x64 asm strange behavior

Question

Can someone help me with understanding x64 asm behaviour? I cant find any kind of documentation for that.

So, basically, we have:

x64 register RAX.
x32 register EAX.
x16 register AX.
x8 register AL.

and now code:

mov rax, -1 (0xFFFFFFFFFFFFFFFF)
add al, 1

result will be 0xFFFFFFFFFFFFFF00 (correct, al register overflow, but we change only AL 1 byte)

mov rax, -1 (0xFFFFFFFFFFFFFFFF)
add ax, 1

result will be 0xFFFFFFFFFFFF0000 (correct, ax register overflow, but we change only AX 2 bytes)

mov rax, -1 (0xFFFFFFFFFFFFFFFF)
add eax, 1

result will be 0x0000000000000000 (wtf?, eax register overflow, but result change all 8 bytes instead of 4. Why it is 0x0000000000000000 but not 0xFFFFFFFF00000000)

See: https://stackoverflow.com/questions/11177137/why-do-most-x64-instructions-zero-the-upper-part-of-a-32-bit-register — JFMR, Sep 22 '17 at 06:57
note that `add eax, 0` will also clear the most-significant half of `RAX`. — JFMR, Sep 22 '17 at 07:07
Btw there are only half as many bytes than you've written, `al` is 1 byte, `ax` is two bytes, etc. — harold, Sep 22 '17 at 07:13

user3599591 · Answer 1 · 2017-09-22T07:08:17.210

1

32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose register.

Thats is. Thanks!

mov rax, 0x6666666666666666
add eax, 1

result confirm this and will be 0x0000000066666667

edited Sep 22 '17 at 07:08

answered Sep 22 '17 at 07:02

user3599591

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x64 asm strange behavior

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