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.syntax unified
.cpu cortex-m0plus
.thumb
.type register, %function
.global register
// R4 input buffer
// R2 output buffer
register: PUSH {LR}
LDRB R0, [R4]
r_test: CMP R0, 'R
BNE special
ADDS R4, 1 // consume R
BL decimal // consume a decimal number (result in R2)
BEQ validate
POP {PC} // error code already in R0
validate: CMP R2, #12 // general purpose registers 0-12
BLS success
MOVS R0, #7 // invalid register number error code
POP {PC}
special: LDRB R0, [R4] // get two bytes from input stream
LDRB R1, [R4, 1]
LSLS R1, 8
ORRS R0, R1
ADR R2, table // get address of table
MOVS R3, 0 // set table offset to 0
loop: LDRH R1, [R2, R3] // get two bytes from table at current offset
CMP R0, R1 // compare input to table row
BEQ done // if equal then done
ADDS R3, 2 // increment table offset
CMP R3, 6 // compare offset to table size
BLO loop // loop until end of table
MOVS R0, #6 // return code 6 (expected register)
POP {PC}
done: ADDS R4, 2 // consume two chars
LSRS R3, 1 // divide table offset by two to row
ADDS R3, 13 // add 13 to get register number
MOVS R2, R3
success: MOVS R0, #0 // return code 0 (success)
POP {PC}
.align 4
table: .ascii "SP"
.ascii "LR"
.ascii "PC"
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