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.syntax unified
.cpu cortex-m0plus
.thumb
.type assemble, %function
.global assemble
// TODO:
// - make subroutine addresses explicit
// - test each instruction
// - decide on additional push or pops
// - add GO to get_char
assemble: PUSH {LR}
LDR R0, =uart_send
LDR R1, =get_char
MOV R9, R0
MOV R10, R1
MOVS R6, 0
MOVS R0, '
MOV R8, R0
LDR R1, =opcode
BLX R1
MOV R0, R8
BLX R9
MOVS R7, R4
main_loop: LSRS R0, R7, 8 // just peek
BNE skip // if more stuff then skip
MOVS R0, '\r
MOV R8, R0 // set end char to carriage return
skip: UXTB R0, R7 // store lsb in R0
LSRS R1, R0, 4 // upper nibble
CMP R1, 0xC // if 0xxxxxxx or 10xxxxxx
BLO handle_op
CMP R1, 0xE // if 110xyyyy
BLO handle_imm
handle_reg: MOVS R1, (1<<4) // bit 4 mask // if 111xyyyy
ANDS R0, R1 // get bit 4
ADDS R0, 3 // add 3 to it (now 3 or 4)
LDR R1, =register
BLX R1
MOVS R0, 0x0F // lower nibble mask
ANDS R0, R7 // store shift amount in R0
LSLS R4, R0 // shift the result by the shift amount
ORRS R6, R4 // OR the register code into the word under construction
B done_stuff
handle_op: MOVS R2, 9 // shift amount for 7-bit opcode
MOVS R1, (1<<7) // bit 7 mask
TST R0, R1 // check bit 7
BEQ fin // if zero done
BICS R0, R1 // clear bit 7
MOVS R2, 11 // shift amount for 5-bit opcode high
MOVS R1, (1<<5) // bit 5 mask
TST R0, R1 // check bit 5
BEQ fin // if zero done
BICS R0, R1 // clear bit 5
MOVS R2, 6 // shift amount for 5-bit opcode low
fin: LSLS R0, R2
ORRS R6, R0
B here
handle_imm: MOVS R1, 0x0F // lower nibble mask
ANDS R0, R1 // store immediate width in R0
LDR R1, =octal
BLX R1 // result is put in R4
LSLS R0, R7, 27
LSRS R0, 31
MOVS R2, 6
MULS R0, R2 // R0 has shift amount (0 or 6)
LSLS R4, R0 // shift the result by the shift amount
ORRS R6, R4 // OR the immediate into the word under construction
done_stuff: MOV R0, R8 // copy the end_char into R0
BLX R9 // echo the space (or carriage return)
here: LSRS R7, 0x8 // get next parse instruction
BNE main_loop // if it's nonzero there are more things to parse
POP {PC}
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