Binary code to reverse bits

Question

binary code to reverse : must begin at memory location 0x3000.

· After running to completion, the value at memory location 0x3050 must be unchanged,

while memory location 0x3051 must contain the bits of 0x3050 in reverse order.

· It must be possible to rerun by manually resetting the PC to 0x3000. It should not

require that the LC-3 be reinitialized or that any files be reloaded.

For example, suppose that, memory location 0x3050 contains the value:

1111 1010 0000 0000

Then, after running memory location 0x3051 should contain the value:

0000 0000 0101 1111

A clear, concise program should be able to do this using fewer than 15 instructions.

Answer 1

.ORIG x3000

LD R0, x3050 ; Load the value to be reversed into R0

LD R1, #15 ; Initialize the counter register R1 to 15

LD R2, #0 ; Initialize the accumulator register R2 to 0

LOOP ; Loop until the counter register reaches 0

BRzp DONE

ASR R0, R0, #1 ; Shift the value in R0 to the right by 1 bit

AND R2, R2, R0 ; Store the least significant bit of R0 in the accumulator

ASL R2, R2, #1 ; Shift the accumulator to the left by 1 bit

SUB R1, R1, #1 ; Decrement the counter register

BR LOOP ; Branch back to the loop

ST R2, x3051 ; DONE: The reversed value is now in the accumulator

HALT

.END