# 6.7. Another FunctionΒΆ

The sample below has a second demonstration of using the calling conventions - it contains an unsigned division function `udiv`

that is used to figure out the coins to use to make change. The `udiv`

function takes two parameters, the number to divide and the divisor, passed in `r0`

and `r1`

and returns two values, the quotient and the remainder, in `r0`

and `r1`

.

Note

Since parameters are just register numbers, documentation of how the registers are being used is critical!

Like the last one, this program avoids storing and reloading registers by keeping long term values in r4+ and using r0-r3 for all the temporary work in the function.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 | ```
/*
Demonstrates using unsigned division function to make ideal change for
the number of cents loaded in r0.
Answer provided as:
r6: quarters
r7: dimes
r8: nickles
r9: pennies
*/
.text
.global _start
_start:
@Initial number of cents
MOV r0, #92
@25 cents per quarter
MOV r1, #25
BL udiv @ cents / 25
@r0 now has quarters and r1 leftover cents
MOV r6, r0 @r6 = num quarters
MOV r0, r1 @put leftover cents back into r0
@10 cents per dime
MOV r1, #10
BL udiv @leftover cents / 10
@r0 now has dimes needed and r1 leftover cents
MOV r7, r0 @r7 = num dimes
MOV r0, r1 @put leftover cents back into r0
@5 cents per nickle
MOV r1, #5
BL udiv @leftover cents / 5
@r0 now has nickles needed and r1 leftover cents
MOV r8, r0 @r8 = num nickles
MOV r9, r1 @r9 = pennies = leftover cents
end:
B end @stop here
@----------------------------------------------------------------------
/*Unsigned division - assumes both values are unsigned integers
Params:
r0 = number
r1 = divisor
Return:
r0 = quotient
r1 = remainder
*/
udiv:
MOV r2, #0 @temp_quotient
B udiv_test
udiv_loop_start:
SUB r0, r0, r1 @number -= divisor
ADD r2, r2, #1 @quotient++
udiv_test:
CMP r0, r1 @is remaining value > divisor?
@Can't use GE for unsigned values
BCS udiv_loop_start @if higher or same continue loop
MOV r1, r0 @remainder = leftover part of number
MOV r0, r2 @quotient = temp_quotient
MOV pc, lr @return
``` |

You have attempted of activities on this page