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Optimizing code for one off variable

Nicholas Poh

Hi,

Taking the code snippet below,

char* ptr  = (char*)0x0000;
short size = 0x100;
short sum  = 0x00;

do {
   sum += *ptr++;
} while (size--)

This will cause C51 to allocate "ptr" and "size" in RAM.

        MOV     R3,#00H
        MOV     R2,#00H
        MOV     R1,#00H
        MOV     DPTR,#pre?1251
        MOVX    @DPTR,PR0

        MOV     DPTR,#size?1252
        MOV     A,#01H
        MOVX    @DPTR,A
        INC     DPTR
        CLR     A
        MOVX    @DPTR,A

and everytime "size" is decremented, or "ptr" is incremented, the values are loaded back into the RAM.

Is there a way to force C51 to not use RAM but use the registers only as "ptr" and "size" will never be used anymore after that. What I mean is, is there a way to force C51 to compile to something like this,

mov dptr, #WORD0(00h)
mov r4, #01h ; r4:r5 = size
mov r5, #00h
mov r6, #00h ; r6:r7 = sum
mov r7, #00h

loop:
movx A, @dptr+#000h
addw 006h, A ; r6:r7 += A

inc  dptr
subw 004h, 001h ; r4:r5--
jnc  loop

; add code to store r6:r7 into RAM

Any advice would be greatly appreciated. Thanks in advance.

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