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.syntax unified
.cpu cortex-m0plus
.thumb
.type decimal, %function
.global decimal
// R4 input stream
// R2 output value
decimal: LDRB R0, [R4] // get a char
CMP R0, '0 // check if zero
BNE notzero
ADDS R4, 1 // consume the char
MOVS R2, 0 // return value of zero
success: MOVS R0, #0 // return code zero (success)
BX LR
notzero: CMP R0, '1 // if not [1-9] then error
BLO bad
CMP R0, '9
BHI bad
ADDS R4, 1 // consume the first digit
SUBS R0, '0 // calculate the value
MOVS R2, R0 // store it in R2
loop: LDRB R0, [R4] // get another char
CMP R0, '0 // if not [0-9] then done
BLO success
CMP R0, '9
BHI success
ADDS R4, 1 // consume the additional digit
SUBS R0, '0 // calculate the value
MOVS R3, 10 // base 10
MULS R2, R3 // shift result by one decimal place
ADDS R2, R0 // accumulate into R2
B loop // keep getting digits
bad: MOVS R0, #5 // return code 5 (not a digit)
BX LR
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