NASM Manual

SUPPORTED OPCODES BY UNCC:

B.4 x86 Instruction Set

B.4.1 `AAA`, `AAS`,`AAM`,`AAD`: ASCII Adjustments

AAA                           ; 37                   [8086]

AAS                           ; 3F                   [8086]

AAD                           ; D5 0A                [8086] 
AAD imm                       ; D5 ib                [8086]

AAM                           ; D4 0A                [8086] 
AAM imm                       ; D4 ib                [8086]

These instructions are used in conjunction with the add, subtract, multiply and divide instructions to perform binary-coded decimal arithmetic in unpacked (one BCD digit per byte - easy to translate to and from ASCII, hence the instruction names) form. There are also packed BCD instructions DAA and DAS: see section B.4.57.

AAA (ASCII Adjust After Addition) should be used after a one-byte ADD instruction whose destination was the AL register: by means of examining the value in the low nibble of AL and also the auxiliary carry flag AF, it determines whether the addition has overflowed, and adjusts it (and sets the carry flag) if so. You can add long BCD strings together by doing ADD/AAA on the low digits, then doing ADC/AAA on each subsequent digit.
AAS (ASCII Adjust AL After Subtraction) works similarly to AAA, but is for use after SUB instructions rather than ADD.
AAM (ASCII Adjust AX After Multiply) is for use after you have multiplied two decimal digits together and left the result in AL: it divides AL by ten and stores the quotient in AH, leaving the remainder in AL. The divisor 10 can be changed by specifying an operand to the instruction: a particularly handy use of this is AAM 16, causing the two nibbles in AL to be separated into AH and AL.
AAD (ASCII Adjust AX Before Division) performs the inverse operation to AAM: it multiplies AH by ten, adds it to AL, and sets AH to zero. Again, the multiplier 10 can be changed.

B.4.2 `ADC`: Add with Carry

ADC r/m8,reg8                 ; 10 /r                [8086] 
ADC r/m16,reg16               ; o16 11 /r            [8086] 
ADC r/m32,reg32               ; o32 11 /r            [386]

ADC reg8,r/m8                 ; 12 /r                [8086] 
ADC reg16,r/m16               ; o16 13 /r            [8086] 
ADC reg32,r/m32               ; o32 13 /r            [386]

ADC r/m8,imm8                 ; 80 /2 ib             [8086] 
ADC r/m16,imm16               ; o16 81 /2 iw         [8086] 
ADC r/m32,imm32               ; o32 81 /2 id         [386]

ADC r/m16,imm8                ; o16 83 /2 ib         [8086] 
ADC r/m32,imm8                ; o32 83 /2 ib         [386]

ADC AL,imm8                   ; 14 ib                [8086] 
ADC AX,imm16                  ; o16 15 iw            [8086] 
ADC EAX,imm32                 ; o32 15 id            [386]

ADC performs integer addition: it adds its two operands together, plus the value of the carry flag, and leaves the result in its destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent ADC instruction.

In the forms with an 8-bit immediate second operand and a longer first operand, the second operand is considered to be signed, and is sign-extended to the length of the first operand. In these cases, the BYTE qualifier is necessary to force NASM to generate this form of the instruction.

To add two numbers without also adding the contents of the carry flag, use ADD (section B.4.3).

B.4.3 `ADD`: Add Integers

ADD r/m8,reg8                 ; 00 /r                [8086] 
ADD r/m16,reg16               ; o16 01 /r            [8086] 
ADD r/m32,reg32               ; o32 01 /r            [386]

ADD reg8,r/m8                 ; 02 /r                [8086] 
ADD reg16,r/m16               ; o16 03 /r            [8086] 
ADD reg32,r/m32               ; o32 03 /r            [386]

ADD r/m8,imm8                 ; 80 /0 ib             [8086] 
ADD r/m16,imm16               ; o16 81 /0 iw         [8086] 
ADD r/m32,imm32               ; o32 81 /0 id         [386]

ADD r/m16,imm8                ; o16 83 /0 ib         [8086] 
ADD r/m32,imm8                ; o32 83 /0 ib         [386]

ADD AL,imm8                   ; 04 ib                [8086] 
ADD AX,imm16                  ; o16 05 iw            [8086] 
ADD EAX,imm32                 ; o32 05 id            [386]

ADD performs integer addition: it adds its two operands together, and leaves the result in its destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected and can be used by a subsequent ADC instruction.

