9. 68hc12 Backend

This chapter documents the Backend for the 68hc12 microcontroller family.

Note that this backend is not done!

9.1 Additional options for this version

This backend provides the following additional options:

‘-merge-constants’

Place identical floating point constants at the same memory location. This can reduce program size and increase compilation time.

‘-const-in-data’

By default constant data will be placed in a read-only section. Using this option it will be placed in the data section.

‘-no-delayed-popping’

By default arguments of function calls are not always popped from the stack immediately after the call, so that the arguments of several calls may be popped at once. With this option vbcc can be forced to pop them after every function call. This may simplify debugging and very slightly reduce the stack size needed by the compiled program.

‘-no-peephole’

Do not perform peephole-optimizations.

‘-mem-cse’

Try to hold values loaded from memory in registers and reuse them. Due to the small register-set of the hc12 this is disabled by default as it increases register- pressure and tends to spill to the stack.

9.2 ABI

The current version generates assembly output for use with the GNU assembler using the non-banked model.

This backend supports the following registers:

• - d for the accumulator (also used for byte, i.e. a)
• - x for index register x
• - y for index register y
• - sp for the stack-pointer

All registers are scratch (i.e. caller-save).

The first function arguments which has integer or pointer types is passed in the accumulator d or the register pair d/x. The remaining arguments are passed on the stack.

Integers and pointers are returned in d or d/x. All other types are returned by passing the function the address of the result as a hidden argument - so when you call such a function without a proper declaration in scope you can expect a crash.

The elementary data types are represented like:

    type        size in bits        alignment in bytes

    char                8                       1
    short              16                       2
    int                16                       2
    long                 currently not supported
    near pointers      16                       2
    far pointers         currently not supported
    huge pointers        currently not supported
    float                currently not supported
    double               currently not supported

9.3 Target-specific variable-attributes

The hc12-backend offers the following variable attributes:

__interrupt

Return with rfi rather than blr.

__section("name","attr")

Place this function/object in section "section" with attributes "attr".

9.4 Predefined Macros

This backend defines the following macros:

__HC12__

9.5 Stack

If the ‘-stack-check’ option is used, every function-prologue will call the function __stack_check with the stacksize needed by this function in register y. This function has to consider its own stacksize and must restore all registers.

9.6 Stdarg

To be written...

9.7 Known Problems

• - this backend is far from done...

This document was generated by vb on January 3, 2015 using texi2html 1.82.