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Update asm.md

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fix typo
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walkerlala 2017-02-18 19:16:45 +08:00 committed by GitHub
parent c9a9daa15f
commit 79b469fc60
1 changed files with 6 additions and 6 deletions
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Theory/asm.md
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@ -159,7 +159,7 @@ As mentioned above, the "clobbered" part should contain a comma-separated list o
Consider the example from before, but we will add an additional, simple assembler instruction:
```C
__asm__("movq $100, %%rdx\t\n"
__asm__("movq $100, %%edx\t\n"
"addl %1,%2" : "=r" (sum) : "r" (a), "0" (b));
```
@ -169,15 +169,15 @@ If we look at the assembly output
0000000000400400 <main>:
400400: ba 05 00 00 00 mov $0x5,%edx
400405: b8 0a 00 00 00 mov $0xa,%eax
40040a: 48 c7 c2 64 00 00 00 mov $0x64,%rdx
40040a: 48 c7 c2 64 00 00 00 mov $0x64,%edx
400411: 01 d0 add %edx,%eax
```
we see that the `%edx` register is overwritten with `0x64` or `100` and the result will be `115` instead of `15`. Now if we add the `%rdx` register to the list of "clobbered" register
we see that the `%edx` register is overwritten with `0x64` or `100` and the result will be `115` instead of `15`. Now if we add the `%edx` register to the list of "clobbered" register
```C
__asm__("movq $100, %%rdx\t\n"
"addl %1,%2" : "=r" (sum) : "r" (a), "0" (b) : "%rdx");
__asm__("movq $100, %%edx\t\n"
"addl %1,%2" : "=r" (sum) : "r" (a), "0" (b) : "%edx");
```
and look at the assembler output again
@ -186,7 +186,7 @@ and look at the assembler output again
0000000000400400 <main>:
400400: b9 05 00 00 00 mov $0x5,%ecx
400405: b8 0a 00 00 00 mov $0xa,%eax
40040a: 48 c7 c2 64 00 00 00 mov $0x64,%rdx
40040a: 48 c7 c2 64 00 00 00 mov $0x64,%edx
400411: 01 c8 add %ecx,%eax
```