TMS34010 C Compiler - Al Kossow's Bitsavers

Runtime Environment - Register Conventions
5.2 Register Conventions
Strict conventions associate specific registers with specific operations in the
C environment. If you plan to interface assembly language routines to a C
program, it is important that you understand these register conventions.
5.2.1 Dedicated Registers
The C environment reserves three registers. Do not modify these registers in
any other manner than that described in Section 5.3, Function Structure and
Calling Conventions, page 5-8.
SP
A14 (STK)
A13 (FP)
5.2.2 Using Registers
points to the top of the system stack.
points to the program stack.
points to the beginning of the currently active frame.
In addition, the C compiler assumes certain information about bits in the status
register. Specifically, it assumes that FS1 (field size 1) is 32 within a C function.
FSO, however, can be changed in a function without being restored.
A function can usually use registers AO through A 12, however:
• When a function is called, it must save the contents of each register that
it uses; it must restore these registers before it returns to the caller. Register
A8 is the only exception; its contents do not have to be saved or
restored.
• If a function returns an integer value or a pointer, the value must be
placed in A8.
The code generator uses the A-file registers for the following purposes:
Expression analysis
Return value/Scratch
User register variables
AO through A11
A8
A9,A10,A11,A12,AO,A2,A4,A6
The C compiler doesn't use registers 80 through 814.
Expression-analysis registers are allocated from high to low registers, based
on availability and current use. (All integer expression analysis uses 32-bit
. math.)
Note:
The compiler constantly tracks the contents of registers and attempts to
reuse register data whenever possible. Therefore, it is inadvisable to use
inline assembly language or any other method to modify a register that a
function is using. Use the -r code generator option to produce information
about register use.
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