B.4.4 `ADDPD`: ADD Packed Double-Precision FP Values

ADDPD xmm1,xmm2/mem128        ; 66 0F 58 /r     [WILLAMETTE,SSE2]

ADDPD performs addition on each of two packed double-precision FP value pairs.

   dst[0-63]   := dst[0-63]   + src[0-63], 
   dst[64-127] := dst[64-127] + src[64-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.5 `ADDPS`: ADD Packed Single-Precision FP Values

ADDPS xmm1,xmm2/mem128        ; 0F 58 /r        [KATMAI,SSE]

ADDPS performs addition on each of four packed single-precision FP value pairs

   dst[0-31]   := dst[0-31]   + src[0-31], 
   dst[32-63]  := dst[32-63]  + src[32-63], 
   dst[64-95]  := dst[64-95]  + src[64-95], 
   dst[96-127] := dst[96-127] + src[96-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.6 `ADDSD`: ADD Scalar Double-Precision FP Values

ADDSD xmm1,xmm2/mem64         ; F2 0F 58 /r     [KATMAI,SSE]

ADDSD adds the low double-precision FP values from the source and destination operands and stores the double-precision FP result in the destination operand.

   dst[0-63]   := dst[0-63] + src[0-63], 
   dst[64-127) remains unchanged.

The destination is an XMM register. The source operand can be either an XMM register or a 64-bit memory location.

B.4.7 `ADDSS`: ADD Scalar Single-Precision FP Values

ADDSS xmm1,xmm2/mem32         ; F3 0F 58 /r     [WILLAMETTE,SSE2]

ADDSS adds the low single-precision FP values from the source and destination operands and stores the single-precision FP result in the destination operand.

   dst[0-31]   := dst[0-31] + src[0-31], 
   dst[32-127] remains unchanged.

The destination is an XMM register. The source operand can be either an XMM register or a 32-bit memory location.

B.4.8 `AND`: Bitwise AND

AND r/m8,reg8                 ; 20 /r                [8086] 
AND r/m16,reg16               ; o16 21 /r            [8086] 
AND r/m32,reg32               ; o32 21 /r            [386]

AND reg8,r/m8                 ; 22 /r                [8086] 
AND reg16,r/m16               ; o16 23 /r            [8086] 
AND reg32,r/m32               ; o32 23 /r            [386]

AND r/m8,imm8                 ; 80 /4 ib             [8086] 
AND r/m16,imm16               ; o16 81 /4 iw         [8086] 
AND r/m32,imm32               ; o32 81 /4 id         [386]

AND r/m16,imm8                ; o16 83 /4 ib         [8086] 
AND r/m32,imm8                ; o32 83 /4 ib         [386]

AND AL,imm8                   ; 24 ib                [8086] 
AND AX,imm16                  ; o16 25 iw            [8086] 
AND EAX,imm32                 ; o32 25 id            [386]

AND performs a bitwise AND operation between its two operands (i.e. each bit of the result is 1 if and only if the corresponding bits of the two inputs were both 1), and stores the result in the destination (first) operand. The destination operand can be a register or a memory location. The source operand can be a register, a memory location or an immediate value.

The MMX instruction PAND (see section B.4.202) performs the same operation on the 64-bit MMX registers.

B.4.9 `ANDNPD`: Bitwise Logical AND NOT of Packed Double-Precision FP Values

ANDNPD xmm1,xmm2/mem128       ; 66 0F 55 /r     [WILLAMETTE,SSE2]

ANDNPD inverts the bits of the two double-precision floating-point values in the destination register, and then performs a logical AND between the two double-precision floating-point values in the source operand and the temporary inverted result, storing the result in the destination register.

   dst[0-63]   := src[0-63]   AND NOT dst[0-63], 
   dst[64-127] := src[64-127] AND NOT dst[64-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.10 `ANDNPS`: Bitwise Logical AND NOT of Packed Single-Precision FP Values

ANDNPS xmm1,xmm2/mem128       ; 0F 55 /r        [KATMAI,SSE]

ANDNPS inverts the bits of the four single-precision floating-point values in the destination register, and then performs a logical AND between the four single-precision floating-point values in the source operand and the temporary inverted result, storing the result in the destination register.

