Hello,
I'm learning assembly on the ARM Cortex M4. I'm trying to write a code which will display the first 5 Fibonacci numbers in register R3 then go into an endless loop.
This would be the C code I'm trying to emulate:
int fibbonacci(int n) { if(n == 0){ return 0; } else if(n == 1) { return 1; } else { return (fibbonacci(n-1) + fibbonacci(n-2)); } }
I wrote the following assembly code in Keil:
;Recursive implementation - Print fibonacci sequence for first 5 numbers in R3 ; 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ; R4 = counter __main MOV R0, #4 ;MOV R4, #3 Sum CMP R0, #1 BEQ done SUBS R0, R0, #1 BL Sum ;SUBS R4, #1 ;CMP R4, #0 ;BEQ Stop POP {R1, R2} ADD R0, R1, R2 MOV R3, R0 PUSH {R0, R1} BX LR Stop B Stop done MOV R0, #0 MOV R3, R0 PUSH {R0} MOV R0, #1 MOV R3, R0 PUSH {R0} BX LR ALIGN END
Problem is that I was not able to make the code branch to the Stop loop after the first 4 numbers are displayed into the register R3. What happens is when I reach the 'BX LR' instruction in the 'Sum' subroutine the code keeps branching to the Sum and keeps on printing Fibonacci numbers forever. I could somehow make use of the R4 register to decrement it few times so when we have 4 numbers displayed in register R3 we branch to the stop loop but that is not a very elegant solution.
I just want the number of numbers to be printed in the register R0 and then once the first 4 numbers are displayed into R3 the code should return from the Sum(4) and go to the stop loop below.I know I have to somehow make use of the LR register to achieve this but I was not able to figure it out. Can you please help me by helping me figure out how I could modify this code so that I can achieve what I want? Thank you for reading and sorry for my English I know it is not the best :D