CODEWITHSUNDEEP
assemblyx86bit-shift
user2934615
user2934615

Reputation: 35

How to shift values to eax correctly?

I am using 32-bit x86.

The problem is I cannot shift the bits to the left.

I want to read one byte each time and put it into eax, but I am messing up with shifting. I cannot move the bits into high order. Any idea?

myStr byte "12345678"

mov ecx, offset myStr
mov dl, myStr[ecx]
sub dl,30h ;convert to real digit
mov al, dl
shl al,4
inc ecx

;eax should look like this end of the operation 12345678

Upvotes: 0

Views: 761

Answers (1)

phuclv
phuclv

Reputation: 41794

First mov al, dl will move the entire byte from dl to al, not just a nibble

Second, you just shift al

shl al,4

After the shift, the remaining upper bytes or eax still not be affected. So what you've done is just move sequentially the numbers in myStr to the high nibble of al. That means al will be 16, 32, 48... 128 after each step, and so is eax if the top 3 bytes eax is 0 before. At the end eax should look like 0xXXXXXX80

Upvotes: 1

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