Double Precision Binary to BCD Conversion

tomcrawford

I couldn't find extended precision binary-to-BCD conversion in spin, so here one is. It does up to (2^64)-1. You give it two longs and it return 20 bytes of BCD. I checked it against all 64 powers of two, which it got correct. The method is discussed in the comment section of the code, but basically it repeatedly subtracts powers of 10.

ChrisGadd

There's another method of converting whereby you check all of the nibbles of the result, one nibble at a time. For any nibble that is greater than four, add three. Then shift all of the registers--result and source--up one bit and check again. Repeat until every bit has been shifted into the result.

16-bit example:                       BCD_value           Hex_value
        Start                   0000_0000 0000_0000|0000_0100 1101_0010   1234 = $04D2
   1    Shift left                                 |0000_1001 1010_010
   2    Shift left                                 |0001_0011 0100_10
   3    Shift left                                 |0010_0110 1001_0
   4    Shift left                                 |0100_1101 0010
   5    Shift left                                 |1001_1010 010
   6    Shift left                                1|0011_0100 10
   7    Shift left                               10|0110_1001 0
   8    Shift left                              100|1101_0010
   9    Shift left                             1001|1010_010
        Add 3 to 1st nibble (>4)               1100|1010_010
  10    Shift left                           1_1001|0100_10
        Add 3 to 1st nibble                  1_1100|0100_10
  11    Shift left                          11_1000|1001_0
        Add 3 to 1st nibble                 11_1011|1001_0               
  12    Shift left                         111_0111|0010
        Add 3 to 2nd nibble               1010_0111|0010
        Add 3 to 1st nibble               1010_1010|0010
  13    Shift left                      1 0101_0100|010
        Add 3 to 2nd nibble             1 1000_0100|010
  14    Shift left                     11 0000_1000|10
        Add 3 to 1st nibble            11 0000_1011|10
  15    Shift left                    110 0001_0111|0
        Add 3 to 3rd nibble          1001 0001_0111|0
        Add 3 to 1st nibble          1001 0001_1010|0
  16    Shift left                 1_0010 0011_0100|   = $1234

Attached 64-bit Spin and PASM routines.