Instruction Set CMPS-CMPSB-CMPSW-CMPSD
CMPS/CMPSB/CMPSW/CMPSD -- Compare String Operands
CMPS/CMPSB/CMPSW/CMPSD -- Compare String Operands
Opcode Instruction Clocks Description
A6 CMPS m8,m8 10 Compare bytes ES:[(E)DI] (second
operand) with [(E)SI] (first
operand)
A7 CMPS m16,m16 10 Compare words ES:[(E)DI] (second
operand) with [(E)SI] (first
operand)
A7 CMPS m32,m32 10 Compare dwords ES:[(E)DI]
(second operand) with [(E)SI]
(first operand)
A6 CMPSB 10 Compare bytes ES:[(E)DI] with
DS:[SI]
A7 CMPSW 10 Compare words ES:[(E)DI] with
DS:[SI]
A7 CMPSD 10 Compare dwords ES:[(E)DI] with
DS:[SI]
Operation
IF (instruction = CMPSD) OR
(instruction has operands of type DWORD)
THEN OperandSize = 32;
ELSE OperandSize = 16;
FI;
IF AddressSize = 16
THEN
use SI for source-index and DI for destination-index
ELSE (* AddressSize = 32 *)
use ESI for source-index and EDI for destination-index;
FI;
IF byte type of instruction
THEN
[source-index] - [destination-index]; (* byte comparison *)
IF DF = 0 THEN IncDec = 1 ELSE IncDec = -1; FI;
ELSE
IF OperandSize = 16
THEN
[source-index] - [destination-index]; (* word comparison *)
IF DF = 0 THEN IncDec = 2 ELSE IncDec = -2; FI;
ELSE (* OperandSize = 32 *)
[source-index] - [destination-index]; (* dword comparison *)
IF DF = 0 THEN IncDec = 4 ELSE IncDec = -4; FI;
FI;
FI;
source-index = source-index + IncDec;
destination-index = destination-index + IncDec;
Description
CMPS compares the byte, word, or doubleword pointed to by the source-index
register with the byte, word, or doubleword pointed to by the
destination-index register.
If the address-size attribute of this instruction is 16 bits, SI and DI
will be used for source- and destination-index registers; otherwise ESI and
EDI will be used. Load the correct index values into SI and DI (or ESI and
EDI) before executing CMPS.
The comparison is done by subtracting the operand indexed by
the destination-index register from the operand indexed by the source-index
register.
Note that the direction of subtraction for CMPS is [SI] - [DI] or
[ESI] - [EDI]. The left operand (SI or ESI) is the source and the right
operand (DI or EDI) is the destination. This is the reverse of the usual
Intel convention in which the left operand is the destination and the right
operand is the source.
The result of the subtraction is not stored; only the flags reflect the
change. The types of the operands determine whether bytes, words, or
doublewords are compared. For the first operand (SI or ESI), the DS register
is used, unless a segment override byte is present. The second operand (DI
or EDI) must be addressable from the ES register; no segment override is
possible.
After the comparison is made, both the source-index register and
destination-index register are automatically advanced. If the direction flag
is 0 (CLD was executed), the registers increment; if the direction flag is 1
(STD was executed), the registers decrement. The registers increment or
decrement by 1 if a byte is compared, by 2 if a word is compared, or by 4 if
a doubleword is compared.
CMPSB, CMPSW and CMPSD are synonyms for the byte, word, and
doubleword CMPS instructions, respectively.
CMPS can be preceded by the REPE or REPNE prefix for block comparison of CX
or ECX bytes, words, or doublewords. Refer to the description of the REP
instruction for more information on this operation.
Flags Affected
OF, SF, ZF, AF, PF, and CF as described in Appendix C
Protected Mode Exceptions
#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault
Real Address Mode Exceptions
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH
Virtual 8086 Mode Exceptions
Same exceptions as in Real Address Mode; #PF (fault-code) for a page fault