Converting Binary Coded Decimal

Binary Coded Decimal (BCD) is a method of representing a decimal number as a hexadecimal value using the hex digits 0-9 and ignoring A-F. For example, the number 21 (hex &15) would be represented as &21 (decimal 33). It can be a convenient internal representation of decimal numbers as each hexadecimal digit can be displayed without any further processing. See Wikipedia. BCD values can be decoded with:

   number% = VALSTR$~bcd%

Numbers can be encoded in BCD using:

   bcd% = EVAL("&"+STR$number%)

A 7-bit binary number can be converted to BCD so it can be printed in hex with the following 6502 code:

   \ Convert binary number to BCD
   \ ----------------------------
   \ On entry, A=value 0-99
   \ On exit,  A=BCD value &00-&99
   \           X corrupted
   .BINtoBCD
   TAX             :\ Transfer number to X
   LDA #&99        :\ Start with -1 in BCD form
   SED             :\ Switch to Decimal arithmetic
   .BINtoBCDlp
   CLC:ADC #1      :\ Add 1 with BCD arithmetic
   DEX             :\ Decrement input value in X
   BPL BINtoBCDlp  :\ Loop until input value < 0
   CLD:RTS         :\ Switch back to Binary arithmetic

See also Discussion for an enhanced implementation.

A BCD number can be converted to binary with the following 6502 code:

\ Convert BCD to Binary
 \ --------------------------------------------
 \ On entry, A=BCD value &00-&99
 \ On exit,  X=binary value 0-99
 \           A=corrupted
 .BCDtoBIN
 LDX #&FF        :\ Start with result=-1
 SED             :\ Switch to Decimal arithmetic
 SEC             :\ Prepare for subtraction
 .BCDtoBINlp
 INX             :\ Add 1 to result
 SBC #1          :\ Subtract 1 with BCD arithmetic
 BCS BCDtoBINlp  :\ Loop until BCD value < 0
 CLD:RTS         :\ Switch back to Binary arithmetic

See also Discussion for an enhanced implementation.

Jgharston 21:11, 23 June 2007 (BST) Jgharston (talk) 00:19, 3 October 2016 (UTC)