Compare and Jump
Using the CMP (compare) instruction along with the conditional jump instructions,
we get the if construct of high level languages.
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The CMP Instruction
The CMP Instruction is used to compare two things. It is used such as
cmp ax,6 ; will compare AX and 6
cmp ah,bl ; will compare AH and BL
var1 dw 0
cmp ax,var1 ; will compare AX and the value in var1
Note that CMP doesn't modify any registers or memory.
The Conditional Jumps
Conditional jumps jump to a label if the current condition (set by CMP) is
true. The conditional jumps include:
JMP - Will jump no matter what, doesn't check conditions.
JE - Will jump if compared things are equal.
JNE - Will jump if comparison is not equal.
JA - Will jump if the first thing is greater.
JB - Will jump if the first thing is less.
Here's some sample code:
cmp ax,bx ; compare AX with BX
ja Above ; jump to label Above if AX > BX
je Equal ; jump to label Equal if AX == BX
jb Below ; jump to label Below if AX < BX
Note that you can't use RET with a jump and there are no conditional CALLs, you have to
jump to somewhere that'll do the call and then jump back or something like that.
This will output the numbers from 9 to 0:
cmp ah,0 ; check if AH is 0
je endofit ; if AH is 0 goto endofit
add ah,30h ; turn a digit into it's ASCII by adding 30h
int 21h ; output the digit
pop ax ; get the number back
jmp again ; do it again
There's a better way to do that, if you find it, you don't get
hit with an imaginary trout.
Hope you enjoyed that! :)
This Day In Review
Hope you're having fun, I know I am!
Intro - Day 8