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+; bcd16.s
+; version 20060201
+;
+; Copyright (C) 2006 Damian Yerrick
+;
+; Copying and distribution of this file, with or without
+; modification, are permitted in any medium without royalty provided
+; the copyright notice and this notice are preserved in any source
+; code copies.  This file is offered as-is, without any warranty.
+;
+; @mssola: The memory addresses being used here have been modified so they don't
+; clash with my programs (from $0x to $8x).
+
+.p02
+
+.exportzp bcdNum, bcdResult
+.export bcdConvert
+
+; bcdConvert
+;
+; Given a number in bcdNum (16-bit), converts it to 5 decimal digits
+; in bcdResult.  Unlike most 6502 binary-to-decimal converters, this
+; subroutine doesn't use the decimal mode that was removed from the
+; 2A03 variant of the 6502 processor.
+;
+; For each value of n from 4 to 1, it compares the number to 8*10^n,
+; then 4*10^n, then 2*10^n, then 1*10^n, each time subtracting if
+; possible. After finishing all the comparisons and subtractions in
+; each decimal place value, it writes the digit to the output array
+; as a byte value in the range [0, 9].  Finally, it writes the
+; remainder to element 0.
+;
+; Extension to 24-bit and larger numbers is straightforward:
+; Add a third bcdTable, increase BCD_BITS, and extend the
+; trial subtraction.
+
+; Constants _________________________________________________________
+; BCD_BITS
+;   The highest possible number of bits in the BCD output. Should
+;   roughly equal 4 * log10(2) * x, where x is the width in bits
+;   of the largest binary number to be put in bcdNum.
+; bcdTableLo[y], bcdTableHi[y]
+;   Contains (1 << y) converted from BCD to binary.
+BCD_BITS = 19
+
+; Variables _________________________________________________________
+; bcdNum (input)
+;   Number to be converted to decimal (16-bit little endian).
+;   Overwritten.
+; bcdResult (output)
+;   Decimal digits of result (5-digit little endian).
+; X
+;   Offset of current digit being worked on.
+; Y
+;   Offset into bcdTable*.
+; curDigit
+;   The lower holds the digit being constructed.
+;   The upper nibble contains a sentinel value; when a 1 is shifted
+;   out, the byte is complete and should be copied to result.
+;   (This behavior is called a "ring counter".)
+;   Overwritten.
+; b
+;   Low byte of the result of trial subtraction.
+;   Overwritten.
+bcdNum = $80
+bcdResult = $82
+curDigit = $87
+b = $82
+
+;
+; Completes within 670 cycles.
+;
+
+bcdConvert:
+  lda #$80 >> ((BCD_BITS - 1) & 3)
+  sta curDigit
+  ldx #(BCD_BITS - 1) >> 2
+  ldy #BCD_BITS - 5
+
+@loop:
+  ; Trial subtract this bit to A:b
+  sec
+  lda bcdNum
+  sbc bcdTableLo,y
+  sta b
+  lda bcdNum+1
+  sbc bcdTableHi,y
+
+  ; If A:b > bcdNum then bcdNum = A:b
+  bcc @trial_lower
+  sta bcdNum+1
+  lda b
+  sta bcdNum
+@trial_lower:
+
+  ; Copy bit from carry into digit and pick up
+  ; end-of-digit sentinel into carry
+  rol curDigit
+  dey
+  bcc @loop
+
+  ; Copy digit into result
+  lda curDigit
+  sta bcdResult,x
+  lda #$10  ; Empty digit; sentinel at 4 bits
+  sta curDigit
+  ; If there are digits left, do those
+  dex
+  bne @loop
+  lda bcdNum
+  sta bcdResult
+  rts
+
+bcdTableLo:
+  .byt <10, <20, <40, <80
+  .byt <100, <200, <400, <800
+  .byt <1000, <2000, <4000, <8000
+  .byt <10000, <20000, <40000
+
+bcdTableHi:
+  .byt >10, >20, >40, >80
+  .byt >100, >200, >400, >800
+  .byt >1000, >2000, >4000, >8000
+  .byt >10000, >20000, >40000