Study the following code. It is designed to perform a meaningful function on my_array. If you understand what task it is doing then you will be able to solve this question much quicker.

.section .data
array_length: .byte 8
my_array: .byte 227, 131, 113, 177, 52, 39, 126, 2
result: .space 1

.section .text
.global asm_function

asm_function:
ldi r26, lo8(my_array)
ldi r27, hi8(my_array)

lds r20, array_length
ldi r16, 10

loop_check:
cpi r20, 0
breq after_loop

loop:
ld r17, X+
dec r20

andi r17, 1
cpi r17, 0
breq loop_check
inc r16

jmp loop_check

after_loop:
sts result, r16
ret

.end

What is the value stored in result after this code is run?

Question

Study the following code. It is designed to perform a meaningful function on my_array. If you understand what task it is doing then you will be able to solve this question much quicker.

.section .data
array_length: .byte 8
my_array: .byte 227, 131, 113, 177, 52, 39, 126, 2
result: .space 1

.section .text
.global asm_function

asm_function:
ldi r26, lo8(my_array)
ldi r27, hi8(my_array)

lds r20, array_length
ldi r16, 10

loop_check:
cpi r20, 0
breq after_loop

loop:
ld r17, X+
dec r20

andi r17, 1
cpi r17, 0
breq loop_check
inc r16

jmp loop_check

after_loop:
sts result, r16
ret

.end

What is the value stored in result after this code is run?

BlackTom AI · Accepted Answer

We are given a small AVR-style assembly routine operating on an array of bytes and a single-byte result location. The task is to determine what value gets stored in result after asm_function runs.

First, observe the data: the array my_array contains eight bytes: 227, 131, 113, 177, 52, 39, 126, 2. The variable array_length holds the value 8, so the loop is designed to iterate exactly eight times. The register r16 is initialized to 10, and r20 is loaded with the length of the array (8).

The loop structure follows this pattern: loop_check checks whether there are any items left (r20 == 0). If there are none left, control goes to after_loop. Otherwise, the code enters loop, loads a byte from the memory pointed to by X (which starts at the address of my_array), and increments X (X+ post-increment). Then it decrements r20 to reduce the remaining count.

Next, andi r17, 1 takes the loaded byte r17 and masks all bits except the least significant bit. This effectively computes whether the byte is odd (LSB = 1) or even (LSB = 0). The instruction cpi r17, 0 compares this LSB with 0. If the result is equal (the byte is even), the code branches back to loop_check without modifying r16. If the LSB is 1 (the byte is odd), the code does inc r16 to add 1 to the running total and then jumps back to loop_check.

Thus, for each element of the array, we add 1 to r16 whenever the element is odd, and do nothing when it is even. After processing all eight elements (r20 reaches 0), the code stores the value of r16 into result and returns.

Now we tally the odd values in the array: 227 (odd), 131 (odd), 113 (odd), 177 (odd), 52 (even), 39 (odd), 126 (even), 2 (even). There are five odd numbers in total: 227, 131, 113, 177, and 39.

Since r16 started at 10 and is incremented once for each odd element, the final value placed in result is 10 + 5 = 15.

In summary, the loop counts the number of odd elements in my_array, adds that count to the initial value in r16 (10), and stores the result (15) into result when the loop completes. The surrounding code does not modify result elsewhere, so this is the value that ends up stored by the time ret executes.

ELEC1601 (ND) ELEC9601 (ND) Essentials Quiz 4 - Requires Respondus LockDown Browser

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