I was thinking about the different memory expansion schemes used in the past on 6502 systems. I understand how the Commodore REU DMA expansion works, how GeoRam works, and how Apple II expansion up to 128K works - but I never learned how the larger cards worked on Apple IIe and IIc.

Apple’s user guides for the 1MB cards are hilarious with a technical Appendix B for assembly programming that actually tells you that you don’t need to know how it works!

I know they can be used as RAM disks, but the manual says “sophisticated programs” can directly take advantage of the expansion memory.

Does anyone know how these worked?

I don't know, but here's a guess based upon how the video memory in the TMS9918 works.

Memory map a latch into the 6502's address space to hold the address lines of the expanded memory. You also need to memory map the data lines of the expanded memory into the 6502's address space. The normal read and write cycles to this memory mapped location access data in the expanded memory. The TMS9918 can also auto increment the address latch allowing a sequence of bytes to be read or written.

From what I can find the card has a 3 byte address register which auto increments when a data register is accessed for either reading or writing. The addresses of the registers depends on the slot the card is plugged into but is in the $C000-$C033 area.

Some of code uses absolute indexed X addressing with a base address in the $BFF8-BFFB range so that any dummy reads generated by LDA/STA abs,X will not affect the registers.

I'm not sure this is exactly the same as the official Apple card.

https://github.com/a2retrosystems/mxp/tree/master/slinky/diags
https://github.com/a2retrosystems/mxp/tree/master/slinky/rom

This schematic is for a 256K card with the same kind of interface

http://john.ccac.rwth-aachen.de:8000/patrick/data/256k_RAM.pdf

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

This is great! Thank you

One method to make use of memory beyond what's available is to use overlays.

HyperC uses this method for the Apple II.

Cheers,
Andy