eads@hpldola.HP.COM (Rick Eads) (06/06/91)
Last weekend I was in the Black Hills of South Dakota visiting my sister-in-law and was asked to look at their vintage Apple ][ plus. The system has 48K of RAM and she would like to be able to run some of the 64K software that is still available. I told her she needed to buy a 16K memory board and she would be all set. Now, just for the hell of it, I opened up the PC and low-and-behold there was a Microsoft 16K memory board plugged into one of the slots. "Hmmmmm," I say to myself, "This is strange." Thinking back to the inner-workings of the 6502, it dawned on me that this processor can only directly access 64K total but I've forgotten how the I/O space is mapped. Thus I am not sure if the architecture of this PC is even capable of accessing 64K of RAM without a BIOS modification. My question is: How can I get this Apple ][ plus to use all 64K of RAM in the system, 48K on the motherboard and 16K on the add in card? The intuitive thing is for the system to just work with no additional work required; however, the software designed for the 64K systems do not work. Also, since the entire memory space is divided across the 64K address range of the 6502, it would seem that the PC would have to know to page the RAM in and out of the base 48K RAM space to avoid conflicts with I/O and ROM address space.