CODEWITHSUNDEEP
assembly
PP Laru
PP Laru

Reputation: 121

Assembly addq clarification

enter image description hereI'm reviewing a worksheet for a machine architecture exam and I wanted a clarification on the answer my instructor provided. He's had a couple typos on previous worksheets so I wanted to make sure he was correct here. Take a look at the above picture and notice that he says the destination after the instruction would be 0x100 and the value would be 0x100, but wouldn't the correct answer be destination: 0x100 (value of %rax because of parentheses) and 0x1 (value of %rcx)? Thanks in advance.

EDIT: fixing the picture

Upvotes: 2

Views: 12066

Answers (1)

fuz
fuz

Reputation: 93014

The addq a,b instruction adds the contents of a to b. So if rcx = 0x1, rax = 0x100 and we have 0xff at address 0x100, then add %rcx,(%rax) adds 0x1 in rcx to 0xff at address 0x100, yielding 0x100 at address 0x100 as the worksheet correctly indicates.

Your intuition would be correct if the instruction was movq instead of addq as movq just overwrites a memory location or register instead of adding to it.

Upvotes: 4

Related Questions