SE/30 VRAM Capacity W/ 24 Bit Addressing?

[trag]

Under 24 bit addressing there is 1M of address space assigned to each Slot. Does that mean that a video card would be limited to 1 MB of VRAM when using 24 bit addressing? Or is there some trick one can do such as telling the Mac to treat each address as a pixel (32 bits per address). One would still be limited to 1M pixels, but more color depth would be available. Or is there some other workaround available?

It seems like this would be a strong limit on monitor sizes connected to the SE/30 although, I guess Mode32 could just be a requirement for use with a larger capacity video card. Does Mode32 affect Slot address assignment or just DRAM capacity?

 

[yuhong]

Luckily, Apple derived a solution to this when it created 32-bit QuickDraw, and it is quite simple. Every time 32-bit QuickDraw access VRAM in the slot space, it uses a trap (SwapMMUMode) to temporarily switch the MMU into 32-bit mode, and when they are done, they use the same trap to switch back into 24-bit mode. There is also a trap (StripAddress) to strip out the flags from addresses that come from 24-bit mode so they are usable in 32-bit mode. This generally works, but on the original Mac II ROM, there was a bug that prevented more than 1 MB of slot space from being allocated per card. Apple released revised ROMs in November 1987 to fix this.

 

[trag]

Luckily, Apple derived a solution to this when it created 32-bit QuickDraw, and it is quite simple. Every time 32-bit QuickDraw access VRAM in the slot space, it uses a trap (SwapMMUMode) to temporarily switch the MMU into 32-bit mode, and when they are done, they use the same trap to switch back into 24-bit mode.

Thank you!! I had given up on ever getting an answer.

Do you remember how you learned about this? Volume of Inside Macintosh? Some other reference?

 

[yuhong]

It is often mentioned in articles that describe how to directly access the VRAM (indeed, such articles are the first hits for "swapmmumode quickdraw" in Google), because in that case the apps have to do the SwapMMUMode themselves, unlike when they use QuickDraw routines like CopyBits where QuickDraw does the necessary calls.

 

[trag]

Thank you again. I appreciate the help. If you would like to share your background, I would enjoy reading about it. It sounds as if you may have interesting experience in working on Macintosh hardware/software.