   dst[0-31]   := src[0-31]   AND NOT dst[0-31], 
   dst[32-63]  := src[32-63]  AND NOT dst[32-63], 
   dst[64-95]  := src[64-95]  AND NOT dst[64-95], 
   dst[96-127] := src[96-127] AND NOT dst[96-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.11 `ANDPD`: Bitwise Logical AND For Single FP

ANDPD xmm1,xmm2/mem128        ; 66 0F 54 /r     [WILLAMETTE,SSE2]

ANDPD performs a bitwise logical AND of the two double-precision floating point values in the source and destination operand, and stores the result in the destination register.

   dst[0-63]   := src[0-63]   AND dst[0-63], 
   dst[64-127] := src[64-127] AND dst[64-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.12 `ANDPS`: Bitwise Logical AND For Single FP

ANDPS xmm1,xmm2/mem128        ; 0F 54 /r        [KATMAI,SSE]

ANDPS performs a bitwise logical AND of the four single-precision floating point values in the source and destination operand, and stores the result in the destination register.

   dst[0-31]   := src[0-31]   AND dst[0-31], 
   dst[32-63]  := src[32-63]  AND dst[32-63], 
   dst[64-95]  := src[64-95]  AND dst[64-95], 
   dst[96-127] := src[96-127] AND dst[96-127].

The destination is an XMM register. The source operand can be either an XMM register or a 128-bit memory location.

B.4.13 `ARPL`: Adjust RPL Field of Selector

ARPL r/m16,reg16              ; 63 /r                [286,PRIV]

ARPL expects its two word operands to be segment selectors. It adjusts the RPL (requested privilege level - stored in the bottom two bits of the selector) field of the destination (first) operand to ensure that it is no less (i.e. no more privileged than) the RPL field of the source operand. The zero flag is set if and only if a change had to be made.

B.4.14 `BOUND`: Check Array Index against Bounds

BOUND reg16,mem               ; o16 62 /r            [186] 
BOUND reg32,mem               ; o32 62 /r            [386]

BOUND expects its second operand to point to an area of memory containing two signed values of the same size as its first operand (i.e. two words for the 16-bit form; two doublewords for the 32-bit form). It performs two signed comparisons: if the value in the register passed as its first operand is less than the first of the in-memory values, or is greater than or equal to the second, it throws a BR exception. Otherwise, it does nothing.

B.4.15 `BSF`, `BSR`: Bit Scan

BSF reg16,r/m16               ; o16 0F BC /r         [386] 
BSF reg32,r/m32               ; o32 0F BC /r         [386]

BSR reg16,r/m16               ; o16 0F BD /r         [386] 
BSR reg32,r/m32               ; o32 0F BD /r         [386]

BSF searches for the least significant set bit in its source (second) operand, and if it finds one, stores the index in its destination (first) operand. If no set bit is found, the contents of the destination operand are undefined. If the source operand is zero, the zero flag is set.
BSR performs the same function, but searches from the top instead, so it finds the most significant set bit.

Bit indices are from 0 (least significant) to 15 or 31 (most significant). The destination operand can only be a register. The source operand can be a register or a memory location.

B.4.16 `BSWAP`: Byte Swap

BSWAP reg32                   ; o32 0F C8+r          [486]

BSWAP swaps the order of the four bytes of a 32-bit register: bits 0-7 exchange places with bits 24-31, and bits 8-15 swap with bits 16-23. There is no explicit 16-bit equivalent: to byte-swap AX, BX, CX or DX, XCHG can be used. When BSWAP is used with a 16-bit register, the result is undefined.

B.4.17 `BT`, `BTC`,`BTR`,`BTS`: Bit Test

BT r/m16,reg16                ; o16 0F A3 /r         [386] 
BT r/m32,reg32                ; o32 0F A3 /r         [386] 
BT r/m16,imm8                 ; o16 0F BA /4 ib      [386] 
BT r/m32,imm8                 ; o32 0F BA /4 ib      [386]

BTC r/m16,reg16               ; o16 0F BB /r         [386] 
BTC r/m32,reg32               ; o32 0F BB /r         [386] 
BTC r/m16,imm8                ; o16 0F BA /7 ib      [386] 
BTC r/m32,imm8                ; o32 0F BA /7 ib      [386]

