Run a trace (optionally with a jumper) from the VDP /IRQ pin over to J5 so that the the Frame Flag IRQ status can be checked without using IRQs and without reading register 7.
Any time reading register 7 is read as the frame flag is being asserted, a race condition can cause the flag to be incorrectly reported as 0 and also become reset. Therefore, any code that polls register 7 waiting for the frame flag to become set will not work reliably.
Using the IRQ line on the retro from the VDP will not work properly when the Z80 is operating in interrupt mode 2 (which is optimal when IRQs are generated from either the CTC or SIO.)
By providing a way to poll the IRQ pin on the VDP without it also driving the Z80's IRQ input, an application may poll the the IRQ signal without reading register 7 to reliably synchronize itself with the VDP.
johnwinans
commented
3 months ago
Run a trace (optionally with a jumper) from the VDP /IRQ pin over to J5 so that the the Frame Flag IRQ status can be checked without using IRQs and without reading register 7.
Any time reading register 7 is read as the frame flag is being asserted, a race condition can cause the flag to be incorrectly reported as 0 and also become reset. Therefore, any code that polls register 7 waiting for the frame flag to become set will not work reliably.
Using the IRQ line on the retro from the VDP will not work properly when the Z80 is operating in interrupt mode 2 (which is optimal when IRQs are generated from either the CTC or SIO.)
By providing a way to poll the IRQ pin on the VDP without it also driving the Z80's IRQ input, an application may poll the the IRQ signal without reading register 7 to reliably synchronize itself with the VDP.