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.syntax unified
.cpu cortex-m0plus
.thumb
.type assemble, %function
.global assemble
// TODO:
// - implement opcode parser
// - test each instruction (do this later -- much easier)
// - test 4-bit reg instructions
// - test BEQ overlaps
// - decide on additional push or pops
assemble:
// LDR R3, [SP, 20]
MOVS R6, 0
LDR R7, =0x00C8E893
// LDR R7, =0xE0E3A588
MOVS R0, '
MOV R8, R0
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:
// MOVS R0, 0xFF // lsb mask
// ANDS R0, R7 // store in R0
UXTB R0, R7 // store lsb in R0
LSRS R1, R0, 4 // upper nibble
CMP R1, 0xC // if 0xxxxxxx or 10xxxxxx
BLO opcode
CMP R1, 0xE // if 110xyyyy
BLO handle_imm
handle_reg: // if 111xyyyy
MOVS R1, (1<<4) // bit 4 mask
ANDS R0, R1 // get bit 4
ADDS R0, 3 // add 3 to it (now 3 or 4)
BL register // result is put in R4
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
opcode:
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
BL octal // 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
BL uart_send // 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
done:
MOVS R0, '\n // send newline
BL uart_send
MOVS R0, R6
BL send_hex
B assemble
.type get_char, %function
.global get_char
// R8: end_char
get_char:
PUSH {LR}
BL uart_recv
CMP R0, 025 // ^U (NAK)
BEQ redo_line
// CMP R0, 004 // ^D (EOT)
// BEQ done_for_real
CMP R0, R8
POP {PC}
redo_line:
POP {R0}
B done
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