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     Copyright (C) 2005 Free Software Foundation, Inc.

     Copying and distribution of this file, with or without modification,
     are permitted in any medium without royalty provided the copyright
     notice and this notice are preserved.

--------------------------------------------------------------------------

                        MS1 ABI
                        =========

Sizes and alignments
--------------------

        Type            Size (bytes)    Alignment (bytes)

        char            1               1
        short           2               2
        int             4               4
        unsigned        4               4
        long            4               4 
        long long       8               8
        float           4               4
        double          8               8
        pointers        4               4 

* alignment within aggregates (structs and unions) is as above, with
  padding added if needed
* aggregates have alignment equal to that of their most aligned
  member
* aggregates have sizes which are a multiple of their alignment


Floating point
--------------

All emulated using IEEE floating point conventions.

Registers
----------------

r0              always zero
r1              argument register 1
r2              argument register 2
r3              argument register 3
r4              argument register 4
r5              callee must save
r6              callee must save
r7              call clobbers
r8              call clobbers
r9              call clobbers
r10             call clobbers
r11             function return value
r12             frame pointer
r13             stack pointer
r14             linkage pointer
r15             interrupt pointer

Stack alignment         8 bytes

Structures passed       <= 32 bits as values, else as pointers

The MS1 Stack
---------------

Space is allocated as needed in the stack frame for the following at compile
time:

* Outgoing parameters beyond the fourth

* All automatic arrays, automatic data aggregates, automatic
  scalars which must be addressable, and automatic scalars for
  which there is no room in registers 

* Compiler-generated temporary values (typically when there are
  too many for the compiler to keep them all in registers) 

Space can be allocated dynamically (at runtime) in the stack frame for the
following:

* Memory allocated using the alloca() function of the C library

Addressable automatic variables on the stack are addressed with positive
offsets relative to r12; dynamically allocated space is addressed with positive
offsets from the pointer returned by alloca().

Stack Frame
-----------

        +-----------------------+
        |    Parameter Word 1   |
        +-----------------------+ <-sp
        |    Previous FP        |
        +-----------------------+
        |    Return address     |
        +-----------------------+
        |    Saved Registers    |
        +-----------------------+
        |        ...            |
        +-----------------------+
        |    Local Variables    |
        +-----------------------+ <-fp
        |    Alloca             |
        +-----------------------+
        |        ...            |
        +-----------------------+
        |   Parameter Word 2    |
        +-----------------------+
        |   Parameter Word 1    |
        +-----------------------+ <-sp


Parameter Assignment to Registers
---------------------------------

Consider the parameters in a function call as ordered from left (first
parameter) to right.  GR contains the number of the next available
general-purpose register.  STARG is the address of the next available stack
parameter word.

INITIALIZE:
        Set GR=r1 and STARG to point to parameter word 1.

SCAN:
        If there are no more parameters, terminate.
        Otherwise, select one of the following depending on the type
        of the next parameter:

    SIMPLE ARG:

        A SIMPLE ARG is one of the following:

        * One of the simple integer types which will fit into a
          general-purpose register,
        * A pointer to an object of any type,
        * A struct or union small enough to fit in a register (<= 32 bits)
        * A larger struct or union, which shall be treated as a
          pointer to the object or to a copy of the object.
          (See below for when copies are made.)

        If GR > r4, go to STACK.  Otherwise, load the parameter value into
        general-purpose register GR and advance GR to the next general-purpose
        register.  Values shorter than the register size are sign-extended or
        zero-extended depending on whether they are signed or unsigned.  Then
        go to SCAN.

    DOUBLE or LONG LONG

        If GR > r3, go to STACK.  Otherwise, if GR is odd, advance GR to the
        next register.  Load the 64-bit long long or double value into register
        pair GR and GR+1.  Advance GR to GR+2 and go to SCAN.

    STACK:

        Parameters not otherwise handled above are passed in the parameter
        words of the caller's stack frame.  SIMPLE ARGs, as defined above, are
        considered to have size and alignment equal to the size of a
        general-purpose register, with simple argument types shorter than this
        sign- or zero-extended to this width.  Round STARG up to a multiple of
        the alignment requirement of the parameter and copy the argument
        byte-for-byte into STARG, STARG+1, ...  STARG+size-1.  Set STARG to
        STARG+size and go to SCAN.


Structure passing
-----------------

As noted above, code which passes structures and unions by value is implemented
specially.  (In this section, "struct" will refer to structs and unions
inclusively.)  Structs small enough to fit in a register are passed by value in
a single register or in a stack frame slot the size of a register.  Structs
containing a single double or long long component are passed by value in two
registers or in a stack frame slot the size of two registers.  Other structs
are handled by passing the address of the structure.  In this case, a copy of
the structure will be made if necessary in order to preserve the pass-by-value
semantics.

Copies of large structs are made under the following rules:

                        ANSI mode                       K&R Mode
                        ---------                       --------
Normal param            Callee copies if needed         Caller copies
Varargs (...) param     Caller copies                   Caller copies

In the case of normal (non-varargs) large-struct parameters in ANSI mode, the
callee is responsible for producing the same effect as if a copy of the
structure were passed, preserving the pass-by-value semantics.  This may be
accomplished by having the callee make a copy, but in some cases the callee may
be able to determine that a copy is not necessary in order to produce the same
results.  In such cases, the callee may choose to avoid making a copy of the
parameter.


Varargs handling
----------------

No special changes are needed for handling varargs parameters other than the
caller knowing that a copy is needed on struct parameters larger than a
register (see above).

The varargs macros set up a register save area for the general-purpose
registers to be saved.  Because the save area lies between the caller and
callee stack frames, the saved register parameters are contiguous with
parameters passed on the stack.  A pointer advances from the register save area
into the caller's stack frame.


Function return values
----------------------

        Type            Register
        ----            --------
        int             r11
        short           r11
        long            r11
        long long       stack
        float           r11
        double          stack