5 ms·

Common x86 Calling Conventions

27 points by mirceasoaica 11y ago

vardump 11y ago
Title should be "Microsoft x86 Calling Conventions". The whole article is based on Microsoft-specific conventions and tools. When I saw the following, I stopped reading: MultiplyEaxByTwo: shl eax,2 ret "shl eax, 2" means bit shift left eax by two, or "eax << 2". Also, while the end result is same as with "shl" [1], "sal" [2] should be used for idiomatic [3] assembler: Fixed: CorrectMultiplyEaxByTwo: sal eax, 1 ret [1]: "shl", shift logical left [2]: "sal", shift arithmetic left [3]: http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate#Arithmetic_Shift_Instructions http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate#A...
- rurban 11y ago
  And 32 bit only. It doesn't cover the 64bit conventions which deviates more wildly on Win64 from the common amd64 ABI.
  - masklinn 11y ago
    Talking about 64b, does anyone know why MS decided not to go with the AMD64 abi? Is it because they had their own 64bit CC for e.g. itanium and kept that?
    - 0x0 11y ago
      Looks like they went with extending their existing __fastcall calling convention. http://blogs.msdn.com/b/oldnewthing/archive/2004/01/02/47184.aspx http://blogs.msdn.com/b/oldnewthing/archive/2004/01/02/47184...
  - 0x0 11y ago
    The biggest surprise going to 64bit is probably the stack redzone. It's bitten the linux kernel a few times too, I think.
- 0x0 11y ago
  sar/sal is for signed numbers, shr/shl is for unsigned numbers. Which one you want depends on the types you are working on. Also, "sal" and "shl" perform the exact same operation with the exact same side-effects. Some sources seem to hint that "sal" is the one going out of fashion - https://code.google.com/p/corkami/wiki/x86oddities#sal https://code.google.com/p/corkami/wiki/x86oddities#sal and http://stackoverflow.com/a/8373438/2412419 http://stackoverflow.com/a/8373438/2412419
  - vardump 11y ago
    Oh, I thought SAL only existed as a mnemonic, I was rather surprised to find out SAL actually has x86 instruction encoding different from SHL!
    - userbinator 11y ago
      It's an undocumented one, that just happens to fill the encoding space of: - shift right and insert 0 ("logical right") - shift right and insert sign bit ("arithmetic right") - shift left and insert 0 ("logical left") - shift left and insert... 0 ("arithmetic left") ** My guess is that it causes the hardware to attempt to sign-extend and shift left, but the latter means the former doesn't have any effect. The Z80 has a similar gap in the encoding but rather more unusual undocumented behaviour here, where it would shift left and insert a 1 in the lowest bit.
- userbinator 11y ago
  Well, cdecl is used by GCC too. It also mentions very little (i.e. not at all) about callee/caller-saved registers, which is a very important part of a calling convention. There's another "WTF" moment in the code too: xor eax,eax add eax,[esp+4] add eax,[esp+8] add eax,[esp+12] I was somewhat confused at the diagrams with "low memory" at the top and "high memory" at the bottom...
- to3m 11y ago
  Not like Windows isn't common, though ;)
revelation 11y ago
Thank god for x64 that got rid of this unnecessary cruft.
- cremno 11y ago
  A wild __vectorcall appeared! http://blogs.msdn.com/b/vcblog/archive/2013/07/12/introducing-vector-calling-convention.aspx http://blogs.msdn.com/b/vcblog/archive/2013/07/12/introducin... https://msdn.microsoft.com/en-us/library/dn375768.aspx https://msdn.microsoft.com/en-us/library/dn375768.aspx
- masklinn 11y ago
  It is my understanding that the cruft is microsoft-specific: other systems use cdecl, fastcall and stdcall are related to Windows APIs and techs. And even though AMD published a call convention as part of AMD64, Microsoft ignored it in favour of their own fastcall-derived CC[0], and have added a new one since (vectorcall[1]) [0] https://msdn.microsoft.com/en-us/library/ms235286.aspx https://msdn.microsoft.com/en-us/library/ms235286.aspx [1] https://msdn.microsoft.com/en-us/library/dn375768.aspx https://msdn.microsoft.com/en-us/library/dn375768.aspx
0xFFC 11y ago
Can you introduce good books about these topics?(linker,loader,calling conventions ,dynamic linking etc).I am looking for good book for deeply understanding these topics(given that traditional OS books just talks about theory)
- mdaniel 11y ago
  I can't comment on its "goodness" but the textbook that sprang to mind is Linkers and Loaders: http://www.amazon.com/Linkers-Kaufmann-Software-Engineering-Programming/dp/1558604960 http://www.amazon.com/Linkers-Kaufmann-Software-Engineering-... The bad news is that it's from '99 but, as far as the foundations go, I can't imagine linkers back then were that different.

