LC3 machine code for reverse the bits

Question

Complete the machine code program in reverse.bin:

· Your program’s machine code must begin at memory location 0x3000.

· After running your program 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 your program 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, prior to running your program, memory location 0x3050 contains the value:

1111 1010 0000 0000

Then, after running your program to completion, 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 instructructions

Answer 1

This question is asking you to write an LC-3 machine code program that takes a value from memory, reverses its bits, and stores the result in another memory location without changing the original value.

Let’s understand this in simple terms.

Imagine you have a 16-bit number stored at memory location 0x3050. Your job is to flip the order of the bits and store the new value at 0x3051. The original value must remain unchanged.

For example, if the value at 0x3050 is:

1111 1010 0000 0000

Then reversing the bits means reading from right to left:

0000 0000 0101 1111

This reversed value should be stored at 0x3051.

To solve this problem in LC-3, the idea is to process one bit at a time. First, load the original value into a register. Then, create another register to build the reversed result. You repeatedly check the least significant bit (the rightmost bit) of the original number. Each time, you shift the result left and insert that bit. Then shift the original number right and repeat this process 16 times.

In simpler terms, you are:

1. Taking one bit from the original number
2. Adding it to the new number in reverse order
3. Repeating this until all bits are processed

A key point from an exam perspective is that the original value at 0x3050 must not be modified, so you should always work on a copy of it. Also, your program should be able to run again without resetting memory, so avoid overwriting important data.

This approach ensures that the program remains efficient and within the instruction limit, while correctly reversing all 16 bits.