I am mildly curious that though the 6502 provides BCD arithmetic which would be useful for implementing decimal floating point, Commodore BASIC uses, like all (?) Micro-Soft BASIC, binary floating point instead.

Are there any easy example that show precision errors in Commodore BASIC, that would not be present if it would be based on decimal FP?

A classic test of the difference is 0.1 + 0.2 = 0.3; this evaluates to false in pretty much every modern language (since almost all of them use the IEEE floating point that is built into modern hardware). There is even a website devoted to this oddity: 0.30000000000000004.com

I tried this on a C64 emulator (which uses the same BASIC as the PET) and to my astonishment, it correctly evaluated to true. So did some other obvious tests like 0.1 * 10 = 1 and 0.1 + 0.9 = 1 but they worked as well.

What test would give a wrong answer on Commodore BASIC? That is, I'm not asking for a way to get it to demonstrate rounding errors per se; that much is trivial. I'm asking for a way to get it to give a wrong answer, not because it lacks infinite precision, but specifically for(simple) cases where decimal arithmetic would give the right answer. Some Commodore BASIC (MS-BASIC) equivalent to the 0.1 + 0.2 = 0.3 test on IEEE 754.

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    It might be worth to remember that the PET at first was A machine, not a business or home. If at all it was meant as a hobby computer. The differentiation in business or home is something that only evolved later - and with it's missing colour and sound abilities it moved into a (more) business range.
    – Raffzahn
    Commented Jan 27, 2019 at 12:52
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    @Raffzahn True! I tried it on an Apple II emulator just now and it passes the test, though iirc the Apple II ended up using Microsoft BASIC just like Commodore so that's not surprising. I'll try it on some others if I can find emulators with working keyboards.
    – rwallace
    Commented Jan 27, 2019 at 13:00
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    This question is out of the ordinary, and I must admit I really like it. Not at least as it looks for easy repeatable tests over some lengthy explanation. A true Engineering aproach :)) Would you mind to rephrase it (mostly the first paragraph) a bit to focus on the core issue of PET/Commodore FP - as the title already expresses it quite good (maybe also adding "... precision" in the title) I can do as well if you like me to. That way it'll make a great stop for others searching for hints in the same direction.
    – Raffzahn
    Commented Jan 27, 2019 at 14:47
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    @Raffzahn Thanks! The rephrase is an interesting idea, sure, please go ahead.
    – rwallace
    Commented Jan 27, 2019 at 15:25
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    @tofro But that wasn't the question. It's not about if the representation with 10 decimal digits is the same but if the actual numbers are the same. So don't print and visually compare the output but actually compare the numbers and print the outcome of that comparison.
    – BlackJack
    Commented Jan 27, 2019 at 17:56

6 Answers 6


This example reveals a rounding error under Commodore BASIC V2.0:

  A=0.3:B=0.6:IF A+B<>0.9 THEN PRINT A+B-0.9

Running this on a C64 yields a difference of 2.32830644e-10. Other pairs that fail are 0.4+0.5, 0.6+0.1 and 0.8+0.1. Please note that also the order in which the numbers are summed up affects the result. 0.6+0.1-0.7 yields a difference, while 0.1+0.6-0.7 results to 0.


Here is my favourite example for this problem. I often use it to show Excel's mathematical shortcomings, but not surprisingly it works the same in the C64:

10 A = 0.1
20 B = 0.1
30 FOR I = 1 TO 10
40 D = B
50 B = 20 * A - 19 * B
70 A = D

In every iteration, the algorithm should be doing 20 * 0.1 - 19 * 0.1 = 0.1, but the output on this simulator is


Might I suggest you try 0.11+0.12?

