have written this ARM assembly code. It is supposed to put the Fibonacci sequence numbers in R4 register. I'm trying to implement this C code in assembly:
int fibbonacci(int n) { if(n == 0) return 0; else if(n == 1) return 1; else return (fibbonacci(n-1) + fibbonacci(n-2)); }
This is the assembly code I have written:
AREA |.text|, CODE, READONLY EXPORT Fib Fib ;R0(n) - store the first n numbers in the Fibonacci sequence ;R1 - address of the first element in the array to store the Fibonacci sequence MOV R4, #0 MOV R5, #0 MOV R6, #1 FiboR CMP R0, #1 PUSHEQ {R5, R6, LR} BEQ Return SUBS R0, R0, #1 BL FiboR ADD R4, R2, R3 PUSH {R4, R2, LR} Return POP {R2, R3, PC} ALIGN END
It is not behaving exactly like that posted C code so far because I need to keep working on it and make it return to main after it prints the first 5 numbers. However for now I need to fix something.
When my code reaches this line:
PUSH {R4, R2, LR}
It's supposed to push values R4, R2 and LR on the stack which at that moment are: 1, 0, LR. Then the code goes down into the 'Return' subroutine in which I'm doing this:
POP {R2, R3, PC}
So at this moment R2 should be loaded with value 1, R3 with value 0 and PC with value of LR so I return to this line:
ADD R4, R2, R3
The code does return to the ADD line however the value of R2 is not loaded with the value 1 as I expected it to. My code stays here in an infinite loop and I keep pushing {1, 0, LR} on the stack but when I try to pop these values into R2, R3 and PC apparently they are not getting popped as I would expect them to at least value 1 does not go into R2 and R2 keeps it's value of 0 forever.
Can you please help me what am I missing here? Thank you for reading!
Sorry, got that reversed. The PUSH is done with lowest numbered register pushed to the lowest address, so you get R14, R4, R2 push order. See
www.keil.com/.../armasm_dom1361289885653.htm
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