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.syntax unified
.cpu cortex-m0plus
.thumb
.type octal, %function
.global octal
octal: PUSH {LR} // 0c0000 0xB500 ; SAVE LINK REGISTER
MOVS R5, R0 // 0x0002 0x0005 ; SAVE BIT WIDTH
BNE 30f // 0x0004 0xD1 ; SKIP IF BIT WIDTH != 0
10: BLX R10 // 0x0006 0x ; GET CHAR
CMP R0, '0 // 0x000A 0x2830 ; COMPARE TO '0 CHAR
BNE 10b // 0x000C 0xD1 ; GET ANOTHER CHAR IF NOT '0
BLX R9 // 0x000E 0x ; OTHERWISE ECHO '0 CHAR
MOVS R4, 0 // 0x0012 0x2400 ; SET RESULT TO 0
20: BLX R10 // 0x0014 0x ; GET CHAR
BNE 20b // 0x0018 0xD1 ; LOOP IF NOT END CHAR
POP {PC} // 0x001A 0xBD00 ; RETURN IF END CHAR
30: MOVS R0, 3 // 0x001C 0x2003 ; DEFAULT TO 3
CMP R5, 3 // 0x001E 0x2D03 ; CHECK BIT WIDTH
BHS 40f // 0x0020 0xD2 ; IF BIT WIDTH >= 3 KEEP R0 == 3
MOVS R0, R5 // 0x0022 0x0028 ; IF BIT WIDTH < 3 COPY TO R0
40: MOVS R4, 1 // 0x0024 0x2401 ; 2^0
LSLS R4, R0 // 0x0026 0x4084 ; 2^(MAX(BIT WIDTH, 3))
ADDS R4, ('0 - 1) // 0x0028 0x342F ; '1 OR '3 or '7
50: BLX R10 // 0x002A 0x ; GET FIRST CHAR
CMP R0, '0 // 0x002E 0x2830 ; COMPARE WITH '0
BLO 50b // 0x0030 0xD3 ; RETRY IF BELOW OCTAL RANGE
CMP R0, R4 // 0x0032 0x42A0 ; COMPARE WITH END CHAR
BHI 50b // 0x0034 0xD8 ; RETRY IF ABOVE RANGE
BLX R9 // 0x0036 0x ; ECHO IF IN RANGE
SUBS R0, '0 // 0x003A 0x3830 ; GET VALUE FROM CHAR
MOVS R4, R0 // 0x003C 0x0004 ; STORE VALUE IN RESULT
MOVS R1, 0 // 0x003E 0x2100 ; BIT WIDTH FOR FIRST CHAR
60: ADDS R1, 1 // 0x0040 0x3101 ; INCREMENT FIRST CHAR WIDTH
LSRS R0, 1 // 0x0042 0x0840 ; SHIFT RIGHT ONE BIT
BNE 60b // 0x0044 0x ; IF NONZERO STAY IN LOOP
SUBS R5, R1 // 0x0046 0x1A6D ; SUBTRACT FIRST CHAR WIDTH FROM TOTAL
70: CMP R5, 3 // 0x0048 0x2D03 ; COMPARE BIT WIDTH TO 3
BLO 80f // 0x004A 0xD3 ; EXIT LOOP IF LESS THAN 3
BLX R10 // 0x004C 0x ; GET ANOTHER CHAR
BEQ 90f // 0x0050 0xD0 ; IF END CHAR EXIT LOOP
CMP R0, '0 // 0x0052 0x2830 ; BOTTOM OF OCTAL RANGE
BLO 70b // 0x0054 0xD3 ; RETRY IF BELOW
CMP R0, '7 // 0x0056 0x2837 ; TOP OF OCTAL RANGE
BHI 70b // 0x0058 0xD8 ; RETRY IF ABOVE
BLX R9 // 0x005A 0x ; ECHO IF IN RANGE
SUBS R0, '0 // 0x005E 0x3830 ; GET VALUE FROM CHAR
LSLS R4, 3 // 0x0060 0x00E4 ; SHIFT OLD VALUE BY ONE DIGIT
ADDS R4, R0 // 0x0062 0x1824 ; ACCUMULATE INTO VALUE
SUBS R5, 3 // 0x0064 0x3D03 ; DECREMENT BIT WIDTH BY 3
B 70b // 0x0066 0xD0 ; LOOP FOR ANOTHER DIGIT
80: BLX R10 // 0x0068 0x ; GET A CHAR
BNE 80b // 0x006C 0xD1 ; LOOP UNTIL END CHAR
90: POP {PC} // 0x006E 0xBD00 ; RETURN
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