Assembly之instruction之CMP

CMP[.W]　　 Compare source and destination
CMP.B　　　  Compare source and destination

Syntax　　　  CMP　　 src,dst 　 or 　CMP.W src,dst
　　　　　　　 CMP.B　 src,dst

Operation 　　dst + .NOT.src + 1　or　　(dst − src)

Description 　　

　　The source operand is subtracted from the destination operand. This is accomplished by adding the 1s complement of the source operand plus 1. The two operands are not affected and the result is not stored; only the status bits are affected.

Status Bits 　　N: Set if result is negative, reset if positive (src >= dst)
　　　　　　　　Z: Set if result is zero, reset otherwise (src = dst)
　　　　　　　　C: Set if there is a carry from the MSB of the result, reset otherwise
　　　　　　　　V: Set if an arithmetic overflow occurs, otherwise reset

Mode Bits 　　OSCOFF, CPUOFF, and GIE are not affected.

Example 　　　　R5 and R6 are compared. If they are equal, the program continues at the label EQUAL.

1 　CMP R5,R6 　　; R5 = R6?

2 　JEQ EQUAL 　　; YES, JUMP

 

Example 　　　　Two RAM blocks are compared. If they are not equal, the program branches to the label ERROR.

1     MOV #NUM,R5       ; number of words to be compared

2     MOV #BLOCK1,R6    ; BLOCK1 start address in R6

3     MOV #BLOCK2,R7    ; BLOCK2 start address in R7

4 L$1 CMP @R6+,0(R7)    ; Are Words equal? R6 increments

5     JNZ ERROR         ; No, branch to ERROR

6     INCD R7           ; Increment R7 pointer

7     DEC R5            ; Are all words compared?

8     JNZ L$1           ; No, another compare                                    

 

Example 　　The RAM bytes addressed by EDE and TONI are compared. If they are equal, the program continues at the label EQUAL.

1 CMP.B EDE,TONI     ; MEM(EDE) = MEM(TONI)?

2 JEQ EQUAL     　　  ; YES, JUMP