[Solved] Consider this ARM assembly snippet for a recursive factorial function:factorial: CMP R0, #1 BLE end_factorial PUSH {LR} SUB R0, R0, #1 BL factorial POP {LR} MUL R0, R0, R1 end_factorial:... | ARM Architecture

ARM Architecture - Subroutines and Stack

Consider this ARM assembly snippet for a recursive factorial function:

factorial:
    CMP R0, #1
    BLE end_factorial
    PUSH {LR}
    SUB R0, R0, #1
    BL factorial
    POP {LR}
    MUL R0, R0, R1
end_factorial:
    MOV R1, R0
    BX LR

What will be the value in R1 after calling factorial with R0 = 3?

A0

B3

C1

D6

Step-by-Step Solution

Solution:

Step 1: Trace the recursive factorial
The function saves LR, decrements R0, calls factorial recursively, restores LR, then multiplies R0 by R1.
Step 2: Calculate factorial for R0=3
Call factorial(3): R0=3, not base case, PUSH LR, SUB R0=2, BL factorial(2). factorial(2): PUSH LR, SUB R0=1, BL factorial(1). factorial(1): base case, sets R1=1, returns. factorial(2): POP LR, MUL R0=1 * R1=1 =1, MOV R1=1, return. factorial(3): POP LR, MUL R0=2 * R1=1=2, MOV R1=2, return.
Step 3: Correction
However, the MUL uses R0 and R1, but R0 was decremented before recursive call, so the multiplication is (n-1) * fact(n-1), which is incorrect for factorial.
Step 4: Final value
Tracing carefully, the final R1 will be 2, which is incorrect factorial of 3 (should be 6). But the code as given will produce 2.
Final Answer:
2 -> Option D

Quick Trick: SUB decrements R0 before call; MUL uses decremented R0 causing incorrect factorial [Check carefully]

Common Mistakes:

Assuming correct factorial computation (expecting 6)
Confusing R0 and R1 values
Missing base case return value

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More ARM Architecture Quizzes

Consider this ARM assembly snippet for a recursive factorial function:

Step 1: Trace the recursive factorial

Step 2: Calculate factorial for R0=3

Step 3: Correction

Step 4: Final value

Final Answer:

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