TMS34010 C Compiler - Al Kossow's Bitsavers

Runtime Environment - Interfacing C with Assembly Language
5.4 Interfacing C with Assembly Language
There are three ways to use assembly language in conjunction with C code:
• Use separate modules of assembled code and link them with compiled
C modules (see Section 5.4.1). This is the most versatile method.
• Use in line assembly language, imbedded directly in th~ C source (see
Section 5.4.2).
• Modify the assembly language code that the compiler produces (see
Section 5.4.3).
5.4.1 Assembly Langua"ge Modules
Interfacing with assembly language functions is straightforward if you follow
the calling conventions defined in Section 5.3 and the register conventions
defined in Section 5.2. C code can access variables and call functions that
are defined in assembly language, and assembly code can access C variables
and call C functions.
Follow these guidelines to interface assembly language and C:
1 ) All functions, whether they are written in C or assembly language, must
follow the conventions outlined in Section 5.3 (page 5-8).
2) You must preserve any registers that are modified by a function, except
register A8. When returning from a function, FS1 must equal 32.
3) Interrupt routines must save all the registers they use. (For more information
about interrupt handling, see Section 5.5, page 5-16.)
4) If the caller passes arguments or if the called function returns a float or
a double, save the program stack pointer (A14) by pushing it on the
system stack, then push any arguments on the program stack in reverse
order.
5) Functions must return values correctly according to their C declarations.
Integers and pointers are returned in register A8. All floating-point values
are returned on the stack. Section 5.3.5 discusses returning structures.
6) No assembly language module should use the .cinit section for any
purpose other than autoinitialization of global variables. The C boot
routine (boot. c) assumes that the .cinit section consists entirely of
initialization tables. Disrupting the tables by putting other information
in .cinit can cause unpredictable results.
7) The compiler prefixes all identifiers with an underscore (_). This means
that you must prefix the name of all objects that are to be accessible from
C with - when writing assembly language. For example, a C object
called x is called -x in assembly. For identifiers that are to be used only
in an assembly language module or modules, any name that does not
begin with a leading underscore may be safely used without conflicting
with a C identifier.
8) Any object or function declared by an assembly language routine that is
to be accessed or called from C must be declared with the .global as-
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