BTR r/m16,reg16               ; o16 0F B3 /r         [386] 
BTR r/m32,reg32               ; o32 0F B3 /r         [386] 
BTR r/m16,imm8                ; o16 0F BA /6 ib      [386] 
BTR r/m32,imm8                ; o32 0F BA /6 ib      [386]

BTS r/m16,reg16               ; o16 0F AB /r         [386] 
BTS r/m32,reg32               ; o32 0F AB /r         [386] 
BTS r/m16,imm                 ; o16 0F BA /5 ib      [386] 
BTS r/m32,imm                 ; o32 0F BA /5 ib      [386]

These instructions all test one bit of their first operand, whose index is given by the second operand, and store the value of that bit into the carry flag. Bit indices are from 0 (least significant) to 15 or 31 (most significant).

In addition to storing the original value of the bit into the carry flag, BTR also resets (clears) the bit in the operand itself. BTS sets the bit, and BTC complements the bit. BT does not modify its operands.

The destination can be a register or a memory location. The source can be a register or an immediate value.

If the destination operand is a register, the bit offset should be in the range 0-15 (for 16-bit operands) or 0-31 (for 32-bit operands). An immediate value outside these ranges will be taken modulo 16/32 by the processor.

If the destination operand is a memory location, then an immediate bit offset follows the same rules as for a register. If the bit offset is in a register, then it can be anything within the signed range of the register used (ie, for a 32-bit operand, it can be (-2^31) to (2^31 - 1)

B.4.18 `CALL`: Call Subroutine

CALL imm                      ; E8 rw/rd             [8086] 
CALL imm:imm16                ; o16 9A iw iw         [8086] 
CALL imm:imm32                ; o32 9A id iw         [386] 
CALL FAR mem16                ; o16 FF /3            [8086] 
CALL FAR mem32                ; o32 FF /3            [386] 
CALL r/m16                    ; o16 FF /2            [8086] 
CALL r/m32                    ; o32 FF /2            [386]

CALL calls a subroutine, by means of pushing the current instruction pointer (IP) and optionally CS as well on the stack, and then jumping to a given address.

CS is pushed as well as IP if and only if the call is a far call, i.e. a destination segment address is specified in the instruction. The forms involving two colon-separated arguments are far calls; so are the CALL FAR mem forms.

The immediate near call takes one of two forms (call imm16/imm32, determined by the current segment size limit. For 16-bit operands, you would use CALL 0x1234, and for 32-bit operands you would use CALL 0x12345678. The value passed as an operand is a relative offset.

You can choose between the two immediate far call forms (CALL imm:imm) by the use of the WORD and DWORD keywords: CALL WORD 0x1234:0x5678) or CALL DWORD 0x1234:0x56789abc.

The CALL FAR mem forms execute a far call by loading the destination address out of memory. The address loaded consists of 16 or 32 bits of offset (depending on the operand size), and 16 bits of segment. The operand size may be overridden using CALL WORD FAR mem or CALL DWORD FAR mem.

The CALL r/m forms execute a near call (within the same segment), loading the destination address out of memory or out of a register. The keyword NEAR may be specified, for clarity, in these forms, but is not necessary. Again, operand size can be overridden using CALL WORD mem or CALL DWORD mem.

As a convenience, NASM does not require you to call a far procedure symbol by coding the cumbersome CALL SEG routine:routine, but instead allows the easier synonym CALL FAR routine.

The CALL r/m forms given above are near calls; NASM will accept the NEAR keyword (e.g. CALL NEAR [address]), even though it is not strictly necessary.

B.4.19 `CBW`, `CWD`,`CDQ`,`CWDE`: Sign Extensions

CBW                           ; o16 98               [8086] 
CWDE                          ; o32 98               [386]

CWD                           ; o16 99               [8086] 
CDQ                           ; o32 99               [386]

All these instructions sign-extend a short value into a longer one, by replicating the top bit of the original value to fill the extended one.

CBW extends AL into AX by repeating the top bit of AL in every bit of AH. CWDE extends AX into EAX.CWD extends AX into DX:AX by repeating the top bit of AX throughout DX, and CDQ extends EAX into EDX:EAX.

