to Top
VERR - Verify Read (286+ protected)
Usage: VERR src
Modifies flags: ZF
Verifies the specified segment selector is valid and is readable
at the current privilege level. If the segment is readable,
the Zero Flag is set, otherwise it is cleared.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 14 10 11 3
mem16 - 16 11 11 5
Back to Top
VERW - Verify Write (286+ protected)
Usage: VERW src
Modifies flags: ZF
Verifies the specified segment selector is valid and is ratable
at the current privilege level. If the segment is writable,
the Zero Flag is set, otherwise it is cleared.
Clocks Size
Operands 808x 286 386 486 Bytes
reg16 - 14 15 11 3
mem16 - 16 16 11 5
Back to Top
WAIT/FWAIT - Event Wait
Usage: WAIT
FWAIT
Modifies flags: None
CPU enters wait state until the coprocessor signals it has finished
its operation. This instruction is used to prevent the CPU from
accessing memory that may be temporarily in use by the coprocessor.
WAIT and FWAIT are identical.
Clocks Size
Operands 808x 286 386 486 Bytes
none 4 3 6+ 1-3 1
Back to Top
WBINVD - Write-Back and Invalidate Cache (486+)
Usage: WBINVD
Modifies flags: None
Flushes internal cache, then signals the external cache to write
back current data followed by a signal to flush the external cache.
Clocks Size
Operands 808x 286 386 486 Bytes
none - - - 5 2
Back to Top
XCHG - Exchange
Usage: XCHG dest,src
Modifies flags: None
Exchanges contents of source and destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 4 3 3 3 2
mem,reg 17+EA 5 5 5 2-4 (W88=25+EA)
reg,mem 17+EA 5 5 3 2-4 (W88=25+EA)
accum,reg 3 3 3 3 1
reg,accum 3 3 3 3 1
Back to Top
XLAT/XLATB - Translate
Usage: XLAT translation-table
XLATB (masm 5.x)
Modifies flags: None
Replaces the byte in AL with byte from a user table addressed by
BX. The original value of AL is the index into the translate table.
The best way to discripe this is MOV AL,[BX+AL]
Clocks Size
Operands 808x 286 386 486 Bytes
table offset 11 5 5 4 1
Back to Top
XOR - Exclusive OR
Usage: XOR dest,src
Modifies flags: CF OF PF SF ZF (AF undefined)
Performs a bitwise exclusive OR of the operands and returns
the result in the destination.
Clocks Size
Operands 808x 286 386 486 Bytes
reg,reg 3 2 2 1 2
mem,reg 16+EA 7 6 3 2-4 (W88=24+EA)
reg,mem 9+EA 7 7 2 2-4 (W88=13+EA)
reg,immed 4 3 2 1 3-4
mem,immed 17+EA 7 7 3 3-6 (W88=25+EA)
accum,immed 4 3 2 1 2-3
Back to Top
Discuss this article in the forums
Date this article was posted to GameDev.net: 10/25/1999
(Note that this date does not necessarily correspond to the date the article was written)
See Also:
x86 Assembly
© 1999-2011 Gamedev.net. All rights reserved. Terms of Use Privacy Policy
Comments? Questions? Feedback? Click here!
|