Hi, I have to code two programs in 6502, the first one - showing on the screen hexa number - is done, and it works correctly, but the second one - showing decimal number on the screen - isn't working. I'm trying to connect the program, which is working with the functions which let count decimal number to get a working program. What I can't achieve, is output on the screen. The loop seems to be working, so the convertion might be done correctly, but there is no output on the screen. I tried using something similar to pr in hexa, jumping there and storing accumulator in $80, but without any effects. I removed my efforts to readability of my code. Could you tell me what should be changed in BCD code?

HEX:



BCD:

It's not clear to me what your strategy is. First of all, are you attempting to convert binary to ASCII, because if you're converting BCD to ASCII then your first code sample should be fine.

My strategy to convert 8-bit binary to ASCII might be to initialize a counter to -1, then construct a small loop that increments the counter and subtracts 100 decimal from the accumulator until the carry is clear, indicating that we went "one iteration too far". The accumulator would then get the last 100 added back in and the counter would be stored in the text buffer as the "100s" digit (after an ASCII offset adjustment). The counter is reset to -1 and another small loop does the same thing but with 10 decimal instead of 100 decimal. The counter is stored as the "10s" digit (after offset adjustment), and the accumulator gets 10 added back in before being stored as the "1s" digit (again with offset adjustment). No BCD would be necessary in my example.

Mike B.

P.S. You can find an example of what I mean here, starting at the instruction LDY #"0"-1 ... but it is only designed to handle binary numbers less than 100 decimal, and has been "streamlined" in ways that may not be clear to a beginner.

Here is an untested sample of what I'm describing:


Mike B.

And here's a stupidly small tested version from codebase. .A = number in. .Y/.X/.A = 100s/10s/1s ASCII digit out.

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

Nice! Even if we replace your RTS with

... it still ends up at only 30 bytes.
Here's a (tested!!) 34-byte version that removes leading zeros but still prints "0" for zero. It uses some advanced stack and V-flag trickery that I learned from John Brooks, and can be very economically expanded to any input with a width that's a multiple of eight bits:


Mike B.

I don't know about the algorithm, but you need to conclude your BINBCD8 subroutine with RTS instead of BRK or it will never return to its caller, and none of the subsequent caller code will be executed.

Mike B.