27 points by mirceasoaica 11y ago

Title should be "Microsoft x86 Calling Conventions". The whole article is based on Microsoft-specific conventions and tools. When I saw the following, I stopped reading: MultiplyEaxByTwo: shl eax,2 ret "shl eax, 2" means bit shift left eax by two, or "eax << 2". Also, while the end result is same as with "shl" [1], "sal" [2] should be used for idiomatic [3] assembler: Fixed: CorrectMultiplyEaxByTwo: sal eax, 1 ret [1]: "shl", shift logical left [2]: "sal", shift arithmetic left [3]: http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate#Arithmetic_Shift_Instructions http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate#A...

rurban 11y ago
And 32 bit only. It doesn't cover the 64bit conventions which deviates more wildly on Win64 from the common amd64 ABI.
- masklinn 11y ago
  Talking about 64b, does anyone know why MS decided not to go with the AMD64 abi? Is it because they had their own 64bit CC for e.g. itanium and kept that?
  - 0x0 11y ago
    Looks like they went with extending their existing __fastcall calling convention. http://blogs.msdn.com/b/oldnewthing/archive/2004/01/02/47184.aspx http://blogs.msdn.com/b/oldnewthing/archive/2004/01/02/47184...
- 0x0 11y ago
  The biggest surprise going to 64bit is probably the stack redzone. It's bitten the linux kernel a few times too, I think.
0x0 11y ago
sar/sal is for signed numbers, shr/shl is for unsigned numbers. Which one you want depends on the types you are working on. Also, "sal" and "shl" perform the exact same operation with the exact same side-effects. Some sources seem to hint that "sal" is the one going out of fashion - https://code.google.com/p/corkami/wiki/x86oddities#sal https://code.google.com/p/corkami/wiki/x86oddities#sal and http://stackoverflow.com/a/8373438/2412419 http://stackoverflow.com/a/8373438/2412419
- vardump 11y ago
  Oh, I thought SAL only existed as a mnemonic, I was rather surprised to find out SAL actually has x86 instruction encoding different from SHL!
  - userbinator 11y ago
    It's an undocumented one, that just happens to fill the encoding space of: - shift right and insert 0 ("logical right") - shift right and insert sign bit ("arithmetic right") - shift left and insert 0 ("logical left") - shift left and insert... 0 ("arithmetic left") ** My guess is that it causes the hardware to attempt to sign-extend and shift left, but the latter means the former doesn't have any effect. The Z80 has a similar gap in the encoding but rather more unusual undocumented behaviour here, where it would shift left and insert a 1 in the lowest bit.
userbinator 11y ago
Well, cdecl is used by GCC too. It also mentions very little (i.e. not at all) about callee/caller-saved registers, which is a very important part of a calling convention. There's another "WTF" moment in the code too: xor eax,eax add eax,[esp+4] add eax,[esp+8] add eax,[esp+12] I was somewhat confused at the diagrams with "low memory" at the top and "high memory" at the bottom...
to3m 11y ago
Not like Windows isn't common, though ;)

revelation 11y ago

Thank god for x64 that got rid of this unnecessary cruft.

cremno 11y ago
A wild __vectorcall appeared! http://blogs.msdn.com/b/vcblog/archive/2013/07/12/introducing-vector-calling-convention.aspx http://blogs.msdn.com/b/vcblog/archive/2013/07/12/introducin... https://msdn.microsoft.com/en-us/library/dn375768.aspx https://msdn.microsoft.com/en-us/library/dn375768.aspx
masklinn 11y ago
It is my understanding that the cruft is microsoft-specific: other systems use cdecl, fastcall and stdcall are related to Windows APIs and techs. And even though AMD published a call convention as part of AMD64, Microsoft ignored it in favour of their own fastcall-derived CC[0], and have added a new one since (vectorcall[1]) [0] https://msdn.microsoft.com/en-us/library/ms235286.aspx https://msdn.microsoft.com/en-us/library/ms235286.aspx [1] https://msdn.microsoft.com/en-us/library/dn375768.aspx https://msdn.microsoft.com/en-us/library/dn375768.aspx

0xFFC 11y ago

Can you introduce good books about these topics?(linker,loader,calling conventions ,dynamic linking etc).I am looking for good book for deeply understanding these topics(given that traditional OS books just talks about theory)

mdaniel 11y ago
I can't comment on its "goodness" but the textbook that sprang to mind is Linkers and Loaders: http://www.amazon.com/Linkers-Kaufmann-Software-Engineering-Programming/dp/1558604960 http://www.amazon.com/Linkers-Kaufmann-Software-Engineering-... The bad news is that it's from '99 but, as far as the foundations go, I can't imagine linkers back then were that different.