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How to get the C-compiler to use RLC and ACC.x

Harjo Otten

Hello,

I would like to know if there's any way I can write C code that will use the assembly instructions RLC and ACC.x efficiently. I would like to some C code that will result in:

MOV A,byBuffer
MOV c,bInput
RLC A
MOV byBuffer,A

The closest I could get was this:

byBuffer=(byBuffer<<1)&bInput;

Which results in:

MOV A,byBuffer
ADD A,ACC
MOV R7,A
MOV C,bInput
CLR A
RLC A
ANL A,R7
MOV byBuffer,A

The same thing happens when I want to use a specific bit from a byte. I have never seen the C-compiler use the ACC.x instruction. It uses solutions like this:

MOV A,R6
MOV R0,AR5
INC R0
SJMP ?C0078
?C0077:
CLR C
RLC A
?C0078:
DJNZ R0,?C0077

Any ideas ????

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  • 0 in reply to Harjo Otten

    First of all, your C code fragment doesn't do what the ASM did. The truly equivalent C fragment would have been:

      byBuffer=(byBuffer<<1) | bInput;

    Note the '|' instead of '&' before bInput.

    Anyway: your original assembly isn't picture-perfect, either, so there's not much reason that the C compiler should generate that for any given piece of C code. 

      MOV	A, byBuffer
  RL	A
  MOV	ACC.7, bInput
  MOV	byBuffer, A

    would have been better, because it doesn't pollute whatever
    was in the carry bit before.

Reply
  • 0 in reply to Harjo Otten

    First of all, your C code fragment doesn't do what the ASM did. The truly equivalent C fragment would have been:

      byBuffer=(byBuffer<<1) | bInput;

    Note the '|' instead of '&' before bInput.

    Anyway: your original assembly isn't picture-perfect, either, so there's not much reason that the C compiler should generate that for any given piece of C code. 

      MOV	A, byBuffer
  RL	A
  MOV	ACC.7, bInput
  MOV	byBuffer, A

    would have been better, because it doesn't pollute whatever
    was in the carry bit before.

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