SGC ROM position RFC

[Dave]

Hi folks.

Just want some feedback. I'm putting this in the Software section because it's about loading software in the memory map. The hardware behind it is secondary.

On a running Super Gold Card, there is an unused area from the top of the vector table ending at $3FF up to the bottom of the IO area at $18000. This is 96K that appears unused in the SGC memory map.

If I map SRAM into $00000 to hold the vector table, I could map SRAM up to the start of the internal IO area. Then, if users wish, they could copy ROM images into there arbitrarily and call them. This could free up as much as 95K in main memory. The internal IO notch is really very small. There is another 16K above that to the base of video RAM at $20000 that could be SRAM too.

I see people may like to have a boot script that copies ROMs into that extra space, calls them to link them in, then continues on.

Is that useful? Does it break anything? Would you use it? If you don't care, does it make any difference?

Feedback!

[swensont]

With the ZX81 and the T/S 2068, understanding the memory map is important, as there is little memory and you need to know how to use it best. On the QL, I've never really had to look at the memory map. When I had a BBQL, I just had to worry about the amount of memory, but not where things were stored. With emulators, and a ton of memory, I never have to worry about memory at all.

Tim

[Dave]

I feel this. I do.

If this idea would cost one penny I probably wouldn’t have mentioned it. It is cheaper. If it is unused it makes no difference.

Do you think it could break anything?

[janbredenbeek]

The area between $10000 and $17FFF is actually used to store half of the (S)GC ROM, which is copied to RAM on startup. The rest of the (S)GC ROM is copied to $1C200-$1CFFF (mainly for patches to the QDOS or Minerva ROM) or used to boot the system.

You can actually run any supported version of QDOS/Minerva by loading the images of the QDOS/Minerva + (S)GC 2.49 into RAM above $50000, doing a few POKEs and CALLing the (S)GC ROM, as described here. Beware though that the first two vectors at $0 and $4 are immutable, even with RAM switched on, so if your custom ROM has a different boot address you will get into trouble with the various boot utilities.