exxos wrote: ↑Sun Nov 05, 2017 12:42 pm
ah, was thinking more counting in the FPGA, you could output binary count to some spare IO pins.. Then check bits with scope.
That's a good idea, although now I probably won't bother since it's working for normal video (...I will confirm that also for 50Hz tonight).
exxos wrote: ↑Sun Nov 05, 2017 12:42 pm
Either something TOS is doing , or the shifter then. Though troed did mention something screwy about the registers if you tried to write to just 1 8 bit chunk instead of the 16bit chunk at once. Not sure if thats a possible issue with your setup or not..
Maybe you could try TOS1.00 and see what happens ? Maybe Atari were plotting extra video modes in TOS or the shifter during its production.. I guess we need someone to look at the TOS sources to see if anything is going on with other bits or not.
I found the disassembled TOS in the ST Internals book... but it's not OCRed so there's no easy way to search for specific addresses. Is there a better copy out there? I also found a copy of the TOS 2.06 source but it's probably quite different from 1.04, which is the version I have in the machine.
One idea, maybe it's comparing the value read back from the video mode register with the copy in RAM in order to check if the video mode has been changed, and then automatically resetting the machine. That would seem to explain most of that behaviour.