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.syntax unified
.cpu cortex-m0plus
.thumb
.type lookup, %function
.global lookup
// input:
// R0 input string (unboxed)
// output:
// R0 return code
// R1 instruction template
// R2 instruction type
// R3 special code
lookup: PUSH {R4, R5, R6}
ADR R4, op_table// op table base address
MOVS R5, 0 // begin pointer
MOVS R6, 7 // end pointer
loop: CMP R5, R6 // if begin == end then range = 0
BEQ error
ADDS R2, R5, R6 // get sum
LSRS R2, 1 // divide by two
LSLS R3, R2, 3 // times 8 (index to offset)
LDR R1, [R4, R3]// read opcode from optable
CMP R0, R1 // compare opcode to input
BLO lower // if lower
BHI higher // if higher
found: LSLS R2, 3 // times 8 (index to offset)
ADDS R4, R2 // select row
MOVS R0, 0 // success return code
LDRH R1, [R4, 4] // get machine code template
LDRB R2, [R4, 6] // get instruction type
LDRB R3, [R4, 7] // get special code
POP {R4, R5, R6}
BX LR // result in R2
lower: MOVS R6, R2 // update end pointer
B loop
higher: ADDS R5, R2, 1 // update begin pointer
B loop
error: MOVS R0, 4 // return code 4 (opcode not found)
POP {R4, R5, R6}
BX LR
.align 8
op_table:
.ascii "ADD3"; .hword 0x1C00; .byte 0x01, 0x03 // instr type 1, special code 3 (imm width)
.ascii "ADD8"; .hword 0x3000; .byte 0x02, 0x08 // instr type 2, special code 8 (imm width)
.ascii "ASPI"; .hword 0xB000; .byte 0x04, 0x07 // instr type 4, special code 7
.ascii "ASRI"; .hword 0x1000; .byte 0x01, 0x05 // instr type 1, special code 5 (imm width)
.ascii "ADCS"; .hword 0x4140; .byte 0x00, 0x00 // instr type 0, special code 0
.ascii "ADDS"; .hword 0x1800; .byte 0x03, 0x00 // instr type 3, special code 0
.ascii "BKPT"; .hword 0xBE00; .byte 0x04, 0x08 // instr type 4, special code 8
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