8051 Microcontroller, Internals, Instructions, Programming and Interfacing, 2nd Edition

87

DEC R0 ; point to next source

DEC R1 ; point to next destination

DJNZ R7, SHIFT ; loop on eight times

; R7 indicates that the operation is over.

; Next instruction is used to terminate the program.

OVER: SJMP OVER ; terminate here.

Please note that had it been a case of shifting down, pointers would have been loaded for the other end (50H

and 4FH) and would have been incremented after every iteration.

C-version

#include <regx51.h>

void main(void)

{

unsigned char *src, *dst;

signed char ptr; // note we take signed data type here

src = 0x50; // point fi rst location of source

dst = 0x51; // point fi rst location of destination

for (ptr = 7; ptr >= 0; ptr --)

{

dst[ptr] = src[ptr];

}

while(1);

}

7.4 | Count No. of Nulls ...