apple2_print_fixed16_fraction/README.md
2017-06-11 07:45:23 -07:00

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# Print Fixed Point #.16 Fraction
# Theory
Printing the fractional value of a 16-bit fixed point number in decimal is rather trivial.
For example, the number `0.7106` in 16-bit fixed point is 0.71506 * 65536 = 46,569 = `$B5E9`
How do we convert the number `$B5E9` back into `.7106` ? By _digit peeling._
1. Multipy N by 10
2. Print the Integer Part digit
3. Set the Integer Part to zero
5. Have we printed the number of decimal digits we want? If no then goto step 1
Example:
```
0.7106 * 65536 = $B5E9
'.'
$B5E9 * $A = $7.1B1A
$1B1A * $A = $1.0F04
$0F04 * $A = $0.9628
$9628 * $A = $5.DD90
$5DD9 * $A = $8.A7A0
0.71058 _______________________^
```
An optimization for N*10 is that this is equivalent to: `N*8 + N*2`
# Application
## Assembly (Source)
See file [print_fract.s](print_fract.s)
## Binary (Executable)
```
CALL-151
0800:a2 b5 a0 e9 20 1a 08 a9 ae 20 ed fd a2 00 bd 76
0810:08 20 ed fd e8 e0 05 d0 f5 60 8e 71 08 8c 72 08
0820:20 6a 08 aa 20 60 08 20 54 08 20 60 08 20 60 08
0830:20 44 08 ad 70 08 09 b0 9d 76 08 20 6a 08 e8 e0
0840:05 d0 e1 60 18 a0 02 b9 70 08 79 73 08 99 70 08
0850:88 10 f4 60 a0 02 b9 70 08 99 73 08 88 10 f7 60
0860:0e 72 08 2e 71 08 2e 70 08 60 a9 00 8d 70 08 60
0870:00 00 00 00 00 00 00 00 00 00 00
800G
E000G
```
## Output
```
.71058
```