Write a subroutine MEMCMP for performing a byte-by-byte comparison of two sequences of
bytes in the main memory. The subroutine should accept three input parameters in registers
representing the first address, the second address, and the length of the sequences to be compared. It
should use a register to return the count of the number of comparisons that do not match.
To write a subroutine called MEMCMP for performing a byte-by-byte comparison of two sequences of bytes in the main memory, you can follow these steps:
1. Initialize a counter register with a value of 0 to keep track of the number of comparisons that do not match.
2. Load the first address of the first sequence from the input parameter into a register.
3. Load the first address of the second sequence from the input parameter into another register.
4. Load the length of the sequences to be compared from the input parameter into another register to use it as a loop counter.
5. Start a loop that will iterate until the length reaches zero.
6. Inside the loop, load a byte from the first sequence using the first address register and store it in a temporary register.
7. Load a byte from the second sequence using the second address register and store it in another temporary register.
8. Compare the two bytes using a conditional branch instruction. If they are not equal, increment the counter register by 1.
9. Increment both address registers to move to the next byte position in the sequences.
10. Decrement the length register by 1 to track the remaining length.
11. Repeat steps 6-10 until the length reaches zero.
12. After the loop ends, the counter register will contain the count of the number of comparisons that do not match.
13. Return the value of the counter register as the result of the subroutine.
Here's an example implementation of the MEMCMP subroutine in assembly language:
```
MEMCMP:
mov edx, 0 ; Counter register for the number of mismatches
mov edi, [esp + 4] ; First address
mov esi, [esp + 8] ; Second address
mov ecx, [esp + 12] ; Length of sequences
cmp ecx, 0 ; Check if length is zero
je end
loop_start:
mov al, byte [edi] ; Load byte from first sequence
mov bl, byte [esi] ; Load byte from second sequence
cmp al, bl ; Compare the two bytes
jne mismatch ; Jump if they are not equal
inc edi ; Increment first address
inc esi ; Increment second address
dec ecx ; Decrement length
cmp ecx, 0 ; Check if length is zero
jne loop_start ; Repeat loop if not zero
jmp end
mismatch:
inc edx ; Increment mismatch counter
inc edi ; Increment first address
inc esi ; Increment second address
dec ecx ; Decrement length
cmp ecx, 0 ; Check if length is zero
jne loop_start ; Repeat loop if not zero
end:
mov eax, edx ; Return the mismatch counter
ret
```
Make sure to use the correct assembly language syntax for the processor architecture you are targeting.
Here is a step-by-step implementation of the MEMCMP subroutine in assembly language:
1. Start with the subroutine declaration:
```
MEMCMP:
```
2. Save the registers that will be used for temporary storage. In this case, we will use registers r1, r2, and r3:
```
PUSH {r1, r2, r3}
```
3. Load the input parameters into registers:
```
LDR r1, [r0] ; Load first address into r1
LDR r2, [r1] ; Load first byte from the first address
LDR r3, [r4] ; Load second address into r3
LDR r4, [r3] ; Load first byte from the second address
LDR r5, [r2] ; Load the length of the sequences
```
4. Initialize a counter to keep track of the number of comparisons that do not match:
```
MOV r6, #0 ; Initialize the counter to zero
```
5. Loop through the sequences and compare the bytes:
```
LOOP:
CMP r5, #0 ; Check if the length is zero
BEQ DONE ; If the length is zero, exit the loop
LDRB r2, [r1], #1 ; Load the next byte from the first sequence and increment the address
LDRB r4, [r3], #1 ; Load the next byte from the second sequence and increment the address
CMP r2, r4 ; Compare the bytes
BNE NOT_MATCH ; If the bytes do not match, jump to NOT_MATCH
SUBS r5, r5, #1 ; Decrement the length by 1
B LOOP ; Continue the loop
NOT_MATCH:
ADD r6, r6, #1 ; Increment the counter by 1
SUBS r5, r5, #1 ; Decrement the length by 1
B LOOP ; Continue the loop
```
6. Return the counter value in a register:
```
DONE:
MOV r0, r6 ; Move the counter value to r0 to return
```
7. Restore the registers and return from the subroutine:
```
POP {r1, r2, r3}
BX lr ; Return from the subroutine
```
That's it! The subroutine MEMCMP is now implemented and can be used to perform byte-by-byte comparison of two sequences of bytes in main memory.