Zilog ZUSBOPTS Manual do Utilizador descarregar pdf (Página 204)

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101

Calling C Functions from Assembly UM017105-0511

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Zilog Developer Studio II – ZNEO™

User Manual

Preserving Registers

The ZNEO C-Compiler implements a scheme in which the registers R8–R13 are treated as

callee save. The assembly function must preserve any of these registers that it uses. The

assembly function is not expected to save and restore the flag register.

Calling C Functions from Assembly

The C functions that are provided with the compiler library can also be used to add func-

tionality to an assembly program. You can also create your own C functions and call them

from an assembly program.

Because the compiler makes the caller function responsible for saving registers R0–R7

(see the Calling Conventions

section on page 172), if the assembly code is using any of

these functions and needs their contents to be preserved across the C function call, it must

save them before the call and restore them afterwards.

The C-Compiler precedes the function names with an underscore in the generated assem-

bly. See the Function Naming Convention section on page 174.

The following example shows an assembly source file referencing the function

sinf. The

sinf function is defined in the C library.

Assembly File

globals on

xref _sinf

segment near_data

val:dl %3F060A96; 0.523599

res:dl 0

segment code

_main:

pushm <R1>

; save the registers, other than return register, if any in

use

ld R1,val; load the argument

call _sinf; call the c functions

ld res,r0 ; the result is in r0

Note:

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