The original firmware reads SW2 and performs either 1 of 8 primitive tests, or 1 of 8 running modes.

The tests rely on output ports to indicate information, but that requires some LEDs to monitor the states which might be pulsing rapidly. I feel this is rather unsatisfactory.

In a previous job, I worked for a smart card company. Smart cards are small but highly critical single-chip microprocessor systems. Any bugs would be disastrous, so checking smart card security costs literally millions of pounds. I only know a hand-waving amount about they work, but the start-up process is useful.

I propose this start-up test sequence:

• Check registers work
• Check the ROM contents have a valid checksum
• Check the CRTC is present (write/read a register)
• Initialise the CRTC (so that RAM gets refreshed)
• Check the VIA is present (write/read a register)
• Set the video colours
• Check the stack RAM is working (so you can use it for subroutines)
• Check the SCC is present (write/read a register)
• Check the video RAM stores data written to it
• Send failure report to the screen or serial port
• If no failures, boot up as normal

It could write a digit code to the output port to indicate the last test that succeeded, or a binary pattern showing which ones had succeeded or failed.