Ti-83 Emulator question with the ROM

Question

I have been building knowledge of computers and C++ for quite a while now, and I've decided I want to try making an emulator to get an even better understanding. I want to try making a TI-83 Emulator (runs on a Zilog Z80 CPU). I currently have two problems:

The first is that the "PC" register that points to the current instruction is only 16 bits, but the Ti-83 ROM I downloaded is 256Kb. How is 16 bits of data supposed to point to an address beyond ~64Kb?

Secondly, where is the entry point on the ROM? Does the execution just begin at 0x0000?

Thanks, and hopefully you can help me understand a bit on how this works.

Answer 1

There's is most likely a programmable paging register outboard of the processor core that can be set to map a portion of the 256K at a time into part of the 64K address space. You will need to emulate that. Hopefully you can find out about this in official or unofficial documentation. If you have a schematic or PCB it might even be visible as an external PAL or collection of logic chips.

I forget off the top of my head where a z80 starts executing on reset, but I'm sure you will find it in the processor manual, which would be a necessary tool to write an emulator for it.

You'll want to make sure the core used is truly a z80 and not some kind of custom extended version thereof.

And of course I'm sure someone has already done this, so your project is likely to be more about learning - though in the end you might surpass any available solution if you work on it long enough.

Answer 2

The Developer Guide describes how memory is arranged, although it doesn't actually describe how the mapping works.

Short version: the address space is divided into four 16K pages, the first of which always maps page 0 of the 32-page flash ROM.