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+@c
+@c  COPYRIGHT (c) 1988-2002.
+@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
+
+All members of the i960 architecture family support
+two methods for performing procedure calls: a RISC-style
+branch-and-link and an integrated call and return mechanism.
+
+On a branch-and-link, the processor branches to the
+invoked procedure and saves the return address in a register,
+@code{G14}.  Typically, the invoked procedure will not invoke another
+procedure and is referred to as a leaf procedure.  Many
+high-level language compilers for the i960 family recognize leaf
+procedures and automatically optimize them to utilize the
+branch-and-link mechanism.  Branch-and-link procedures are
+invoked using the @code{bal} and @code{balx} instructions and return control
+via the @code{bx} instruction.  By convention, @code{G14} is zero when not in
+a leaf procedure.  It is the responsibility of the leaf
+procedure to clear @code{G14} before returning.
+
+The integrated call and return mechanism also
+branches to the invoked procedure and saves the return address
+as did the branch and link mechanism. However, the important
+difference is that the @code{call}, @code{callx}, and @code{calls} instructions save
+the local register set (@code{R0} through @code{R15}) before transferring
+control to the invoked procedure.  The @code{ret} instruction
+automatically restores the previous local register set.  The
+i960CA provides a register cache which can be configured to
+retain the last five to sixteen recent register caches.  When
+the register cache is full, the oldest cached register set is
+written to the stack.
+
+@section Calling Mechanism
+
+All RTEMS directives are invoked using either a @code{call}
+or @code{callx} instruction and return to the user via the @code{ret}
+instruction.
+
+@section Register Usage
+
+As discussed above, the @code{call} and @code{callx} instructions
+automatically save the current contents of the local register
+set (@code{R0} through @code{R15}).  The contents of the local registers will
+be restored as part of returning to the application.  The
+contents of global registers @code{G0} through @code{G7} are not preserved by
+RTEMS directives.
+
+@section Parameter Passing
+
+RTEMS uses the standard i960 family C parameter
+passing mechanism in which @code{G0} contains the first parameter, @code{G1}
+the second,  and so on  for the remaining parameters.  No RTEMS
+directive requires more than six parameters.
+
+@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.
+
+@section Leaf Procedures
+
+RTEMS utilizes leaf procedures internally to improve
+performance.  This improves execution speed as well as reducing
+stack usage and the number of register sets which must be cached.
+
+