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 101, 197, 17, 180, 16, 127, 113, 146
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 101, 197, 17, 180, 16, 127, 113, 146
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 an assembly-like snippet and asked to determine what value ends up in the memory location labeled 'result' after the code runs. Let me walk through how the code behaves step by step, examining each part of the loop and how it modifies the register values.

- Data section setup:
  - array_length is a byte with value 8, so there are 8 elements in my_array.
  - my_array contains the bytes: 101, 197, 17, 180, 16, 127, 113, 146.
  - result reserves 1 byte of space to store the final value.

- Text section setup:
  - The function asm_function sets up a pointer to the start of my_array in r26:r27 using lo8/hi8 of the label, so X will point to my_array.
  - r20 is loaded with array_length, so r20 starts at 8. This appears to track how many elements remain to process.
  - r16 is initialized to 10. This is the running total we will ultimately store into result.

- Loop control:
  - loop_check compares r20 with 0; if equal, it jumps to after_loop (end of processing).
  - Inside loop, it loads a byte from address X (the current array element) into r17 and post-increments X (X+), then decrements r20 (consuming one element).
  - It then performs andi r17, 1, which masks all bits except the least significant bit. This effectively tests whether the current byte is odd (result 1) or even (result 0).
  - cpi r17, 0 followed by breq loop_check means: if the element is even (LSB = 0), branch back to loop_check without changing r16. If the element is odd (LSB = 1), we do not take the branch and continue to inc r16, then jump back to loop_check.

- Effect per element:
  - For each odd element, r16 is incremented by 1.
  - For each even element, r16 is unchanged.
  - After processing an element, the loop checks if there are more elements (via r20) and continues until r20 reaches 0.

- Applying this to the actual array:
  - 101 is odd -> r16 goes from 10 to 11
  - 197 is odd -> r16 goes from 11 to 12
  - 17 is odd  -> r16 goes from 12 to 13
  - 180 is even -> r16 stays 13
  - 16 is even  -> r16 stays 13
  - 127 is odd  -> r16 goes from 13 to 14
  - 113 is odd  -> r16 goes from 14 to 15
  - 146 is even -> r16 stays 15

- After the loop completes, since r16 started at 10 and we incremented it 5 times (for the 5 odd numbers), r16 ends up at 15. The code then stores this final value into the memory location labeled 'result' via sts result, r16 and returns.

- Summary of the logic and common pitfalls:
  - The code counts odd numbers in the array by masking the LSB of each byte and incrementing a counter for odds.
  - It starts the counter at 10, so the final value is 10 plus the number of odd elements.
  - The number of odd elements in the given array is 5 (101, 197, 17, 127, 113).
  - Therefore, final result should be 10 + 5 = 15, not 5. Any answer claiming 5 ignores the initial offset of 10 and the 5 increments.

- Conclusion about the provided answer:
  - The value stored in result is 15. The option stating 5 would be incorrect because it disregards the initial value of r16 (10) and the five increments added during processing of odd elements.

ELEC1601 (ND) ELEC9601 (ND)

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