philpem
commented
4 years ago This is probably a very simple change to memory.c (specifically the checkMemoryAccess() function)
- Is the memory address within the video RAM region?
- If so - and the 'VIDPAL' config option is enabled - return MEM_ALLOWED.
The VIDPAL was an upgrade to the base 3B1 which sat between the 68010 CPU and the motherboard.
When the VIDPAL detected an access to video memory, it forced the CPU status bits to indicate a supervisor access. Effectively this bypasses memory protection for VRAM, allowing higher video RAM refresh rates.
This was used to great effect with a 3B1 GUI windowing system implementation.
While VIDPAL can be emulated with a software driver, emulating the hardware would provide a clear performance benefit to user code.