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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