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.syntax unified
.cpu cortex-m0plus
.thumb
.type opcode, %function
.global opcode
// 1 unexpected first char
// R4 input buffer
// R2 output buffer
opcode: PUSH {LR}
LDRB R0, [R4] // get a char
CMP R0, 0x41 // A
BLO 1f
CMP R0, 0x5A // Z
BLS 2f
1: MOVS R0, #1 // unexpected char
POP {PC}
2: ADDS R4, 1 // consume the character
STRB R0, [R2] // store in temp buffer
ADDS R2, 1 // advance temp buffer pointer
LDRB R0, [R4] // get another character
BL goodchar // check if valid symbol char
BEQ 2b // if so keep getting chars
MOVS R0, #0 // return code success
STRB R0, [R2] // write null byte
POP {PC}
goodchar: CMP R0, '0
BLO bad
CMP R0, '9
BLS good
CMP R0, 'A
BLO bad
CMP R0, 'Z
BHI bad
good: CMP R0, R0
bad: BX LR
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