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TonyDuell · 10th Jan 2018, 7:22 pm

0xFF is on of the RST (Restart) instructions, basically a 1-byte subroutine call to a subroutine starting at 0x38.

Forcing that on the bus is a useful test in that the processor will keep on pushing 0x39, 0x00 (the return address, low byte first) onto the stack. It will do this throughout memory, so if the video system is memory-mapped (which I would think it is), it will fill it with a repeated 2 byte pattern. 0x39 probably displays as '9' (I am assuming ASCII), 0x00 could be anything. This was actually a suggested test in the technical manual of the TRS-80 Model 1 -- pull the ROMs and see if you got the expected pattern on the display.

And of course after the first 0xFF, all subsequent ones are fetched from address 0x0038 (the start of the 'subroutine'). So provided that address is in the ROM area, it doesn't matter what other devices are on the bus (provided the address decoder is working properly) as they will never be addressed to provide an instruction.

10th Jan 2018, 7:22 pm	#52
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,215	Re: 8085 processor questions 0xFF is on of the RST (Restart) instructions, basically a 1-byte subroutine call to a subroutine starting at 0x38. Forcing that on the bus is a useful test in that the processor will keep on pushing 0x39, 0x00 (the return address, low byte first) onto the stack. It will do this throughout memory, so if the video system is memory-mapped (which I would think it is), it will fill it with a repeated 2 byte pattern. 0x39 probably displays as '9' (I am assuming ASCII), 0x00 could be anything. This was actually a suggested test in the technical manual of the TRS-80 Model 1 -- pull the ROMs and see if you got the expected pattern on the display. And of course after the first 0xFF, all subsequent ones are fetched from address 0x0038 (the start of the 'subroutine'). So provided that address is in the ROM area, it doesn't matter what other devices are on the bus (provided the address decoder is working properly) as they will never be addressed to provide an instruction.