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I'm trying to write memmove in assembly: the problem is that in my section data I've got two line : source and destination, and when I'm trying to pass 20 byte from source to destination while source got just 15 byte the 16th byte that pass from source to destination need to be the first byte of destination but the first byte of source passed,

  1. In the first picture we can see that RAX is on 21 then the data that need to pass is source+21 = 43,
  2. We can see that the result of source+21 is 176(176 = source+0) and not 43 like it is supposed to be,
  3. In the last pic we can see that in RCX there is source + 22 = 122(122= source+1) and suppose to be 161

It's look like the code is cyclic , thanks for your help!

in the first picture we can see that rax is on 21 then the data that need to pass is source+21 = 43 we can see that the result of source+21 is 176 and not 43 like it suppose to be in the last pic we can see that in rcx there is source + 22= 122 and suppose to be 161

zx485
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Doron Slomovits
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  • You seem to be using x86 assembly, so why don't you just use `rep movsb`? – zx485 Apr 13 '22 at 16:59
  • You know you could try to debug this with a list of about 3 items, right? Don't have to go for a big example until you've debugged a small example. – Erik Eidt Apr 13 '22 at 20:24
  • off-topic: Since this is 64-bit code, it's more efficient to use `source(%rax)` 64-bit addressing instead of `source(%eax)` 32-bit addressing. (Both ways use `source` as a 32-bit absolute displacement, but using `%eax` also forces an address-size prefix to truncate the address to 32-bit and zero-extend to 64, instead of just letting the 32-bit absolute address sign-extend to 64-bit.) Also, post your code as text, not only in images. – Peter Cordes Apr 14 '22 at 01:44

1 Answers1

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The source and destination arrays follow each other in memory. There's no gap between them.

The very first transfer that your copying loop does will read the first byte from source (176) and write it over the first byte of destination (43). Hereafter the value 43 is gone forever!
When you inspect source+21, which is the same as destination+0, only the value 176 remains.

The second transfer that your copying loop does will read the second byte from source (122) and write it over the second byte of destination (161). Hereafter the value 161 is gone forever!
When you inspect source+22, which is the same as destination+1, only the value 122 remains.

Sep Roland
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