B.4.20 `CLC`, `CLD`,`CLI`,`CLTS`: Clear Flags

CLC                           ; F8                   [8086] 
CLD                           ; FC                   [8086] 
CLI                           ; FA                   [8086] 
CLTS                          ; 0F 06                [286,PRIV]

These instructions clear various flags. CLC clears the carry flag; CLD clears the direction flag; CLI clears the interrupt flag (thus disabling interrupts); and CLTS clears the task-switched (TS) flag in CR0.

To set the carry, direction, or interrupt flags, use the STC,STD and STI instructions (section B.4.301). To invert the carry flag, use CMC (section B.4.22).

B.4.21 `CLFLUSH`: Flush Cache Line

CLFLUSH mem                   ; 0F AE /7        [WILLAMETTE,SSE2]

CLFLUSH invalidates the cache line that contains the linear address specified by the source operand from all levels of the processor cache hierarchy (data and instruction). If, at any level of the cache hierarchy, the line is inconsistent with memory (dirty) it is written to memory before invalidation. The source operand points to a byte-sized memory location.

Although CLFLUSH is flagged SSE2 and above, it may not be present on all processors which have SSE2 support, and it may be supported on other processors; the CPUID instruction (section B.4.34) will return a bit which indicates support for the CLFLUSH instruction.

B.4.22 `CMC`: Complement Carry Flag

CMC                           ; F5                   [8086]

CMC changes the value of the carry flag: if it was 0, it sets it to 1, and vice versa.

B.4.23 `CMOVcc`: Conditional Move

CMOVcc reg16,r/m16            ; o16 0F 40+cc /r      [P6] 
CMOVcc reg32,r/m32            ; o32 0F 40+cc /r      [P6]

CMOV moves its source (second) operand into its destination (first) operand if the given condition code is satisfied; otherwise it does nothing.

For a list of condition codes, see section B.2.2.

Although the CMOV instructions are flagged P6 and above, they may not be supported by all Pentium Pro processors; the CPUID instruction (section B.4.34) will return a bit which indicates whether conditional moves are supported.

B.4.24 `CMP`: Compare Integers

CMP r/m8,reg8                 ; 38 /r                [8086] 
CMP r/m16,reg16               ; o16 39 /r            [8086] 
CMP r/m32,reg32               ; o32 39 /r            [386]

CMP reg8,r/m8                 ; 3A /r                [8086] 
CMP reg16,r/m16               ; o16 3B /r            [8086] 
CMP reg32,r/m32               ; o32 3B /r            [386]

CMP r/m8,imm8                 ; 80 /0 ib             [8086] 
CMP r/m16,imm16               ; o16 81 /0 iw         [8086] 
CMP r/m32,imm32               ; o32 81 /0 id         [386]

CMP r/m16,imm8                ; o16 83 /0 ib         [8086] 
CMP r/m32,imm8                ; o32 83 /0 ib         [386]

CMP AL,imm8                   ; 3C ib                [8086] 
CMP AX,imm16                  ; o16 3D iw            [8086] 
CMP EAX,imm32                 ; o32 3D id            [386]

CMP performs a `mental' subtraction of its second operand from its first operand, and affects the flags as if the subtraction had taken place, but does not store the result of the subtraction anywhere.

The destination operand can be a register or a memory location. The source can be a register, memory location or an immediate value of the same size as the destination.

B.4.25 `CMPccPD`: Packed Double-Precision FP Compare

CMPPD xmm1,xmm2/mem128,imm8   ; 66 0F C2 /r ib  [WILLAMETTE,SSE2]

CMPEQPD xmm1,xmm2/mem128      ; 66 0F C2 /r 00  [WILLAMETTE,SSE2] 
CMPLTPD xmm1,xmm2/mem128      ; 66 0F C2 /r 01  [WILLAMETTE,SSE2] 
CMPLEPD xmm1,xmm2/mem128      ; 66 0F C2 /r 02  [WILLAMETTE,SSE2] 
CMPUNORDPD xmm1,xmm2/mem128   ; 66 0F C2 /r 03  [WILLAMETTE,SSE2] 
CMPNEQPD xmm1,xmm2/mem128     ; 66 0F C2 /r 04  [WILLAMETTE,SSE2] 
CMPNLTPD xmm1,xmm2/mem128     ; 66 0F C2 /r 05  [WILLAMETTE,SSE2] 
CMPNLEPD xmm1,xmm2/mem128     ; 66 0F C2 /r 06  [WILLAMETTE,SSE2] 
CMPORDPD xmm1,xmm2/mem128     ; 66 0F C2 /r 07  [WILLAMETTE,SSE2]

The CMPccPD instructions compare the two packed double-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an XMM register. The source can be either an XMM register or a 128-bit memory location.

