Re: Unix binaries
On Mon, 31 Aug 2009, Roedy Green wrote:
On Mon, 31 Aug 2009 21:42:08 +0100, Tom Anderson
<twic@urchin.earth.li> wrote, quoted or indirectly quoted someone who
said :
What do you mean by "the assembler API"?
In DOS you call OS services primarily with INT 21, a software
interrupt. BIOS services have other interrupt numbers e.g. INT 13 for
low level floppy and hard disk i/o. Presumably, there is something
similar buried inside the system C methods you call on Unix.
Okay, this is more or less what i understand by "application binary
interface".
At some pint there is usually a flip between user mode and supervisor
mode, and user address space and system address space. If you provide
services in pure user mode, they would not need to be OS functions. They
could just be part of a class library.
I asked about this back 1n 1985 and was told these magic numbers are
never any concern of the programmer, and not to worry my then pretty
little head about them. There was a linking process to C system
methods, and such an interface was private and internal to those
wrappers. The numbers need not be published or stable the way it was in
DOS.
Here's what the System V ABI specification for the x86 [1], which is
something of an ur-text for unix ABIs, on the x86 at least, says (page
3-24):
A program executes the lcall instruction through a system call gate to
change execution modes, and thus the lcall instruction provides the
low-level interface to system calls. For the Intel386, one low-level
interface is defined: _exit(BA_OS).
To ensure a process has a way to terminate itself, the system treats
_exit as a special case. The ABI does not specify the implementation of
other system services. Instead, programs should use the system libraries
that Chapter 6 describes. Programs with other embedded lcall instructions
do not conform to the ABI.
So there isn't really an equivalent to INT21h, because as you say, you're
only supposed to go through the libraries!
tom
[1] http://www.sco.com/developers/devspecs/abi386-4.pdf
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