@c @c COPYRIGHT (c) 1988-1998. @c On-Line Applications Research Corporation (OAR). @c All rights reserved. @c @c $Id$ @c @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.