TI-83 Plus Paging System
------------------------
By Michael Vincent - michael@radicalsoft.org


** INTRODUCTION

The Z80 processor, using 16-bit addressing, only has a 64K address space. In order to access the 512K ROM and the 32K of RAM, swapping of pages is performed. Below is the  layout of the 83+s address space:

----------------------|
0000h 16K ROM page 0  |
      (always)        |
----------------------|3FFFh
4000h 16K RAM pages   |
      0-1 or ROM pages|
      0-31            |
-----------------------7FFFh
8000h 16K RAM pages   |
      0-1 or ROM pages|
      0-31            |
-----------------------BFFFh
C000h 16K RAM page 0  |
      (always)        |
-----------------------FFFFh

So 0000h-3FFFh is always ROM page 0, and C000h to FFFFh is always RAM page 0. For 4000h-7FFFh and 8000h-BFFFh, you can swap any RAM or ROM page in that you desire. Typically though, 8000h-BFFFh is RAM page 1, and a ROM page is swapped in 4000h-7FFFh.


** HOW TO SWAP PAGES IN 4000h-7FFFh AREA

Port 6 controls this area. You output the page you wish to swap in.
For the TI-83 Plus, a RAM page number has bit 6 set. That is what is shown in the examples below. However, for the Silver Edition, it is bit 7. So, that's 40h-41h for the 83+ and 80h-81h on the SE.

For ROM pages, bit 6 of the port is zero and the lower four bits contain the page number.
Example: (swap page $15 of flash into 4000h-7FFFh)

ld a,%00010101
out (6),a

For RAM pages, bit 6 of the port is nonzero, and the lower four bits contain the page number.
Example: (swap page 1 of RAM into 4000h-7FFFh)

ld a,%01000001
out (6),a


** HOW TO SWAP PAGES IN 8000h-BFFFh AREA

Port 7 controls this area. You output the page you wish to swap in.

For ROM pages, bit 6 of the port is zero, and the lower four bits contain the page number.
Example: (swap page $15 of flash into 4000h-7FFFh)

ld a,%00010101
out (7),a

For RAM pages, bit 6 of the port is nonzero, and the lower four bits contain the page number.
Example: (swap page 1 of RAM into 4000h-7FFFh)

ld a,%01000001
out (7),a

** REMINDERS

Remember that the TI-OS (and other ASM programs/applications) expect that 8000h-BFFFh contains RAM page 1. ROM calls such as vputs or puts won't work also (penrol and pencol are addresses in RAM) if you have another page there.

** SILVER EDITION NOTES

The 83+ SE contains a 128K RAM chip, having 8 pages of memory. With the 128K chip, RAM pages 0-7 exist. 0-1 are the same as the normal 83+. Note that on the SE, you can actually change the page swapped into the C000-FFFFh space! Just output the page number (in the range 0-7, no bit 7 set) to port 5.