I am struggling with trying to get SYM BASIC to run on my custom board and am looking for suggestions.

My board has:
- 6502 at 1 Mhz
- RAM at $0000-$1FFF
- EEPROM for monitor at $E000-$FFFF
- 6551 for serial IO at $8000
- EEPROM for 8K BASIC at $C000-$DFFF

The monitor program is a very simplified version of the SYM-1 monitor.
It has simple commands for viewing/editing memory and for starting execution at an address.
The board/monitor seem to work fine with a terminal emulator TERATERM (serial-USB converter).
I can view and edit memory fine as well as run small hand assembled programs.

I looked at the disassembly listing for SYM BASIC (from Davidson) and found the SYM monitor routines it uses.



I added routines in my monitor program to perform the same functions.
I have not implemented the LOAD and SAVE yet though.
And since I do not have write protection on any RAM, the ACCESS routine is not needed.

I located the spots in the BASIC ROM image where the routines are called as follows:



I loaded the BASIC ROM image into a hex editor and put in the addresses of my monitor routines at the appropriate spots.
The EEPROM was then programmed and installed in my board.

Doing a G C000 results in the printing of MEMORY SIZE ? as it should.
Hitting enter at that point results in printing ? SN ERROR and it prompts for MEMORY SIZE again.
It will accept other chars until you press enter, then it throws the same error.

From my monitor I can view the memory of the BASIC EPROM and it appears that it has the correct contents.

I'm wondering what exactly BASIC expects from the monitor routines.

Currently my routines are:
INCHR - waits for an input char and returns (RTS) with the character in A.
INSTAT - checks to see if there is a char available. If no char is available it returns (RTS) with carry clear.
If there is a char, it returns with carry set and character in A.
ASCNIB - converts ASCII char in A to a hex nibble and returns (RTS) with value in A.
OUTCHR - sends out the character in A.
ACCESS - just set to NOPs in the EEPROM.

Since it seems to print output ok I believe the OUTCHR routine is working.
It appears that something is not quite right with INCHR.
Even though the input works when in the monitor.
I've tried about everything I can think of but can't seem to get past the MEMORY SIZE prompt.

My INCHR routine


Has anyone been successful in getting SYM BASIC to run on their board?

Any tips would be appreciated, thanks.

When trouble shooting, it is sometimes easier to find the error when following the code backwards. Look at the code that branches to the Syntax Error and find what is forcing the branch or jump.

List the part of INCHAR that branches to the Syntax Error.

Good suggestion. Also, might be worth checking to see if the original routines preserve any registers which your new ones don't.

Also, perhaps it's worth looking at the Page Zero usage of SymBasic... and compare it to any Page Zero usage your monitor is using. It's also possible that SymBasic might be using some Stack space memory for pointers, variables, etc., which may result in some corruption if the Stack pointer isn't initialized.

_________________
Regards, KM
https://github.com/floobydust

Not sure if this will help but I came across this comparison of Page Zero references for Basics and hex dump for SYM Basic. It's been a while since I looked seriously but I still have my SYM-1 and while I have removed the BASIC ROM to be replaced by SYMbiosys for a CF card Storage system I could replace it and check things out.

Thanks for the suggestions everyone, I have it somewhat solved.

I changed the way I was entering BASIC from my monitor program.
Originally I was using some code from the SYM monitor that would take an address and start execution there, after some stack manipulation, with an RTI.
For now I just put in a simple jmp $C000 and I'm able to get past the MEMORY SIZE, WIDTH prompts and get into the interpreter.
So it must be something stack related, even though it looks like BASIC sets the stack to $FF as one of the first things.

There is still some goofy stuff going on, but I got past the first hurdle.

Thanks again!