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@c  callconv.t,v 1.1 2002/07/30 21:43:53 joel Exp

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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 ARM architecture supports a simple yet

effective call and return mechanism.  A subroutine is invoked

via the branch and link (@code{bl}) instruction.  This instruction

saves the return address in the @code{lr} register.  Returning

from a subroutine only requires that the return address be

moved into the program counter (@code{pc}), possibly with

an offset.  It is is important to

note that the @code{bl} instruction 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 the @code{bl}

instruction and return to the user application via the

mechanism described above.

@section Register Usage

As discussed above, the ARM's call and return mechanism dos

not automatically save any registers.  RTEMS uses the registers

@code{r0}, @code{r1}, @code{r2}, and @code{r3} as scratch registers and

per ARM calling convention, the @code{lr} register is altered

as well.  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 ARM calling conventions are followed and that

the first four arguments are placed in registers @code{r0} through

@code{r3}.  If there are more arguments, than that, then they

are place on the stack.

@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.