Assembly Syscall Question

[Ruslan Ermilov]

On Thu, Jul 31, 2003 at 04:12:27PM -0400, Ryan Sommers wrote:
> When making a system call to the kernel why is it necessary to push the 
> syscall value onto the stack when you don't call another function? 
> 
> Example: 
> 
> access.the.bsd.kernel:
> int 80h
> ret 
> 
> func:
> mov eax, 4    ; Write
> call access.the.bsd.kernel
> ; End 
> 
> Works. However:
> func:
> mov eax, 4    ; Write
> int 80h
> ; End 
> 
> Doesn't. 
> 
This is because in a C library, all system calls are wrapped into
C functions, so the stack looks like this when in the syscall
code in libc:

	return address to a program
	syscall args

So the kernel knows how to account for a return address to access
actual arguments.

So when calling the kernel directly (not through a C library
wrapper function), we need to align the stack to fake the kernel
we're calling it from the syscall code in libc.


Cheers,
-- 
Ruslan Ermilov		Sysadmin and DBA,
ru at sunbay.com		Sunbay Software Ltd,
ru at FreeBSD.org		FreeBSD committer
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