3 bug fixes

[dmantione]

Hello,

The low-level serial API cannot work as it is now, due to a few bugs. 3 bug fixes in the serial code:

• Ciout did store the byte to be transmitted in BSOUR while iec_tx_byte expects the byte to be transmitted in TBTCNT
• iex_tx_flush did call iec_tx_byte, but that is wrong is JiffyDOS is used, the byte needs to be transmitted using JiffyDOS protocol
• jiffydos_tx_byte tries tro avoid changing the VIC-II bank, but did not do that for the last 2 bits.

With these changes the code works and is able to do serial communication.

[dmantione]

Yes certainly.

• In the Open ROMs, the routine iec_tx_byte expects the byte to be transmitted in TBTCNT
• In the original KERNAL, the equivalent routine ISOUR expects the byte to be transmitted in BSOUR

So because the Open ROM implementation uses a different variable than the original ROM, CIOUT has to be adapted as well. If you store in BSOUR while iec_tx_byte expects it in TBTCNT, it just doesn't work.

[FeralChild64]

So the proper fix would be to adapt iec_tx_byte to take the value from BSOUR. Some other tests and adaptations might be needed.

[dmantione]

Yes, if you want to make the code more similar to the original KERNAL, this is what needs to be done. However, I don't believe using TBTCNT is inherently broken: It can work and it makes the code more different from the original, so might be an advantage from a copyrights point of view. It is a matter of taste: If you want all zero page locations to have their original function, iec_tx_byte (and its JiffyDOS counterpart) need to be changed.

[FeralChild64]

If you think this change would be too risky/problematic, we keep it this way for now - but please add appropriate comment in ciout.s and a remark in the STATUS.md document. it should be clear that we have deviated here.

[dmantione]

Changing from TBTCNT to BSOUR would certainly require a lot more extensive testing, for example LOAD/SAVE also uses these routines and would need modification+testing. So this one line change is certainly the fastest and lowest-risk fix at the moment. A good approach would be to keep it this way and migrate if future problems are discovered.

I have added a notice in both ciout.s and STATUS.md.

[FeralChild64]

The chapter is not necessary - a short note in the table below is what we need :)

[FeralChild64]

One more thing - we have a tool which detects similarities between out code and the original ROMs (see https://github.com/MEGA65/open-roms/blob/master/README.md#defending-against-potential-copyright-infringement-claims), to use it one needs to put kernal and basic files to 3rdparty/ROMs directory and execute a make similarity command. I see we now have one similarity in the KERNAL code over 3 bytes long:

$0CD3 = $3FFD : 00 00 00 00 00 00 03

Could you also add an explanation file, similar to https://github.com/MEGA65/open-roms/blob/master/strings/0000000003 (file needs to be named 00000000000003)?