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C51 v8.17 saving/restoring all registers in interrupt function with assembly src

Eckhard Tikwinski

C51 will generate different code for the following function:

void timer1 (void) interrupt 3 {
    second;
#pragma asm
    clr    TF1
#pragma endasm
}


Compiler v8.08 code (edited)

    USING    0
timer1:
; #pragma asm
    clr    TF1
; #pragma endasm
    RETI


Compiler v8.17 code (edited)

    USING    0
timer1:
    PUSH     ACC
    PUSH     B
    PUSH     DPH
    PUSH     DPL
    PUSH     PSW
    MOV      PSW,#00H
    PUSH     AR0
    ...
    PUSH     AR7
    USING    0
; #pragma asm
      clr    TF1
; #pragma endasm
    POP      AR7
    ...
    POP      AR0
    POP      PSW
    POP      DPL
    POP      DPH
    POP      B
    POP      ACC
    RETI


C51 v8.17 generates code to save and restore all registers, as soon as the (very simplified, nonsense) C function contains any assembly source, whereas C51 v8.08 does not. For a function containing equivalent C code only (TF=0), nothing would be saved with either version.

Is there a reason for the different code generated by v8.17 ? How could saving/restoring of the registes be avoided ?

0