ARM assembly print register value in decimal

Question

Hey, I have an exponentiation subroutine that does the calculation and puts the result in r0. Currently I can print the result in hexadecimal, but I also want to print it in decimal. After doing a lot of searching online I haven't found a straightforward way of doing it. Seems like a simple task to do but I can't figure it out.

thanks

Answer 1

I'm tempted to say "link with a C library and call printf". If that's not an option, here is something that probably works, given a "putchar" function that outputs the ASCII character in r0. It repeatedly divides the number by 10, writing the digits into a small buffer on the stack, then outputs them in reverse order. It requires architecture 4M or better for the "umull" instruction.

print_decimal:
        stmfd   sp!, {r4,r5,lr}

        cmp     r0, #0
        moveq   r0, #'0'
        bleq    putchar
        beq     done

        mov     r4, sp
        mov     r5, sp
        sub     sp, sp, #12

        rsblt   r0, r0, #0          ; r0 = abs(r0)
        movlt   lr, #1              ; lr = negative ? 1 : 0
        movgt   lr, #0

        ldr     r1, =0x1999999a     ; r1 = 2^32 / 10

loop:   umull   r2, r3, r0, r1      ; r3 = r0 / 10
        sub     r2, r0, r3, lsl #3
        sub     r2, r2, r3, lsl #1  ; r2 = r0 - 10*r3 = r0 % 10

        add     r2, r2, #'0'
        strb    r2, [r4, #-1]!

        movs    r0, r3
        bne     loop

        cmp     lr, #0
        movne   r0, #'-'
        blne    putchar

write:  ldrb    r0, [r4], #1
        bl      putchar
        cmp     r4, r5
        blt     write

        add     sp, sp, #12
done:
        ldmfd   sp!, {r4,r5,lr}
        mov     r0, #'\n'
        b       putchar