source: rtems/doc/supplements/mips64orion/callconv.t @ f48b1000

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@c  COPYRIGHT (c) 1988-1998.
@c  On-Line Applications Research Corporation (OAR).
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@chapter Calling Conventions

@section Introduction

Each high-level language compiler generates
subroutine entry and exit code based upon a set of rules known
as the compiler's calling convention.   These rules address the
following issues:

@itemize @bullet
@item register preservation and usage
@item parameter passing
@item call and return mechanism
@end itemize

A compiler's calling convention is of importance when
interfacing to subroutines written in another language either
assembly or high-level.  Even when the high-level language and
target processor are the same, different compilers may use
different calling conventions.  As a result, calling conventions
are both processor and compiler dependent.

@section Processor Background

The MC68xxx architecture supports a simple yet
effective call and return mechanism.  A subroutine is invoked
via the branch to subroutine (@code{XXX}) or the jump to subroutine
(@code{XXX}) instructions.  These instructions push the return address
on the current stack.  The return from subroutine (@code{XXX})
instruction pops the return address off the current stack and
transfers control to that instruction.  It is is important to
note that the XXX call and return mechanism does not
automatically save or restore any registers.  It is the
responsibility of the high-level language compiler to define the
register preservation and usage convention.

@section Calling Mechanism

All RTEMS directives are invoked using either a @code{XXX}
or @code{XXX} instruction and return to the user application via the
@code{XXX} instruction.

@section Register Usage

As discussed above, the @code{XXX} and @code{XXX} instructions do
not automatically save any registers.  RTEMS uses the registers
@b{D0}, @b{D1}, @b{A0}, and @b{A1} as scratch registers.  These registers are
not preserved by RTEMS directives therefore, the contents of
these registers should not be assumed upon return from any RTEMS
directive.

@section Parameter Passing

RTEMS assumes that arguments are placed on the
current stack before the directive is invoked via the @code{XXX} or @code{XXX}
instruction.  The first argument is assumed to be closest to the
return address on the stack.  This means that the first argument
of the C calling sequence is pushed last.  The following
pseudo-code illustrates the typical sequence used to call a
RTEMS directive with three (3) arguments:

@example
@group
push third argument
push second argument
push first argument
invoke directive
remove arguments from the stack
@end group
@end example

The arguments to RTEMS are typically pushed onto the
stack using a move instruction with a pre-decremented stack
pointer as the destination.  These arguments must be removed
from the stack after control is returned to the caller.  This
removal is typically accomplished by adding the size of the
argument list in bytes to the current stack pointer.

@section User-Provided Routines

All user-provided routines invoked by RTEMS, such as
user extensions, device drivers, and MPCI routines, must also
adhere to these calling conventions.