6502 : Reliability and diagnostic information

Reliability

Our environment for developing and running 6502 programs is still not reliable.  Since I put the hardware on a stand, physical connectivity has not been a problem.  I am past the stage where single stepping through 6502 programs looking at bus signals using the Arduino Mega is regularly required.  However when I turn the 6502 on, or reboot, the system doesn't always come up properly.

When you start the system  the Mega runs its program which typically stop the 6502 whilst it loads a program into ROM.  It is possible to use an external 5V power supply instead and disconnect both 5v and GND from Mega to breadboard.  This avoids any complications due to the Mega doing unexpected things.  It may be necessary to do a reset for the 6502 to boot normally.

I start a new terminal-based "monitor program called cmdline which has options for checking various system information.  Initially I set up power up self test (POST) subroutines for terminal and LCD but there isn't much testing the 6502 can do at start up without using terminal or LCD for output.  On the basis it is obvious if LCD and terminal are working I dispensed with POST.

cmdline has an "r" option which jumps to $8000 to reboot and initialise the system.  Interestingly this is very dependable, implying that startup problems are power or hardware related and the software is working just fine.

Diagnostic subroutines

Option "d" calls a subroutine to display register values: Accumulator, X, Y, P (status flags), S (stack pointer) and PC (Program Counter).  Program status flags are made available by pushing them on the stacvk (php) and popping them (pla) to the accumulator.  The TSX command provides access to the stack pointer.  The program counter is actually available st compile time and is retrieved by loading a label value.

The term-register subroutine is not very useful at the command line as it just shows values when the 6502 is at the monitor prompt; it is more helpful as a subroutine call when testing a program to print out register values.

Dirk Grappendorf has a monitor program which allows the user to display a range of memory addresses.  It is very useful for looking at page zero ($0000), stack ($0100) and working storage ($0200).  We implement "mhhhh" where "hhhh" is the start address to list.  It is pleasing that we can copy Dirks assembly code to convert hex numbers into our program - I don't want to have to write all the code myself.

I was also able to copy another subroutine PRIMM, courtesy of Lee Davidson at 6502.org.  This allows you to put display text in your program immediately after a subroutine call, which avoids situations where the text and code become separated when copying program snippets around.

One issue we face when using the 6502 system is that we dont know what program is running.  To improve the situation we display the program name and assembly time on the LCD and provide an option to display it on the terminal.
We capture the information  to be added within the formatHexProgram and forward variables which are copied into the writeROM sketch.  The sketch loads the information at $FFE0-$FFF7 in ROM so that subroutines can show details on screen or LCD.