The third operand is an 8-bit immediate value, of which the low 3 bits define the type of comparison. For ease of programming, the 8 two-operand pseudo-instructions are provided, with the third operand already filled in. The Condition Predicates are:

EQ     0   Equal 
LT     1   Less-than 
LE     2   Less-than-or-equal 
UNORD  3   Unordered 
NE     4   Not-equal 
NLT    5   Not-less-than 
NLE    6   Not-less-than-or-equal 
ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

B.4.26 `CMPccPS`: Packed Single-Precision FP Compare

CMPPS xmm1,xmm2/mem128,imm8   ; 0F C2 /r ib     [KATMAI,SSE]

CMPEQPS xmm1,xmm2/mem128      ; 0F C2 /r 00     [KATMAI,SSE] 
CMPLTPS xmm1,xmm2/mem128      ; 0F C2 /r 01     [KATMAI,SSE] 
CMPLEPS xmm1,xmm2/mem128      ; 0F C2 /r 02     [KATMAI,SSE] 
CMPUNORDPS xmm1,xmm2/mem128   ; 0F C2 /r 03     [KATMAI,SSE] 
CMPNEQPS xmm1,xmm2/mem128     ; 0F C2 /r 04     [KATMAI,SSE] 
CMPNLTPS xmm1,xmm2/mem128     ; 0F C2 /r 05     [KATMAI,SSE] 
CMPNLEPS xmm1,xmm2/mem128     ; 0F C2 /r 06     [KATMAI,SSE] 
CMPORDPS xmm1,xmm2/mem128     ; 0F C2 /r 07     [KATMAI,SSE]

The CMPccPS instructions compare the two packed single-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an XMM register. The source can be either an XMM register or a 128-bit memory location.

EQ     0   Equal 
LT     1   Less-than 
LE     2   Less-than-or-equal 
UNORD  3   Unordered 
NE     4   Not-equal 
NLT    5   Not-less-than 
NLE    6   Not-less-than-or-equal 
ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

B.4.27 `CMPSB`, `CMPSW`,`CMPSD`: Compare Strings

CMPSB                         ; A6                   [8086] 
CMPSW                         ; o16 A7               [8086] 
CMPSD                         ; o32 A7               [386]

CMPSB compares the byte at [DS:SI] or [DS:ESI] with the byte at [ES:DI] or [ES:EDI], and sets the flags accordingly. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) SI and DI (or ESI and EDI).

The registers used are SI and DI if the address size is 16 bits, and ESI and EDI if it is 32 bits. If you need to use an address size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.

The segment register used to load from [SI] or [ESI] can be overridden by using a segment register name as a prefix (for example, ES CMPSB). The use of ES for the load from [DI] or [EDI] cannot be overridden.

CMPSW and CMPSD work in the same way, but they compare a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The REPE and REPNE prefixes (equivalently, REPZ and REPNZ) may be used to repeat the instruction up to CX (or ECX - again, the address size chooses which) times until the first unequal or equal byte is found.

B.4.28 `CMPccSD`: Scalar Double-Precision FP Compare

CMPSD xmm1,xmm2/mem64,imm8    ; F2 0F C2 /r ib  [WILLAMETTE,SSE2]

CMPEQSD xmm1,xmm2/mem64       ; F2 0F C2 /r 00  [WILLAMETTE,SSE2] 
CMPLTSD xmm1,xmm2/mem64       ; F2 0F C2 /r 01  [WILLAMETTE,SSE2] 
CMPLESD xmm1,xmm2/mem64       ; F2 0F C2 /r 02  [WILLAMETTE,SSE2] 
CMPUNORDSD xmm1,xmm2/mem64    ; F2 0F C2 /r 03  [WILLAMETTE,SSE2] 
CMPNEQSD xmm1,xmm2/mem64      ; F2 0F C2 /r 04  [WILLAMETTE,SSE2] 
CMPNLTSD xmm1,xmm2/mem64      ; F2 0F C2 /r 05  [WILLAMETTE,SSE2] 
CMPNLESD xmm1,xmm2/mem64      ; F2 0F C2 /r 06  [WILLAMETTE,SSE2] 
CMPORDSD xmm1,xmm2/mem64      ; F2 0F C2 /r 07  [WILLAMETTE,SSE2]

The CMPccSD instructions compare the low-order double-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an XMM register. The source can be either an XMM register or a 128-bit memory location.