I believe IEEE754 will in fact give the right answer on 0.1+0.2=0.3, using standard single precision. It is, however, not difficult to provoke IEEE754 failures, for instance on 0.11+0.12. The C program below show the raw bin32 representations of the relevant IEEE754 numbers, the program output is:

a  :3dcccccd
b  :3e4ccccd
c  :3e99999a
IEEE754 copes
a  :3de147ae
b  :3df5c28f
c  :3e6b851f
IEEE754 fails


#include <stdio.h>
#include <stdint.h>
int main( void ) {
    float a = 0.1;
    float b = 0.2;
    float c = 0.3;
    float apb = a+b;
    printf( "a  :%x\n", *(uint32_t *)&a);
    printf( "b  :%x\n", *(uint32_t *)&b);
    printf( "a+b:%x\n", *(uint32_t *)&apb);
    printf( "c  :%x\n", *(uint32_t *)&c);
    if ( a + b == c ) {
            printf( "IEEE754 copes\n" );
    } else {
            printf( "IEEE754 fails\n" );

    a = 0.11;
    b = 0.12;
    c = 0.23;
    apb = a+b;
    printf( "a  :%x\n", *(uint32_t *)&a);
    printf( "b  :%x\n", *(uint32_t *)&b);
    printf( "a+b:%x\n", *(uint32_t *)&apb);
    printf( "c  :%x\n", *(uint32_t *)&c);
    if ( a + b == c ) {
            printf( "IEEE754 copes\n" );
    } else {
            printf( "IEEE754 fails\n" );

    return 0;
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    Ah! I ran the .1+.2 test on a modern machine in double precision; maybe in some sense, single precision is not precise enough to show the difference; CBM BASIC is closer to single precision. But .11+.12=.23 does indeed fail on a C64.
    – rwallace
    Commented Jan 27, 2019 at 14:04
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    On every half decent implementation, 0.1 + 0.2 will give you something that is very, very close to 0.3. Whether it equals 0.3 is more or less coincidence. The actual precision doesn't matter, double precision will be much much closer to 0.3, but will also make a lot lot smaller difference "not equal".
    – gnasher729
    Commented Jan 27, 2019 at 19:21
  • @gnasher729 Sure... what it equals or not, in the test, is the number produced by the string "0.3", which is 5*2^54 less than 0.3 in double precision, vs 0.1 + 0.2 being 5*2^52 greater.
    – Random832
    Commented Jan 29, 2019 at 4:25

Probably the simplest pattern to look for is 0.1+2-2-0.1. Depending upon precision, 0.1 is going to be (the .... is some number of repetitions of 0011)


Adding 2 is going to reduce the available precision at the low end by five bits (more than a whole repetition of the 0011 pattern), forcing some kind of rounding there. After 2 is subtracted, zeroes would be shifted into the low-order bits, and subtracting 0.1 would leave values in those bits.

When using decimal floating point, all calculations would be exact, and thus the final residue would be zero.

Incidentally, with regard to speed, the 6502 has patented circuitry to perform BCD addition and subtraction as fast as binary (interestingly, the CMOS versions of the chip do not have such circuitry, causing BCD addition and subtraction to be slower), but performing most other kinds of BCD math efficiently requires some substantial lookup tables.


Although not a direct answer to the question, I believe this is still worth pointing out:

Atari BASIC used BCD for its FP implementation. It was free from the sorts of errors being outlined above. I have found several other BCD implementations, but nothing "mainstream" to this degree.

The flip side is that BCD is theoretically slightly slower, you have to be careful turning the BCD mode on and off, and you get slightly fewer digits per 40-bit store.

On top of that, the Atari implementation was notoriously slow, but that was not specifically because it was BCD, it was just bad.

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    Microsoft BASIC on the MSX, Tandy 100 and Apple Mac were all decimal. The MSX outsold the Atari 8-bits several times over: 9 million (MSX; mostly Japan) vs 2 million (Atari 8-bit, worldwide).
    – scruss
    Commented Aug 23, 2020 at 19:09

Another precision test, previously mentioned in a comment by Tim Locke on this site, was published in Antic magazine Vol 1 No.4. It was submitted by one "R. Broucke" (the late Roger A. Broucke, then at UT Austin):

10 S=0
20 X=0
30 FOR N=1 TO 1000
40 S=S+X*X
50 X=X+0.00123
80 PRINT "CORRECT RESULT: 503.54380215, 1.23"

When run on a binary floating point interpreter (cbmbasic) it produces:

 503.543832            1.23000004

On a decimal floating point interpreter (tibasic):

 503.5438022   1.23 

Typically (but not always) binary floating point interpreters add spurious digits after the 1.23 result. Microsoft produced a few interpreters that used decimal floating point around 1983-4. These include BASIC for the Tandy 100 portable and MSX BASIC. Microsoft BASIC for Macintosh included two interpreters, one binary, one decimal.

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