Sega Master System: Back to the Future Part III Black screen

[SN902]

VDP version: NtscMasterSystem1
ROM CRC32: 2D48C1D3
Region in cartridge header: International

[jsgroth]

This depends on PAL/50Hz timings, though even with those it currently fails to boot because of a timing bug. At boot it polls the VDP control register until it sees the frame interrupt flag change from 0 to 1, then it runs a counting loop that lasts for a few hundred lines, then it reads the VDP control register again and expects to see the frame interrupt flag clear. If it's set (which it will be with NTSC timings) then the game goes into a jr -2 infinite loop, which I guess was the developer's way of saying NTSC not supported?

The timing bug is that I trigger the line interrupt slightly later than I set the flag (was necessary to fix some timing tests), and if a game reads the control port between the flag getting set and the interrupt triggering, the flag will immediately get set again which is wrong. I'm going to make it so that the line interrupt is suppressed if a game reads the control port after the flag gets set but before the interrupt triggers. In actual hardware there's probably some Z80 interrupt line latching behavior that I'm not currently emulating.