Assembly language learning (7)

This part is mainly for the usage examples of some instructions of the flag register in Chapter 11 of Wang Shuang's "Assembly Language".

1.adc carry addition instruction exploration:

Be careful to use statements that do not affect the CF bit

Points to note:

1. Subroutine structure:

Put all the registers to be used on the stack first, and pop them off the stack after the program ends

2. Set the value of CF:

Let sub ax,ax clear the value of CF in advance

Use inc and loop to adjust the offset address instead of add to prevent affecting the value of CF

add128: push axpush cxpush sipush dipush bxsub ax,axmov cx,8s: mov ax,[si]adc ax,[di]mov [bx],axinc diinc diinc siinc siinc bxinc bxloop spop bxpop dipop sipop cxpop axret

Use with 2.cmp command

Count the number of bytes with a value of 8 in the data segment.

The idea is to skip the count part if not equal.

 mov bx,0mov ax,0mov cx,8s: cmp byte ptr [bx],8jne nextinc axnext: inc bxloop s

Number of bytes whose statistical value is greater than 8: jna next

Number of bytes whose statistical value is less than 8: jnb next

3. Use of string instructions

 mov ax,datamov ds,axmov es,axmov di,0mov si,16mov cx,8cldrep movsw

4. About popf and pushf

Through these two instructions, you can fetch or modify the value of the flag register

After the following program is executed, what is ax?

 mov ax,0push axpopf ;The flag register is 0mov ax,0fff0hadd ax,0010h ;ax = 0 ZF is 0, PF is 1, CF is 1, OF is 0, SF is 0pushfpop ax ;ax is equal to the value of the flags registerand al,11000101Band ah,00001000B

Get ax=00000000_01000101ie 0045h