EQ     0   Equal 
LT     1   Less-than 
LE     2   Less-than-or-equal 
UNORD  3   Unordered 
NE     4   Not-equal 
NLT    5   Not-less-than 
NLE    6   Not-less-than-or-equal 
ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

B.4.29 `CMPccSS`: Scalar Single-Precision FP Compare

CMPSS xmm1,xmm2/mem32,imm8    ; F3 0F C2 /r ib  [KATMAI,SSE]

CMPEQSS xmm1,xmm2/mem32       ; F3 0F C2 /r 00  [KATMAI,SSE] 
CMPLTSS xmm1,xmm2/mem32       ; F3 0F C2 /r 01  [KATMAI,SSE] 
CMPLESS xmm1,xmm2/mem32       ; F3 0F C2 /r 02  [KATMAI,SSE] 
CMPUNORDSS xmm1,xmm2/mem32    ; F3 0F C2 /r 03  [KATMAI,SSE] 
CMPNEQSS xmm1,xmm2/mem32      ; F3 0F C2 /r 04  [KATMAI,SSE] 
CMPNLTSS xmm1,xmm2/mem32      ; F3 0F C2 /r 05  [KATMAI,SSE] 
CMPNLESS xmm1,xmm2/mem32      ; F3 0F C2 /r 06  [KATMAI,SSE] 
CMPORDSS xmm1,xmm2/mem32      ; F3 0F C2 /r 07  [KATMAI,SSE]

The CMPccSS instructions compare the low-order single-precision FP values in the source and destination operands, and returns the result of the comparison in the destination register. The result of each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an XMM register. The source can be either an XMM register or a 128-bit memory location.

EQ     0   Equal 
LT     1   Less-than 
LE     2   Less-than-or-equal 
UNORD  3   Unordered 
NE     4   Not-equal 
NLT    5   Not-less-than 
NLE    6   Not-less-than-or-equal 
ORD    7   Ordered

For more details of the comparison predicates, and details of how to emulate the "greater-than" equivalents, see section B.2.3

B.4.30 `CMPXCHG`, `CMPXCHG486`: Compare and Exchange

CMPXCHG r/m8,reg8             ; 0F B0 /r             [PENT] 
CMPXCHG r/m16,reg16           ; o16 0F B1 /r         [PENT] 
CMPXCHG r/m32,reg32           ; o32 0F B1 /r         [PENT]

CMPXCHG486 r/m8,reg8          ; 0F A6 /r             [486,UNDOC] 
CMPXCHG486 r/m16,reg16        ; o16 0F A7 /r         [486,UNDOC] 
CMPXCHG486 r/m32,reg32        ; o32 0F A7 /r         [486,UNDOC]

These two instructions perform exactly the same operation; however, apparently some (not all) 486 processors support it under a non-standard opcode, so NASM provides the undocumented CMPXCHG486 form to generate the non-standard opcode.

CMPXCHG compares its destination (first) operand to the value in AL, AX or EAX (depending on the operand size of the instruction). If they are equal, it copies its source (second) operand into the destination and sets the zero flag. Otherwise, it clears the zero flag and copies the destination register to AL, AX or EAX.

The destination can be either a register or a memory location. The source is a register.

CMPXCHG is intended to be used for atomic operations in multitasking or multiprocessor environments. To safely update a value in shared memory, for example, you might load the value into EAX, load the updated value into EBX, and then execute the instruction LOCK CMPXCHG [value],EBX. If value has not changed since being loaded, it is updated with your desired new value, and the zero flag is set to let you know it has worked. (The LOCK prefix prevents another processor doing anything in the middle of this operation: it guarantees atomicity.) However, if another processor has modified the value in between your load and your attempted store, the store does not happen, and you are notified of the failure by a cleared zero flag, so you can go round and try again.

