Re: Kernel Calling Conventions


"Sprunk, Stephen" <spamtrap@xxxxxxxxxx> wrote in message
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"Rod Pemberton" <spamtrap@xxxxxxxxxx> wrote in message
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"Kroll" <spamtrap@xxxxxxxxxx> wrote in message
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Greetings all,
I was reading the "FreeBSD Assembly Language Programming" tutorial
(http://www.int80h.org/bsdasm/), when I came across something that
piqued my curiosity.

The C calling convention is touted as being more convenient, and
superior to the calling convention used by linux and microsoft of
passing arguments within registers. I was wondering if this is just
a
bias on the part of the writer or what.

What are the actual advantages of popping the arguments onto to the
stack as opposed to passing them via registers?

I don't think one is necessarily better than the other. For the C
language, I'd say that the C calling convention is best. But, for
assembly programming, I think passing in registers is the more
convenient.

If you're trying to design a one-size-fits-all calling convention, the
main
consideration is the number of GPRs. x86 only has 6 usable GPRs (5 for
PIC)
during a call, so if you pass in registers most of the time the callee
will
have to immediately spill them; you might as well let the caller spill
them
and improve OOO performance.

Actually, the number of GPR's is compiler specific too. GCC style compilers
use a register based stackframe consuming two registers: EBP and ESP.
However, OW doesn't use a register based stackframe. It uses the pop
immediate value of the RET instruction to pull unused arguments off the
stack.


Rod Pemberton

PS. I think you sent that across as HTML instead of plain text. I had to
manually correct the quoting...



.

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