Posted By
JamesD
on 2016-09-20
02:29:09
|  Re: What would I do differently in the Plus/4
I haven't looked at the 3+1 memory map. But, if the 3+1 software copied itself from ROM to RAM, I would think it could have altered the memory map to 64K RAM or could access the kernel when I/O as needed. 99 lines of text is about 4 pages and is less than 5K.
A RAM expansion shouldn't be that difficult, but you need an input to disable internal memory mapping to the build in RAM and ROM. Basically, you intercept the chip selects. or something similar.
Then the expansion can put RAM anywhere it wants. It's going to require some logic to deal with that.
The actual paging circuit for external RAM could be on the expansion itself. Dealing with refresh for DRAMs... I don't know enough about how that works to say how to do it. Now you'd use a big SRAM.
Back then it would be 2 64Kx4 DRAMs. That's not very big board wise so it begs for even larger expansions.
I think 128K became the new 64K shortly after Apple shipped the IIe. You didn't have to ship with it, just support it. The C64 got around it by mostly being a video game with a keyboard. Kids got a videogame and parents thought it would help with their homework, so better than a videogame!
*edit*
If the 3+1 software is moving from ROM to RAM, it could be stored in ROM in compressed form. Then maybe the programs could have fit in ROM without being stripped down.
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