B.4.31 `CMPXCHG8B`: Compare and Exchange Eight Bytes

CMPXCHG8B mem                 ; 0F C7 /1             [PENT]

This is a larger and more unwieldy version of CMPXCHG: it compares the 64-bit (eight-byte) value stored at [mem] with the value in EDX:EAX. If they are equal, it sets the zero flag and stores ECX:EBX into the memory area. If they are unequal, it clears the zero flag and stores the memory contents into EDX:EAX.

CMPXCHG8B can be used with the LOCK prefix, to allow atomic execution. This is useful in multi-processor and multi-tasking environments.

B.4.32 `COMISD`: Scalar Ordered Double-Precision FP Compare and Set EFLAGS

COMISD xmm1,xmm2/mem64        ; 66 0F 2F /r     [WILLAMETTE,SSE2]

COMISD compares the low-order double-precision FP value in the two source operands. ZF, PF and CF are set according to the result. OF, AF and AF are cleared. The unordered result is returned if either source is a NaN (QNaN or SNaN).

The destination operand is an XMM register. The source can be either an XMM register or a memory location.

The flags are set according to the following rules:

   Result          Flags        Values

   UNORDERED:      ZF,PF,CF <-- 111; 
   GREATER_THAN:   ZF,PF,CF <-- 000; 
   LESS_THAN:      ZF,PF,CF <-- 001; 
   EQUAL:          ZF,PF,CF <-- 100;

B.4.33 `COMISS`: Scalar Ordered Single-Precision FP Compare and Set EFLAGS

COMISS xmm1,xmm2/mem32        ; 66 0F 2F /r     [KATMAI,SSE]

COMISS compares the low-order single-precision FP value in the two source operands. ZF, PF and CF are set according to the result. OF, AF and AF are cleared. The unordered result is returned if either source is a NaN (QNaN or SNaN).

The destination operand is an XMM register. The source can be either an XMM register or a memory location.

The flags are set according to the following rules:

   Result          Flags        Values

   UNORDERED:      ZF,PF,CF <-- 111; 
   GREATER_THAN:   ZF,PF,CF <-- 000; 
   LESS_THAN:      ZF,PF,CF <-- 001; 
   EQUAL:          ZF,PF,CF <-- 100;

B.4.34 `CPUID`: Get CPU Identification Code

CPUID                         ; 0F A2                [PENT]

CPUID returns various information about the processor it is being executed on. It fills the four registers EAX,EBX,ECX and EDX with information, which varies depending on the input contents of EAX.

CPUID also acts as a barrier to serialise instruction execution: executing the CPUID instruction guarantees that all the effects (memory modification, flag modification, register modification) of previous instructions have been completed before the next instruction gets fetched.

The information returned is as follows:

If EAX is zero on input, EAX on output holds the maximum acceptable input value of EAX, and EBX:EDX:ECX contain the string "GenuineIntel" (or not, if you have a clone processor). That is to say, EBX contains "Genu" (in NASM's own sense of character constants, described in section 3.4.2), EDX contains "ineI" and ECX contains "ntel".
If EAX is one on input, EAX on output contains version information about the processor, and EDX contains a set of feature flags, showing the presence and absence of various features. For example, bit 8 is set if the CMPXCHG8B instruction (section B.4.31) is supported, bit 15 is set if the conditional move instructions (section B.4.23 and section B.4.72) are supported, and bit 23 is set if MMX instructions are supported.
If EAX is two on input, EAX,EBX,ECX and EDX all contain information about caches and TLBs (Translation Lookahead Buffers).

For more information on the data returned from CPUID, see the documentation from Intel and other processor manufacturers.

B.4.35 `CVTDQ2PD`: Packed Signed INT32 to Packed Double-Precision FP Conversion

CVTDQ2PD xmm1,xmm2/mem64      ; F3 0F E6 /r     [WILLAMETTE,SSE2]

CVTDQ2PD converts two packed signed doublewords from the source operand to two packed double-precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an XMM register or a 64-bit memory location. If the source is a register, the packed integers are in the low quadword.

B.4.36 `CVTDQ2PS`: Packed Signed INT32 to Packed Single-Precision FP Conversion

CVTDQ2PS xmm1,xmm2/mem128     ; 0F 5B /r        [WILLAMETTE,SSE2]

CVTDQ2PS converts four packed signed doublewords from the source operand to four packed single-precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an XMM register or a 128-bit memory location.