Linking Calculators

The ti86's processor can address up to 65536 bytes of memory. Each byte has an address from $0000 to $ffff. But the ti86 has far more memory than 64Kb. This works as follows.

• The area of memory between $0000 and $3fff is always the same memory. It is ROM and contains the basic stuff that keeps the calculator alive.
• The area of memory between $4000 and $7fff is also normally ROM. It contains the code for most ROM calls. Port 5 enables a different memory 'page' to be loaded into this area.
• The area of memory between $8000 and $bfff is normally RAM. It contains various locations of memory that are used for many different purposes. Port 6 enables a different memory page to be loaded into this area.
• The area of memory from $c000 to $ffff is fixed. It is always RAM. It contains the memory locations called by the various user routines and the default video memory.

But exactly what are memory pages? Think of it like this. The area of memory between $8000 and $bfff is read from a specific page of memory in a book of memory. If you turn the page in the book the memory between $8000 and $bfff will contain different data, but it can still be read and written to. If the page is then set back to the original page then you can read the initial data again. Writing a value onto one memory page will not affect any other page. To read and write from a page you must 'turn' to that page.

Port 5 is used to control which page is loaded between $4000 and $7fff. Port 6 is used to control which page is loaded between $8000 and $bfff. These ports can be read from and written to, so you can find the current memory page. The different pages are set (or read) by the values written (or read from) these two ports. The values are as follows.

• Values between $00 and $0f load ROM pages into the area. Pages $0e and $0f do not seem to have anything on them.
• Values between $40 and $47 load RAM pages into the area. All pages can contain information, but page $41 has little on it - the areas from $9000 to $afff (or $5000 to $6fff) are safe to use, you can use the entire page if you aren't running the program from a shell or using floating point maths.

Interestingly, if you write, for example, $41 to both ports 5 and 6 and then set the value of, say, $8000 the value of $4000 will also change. Neat, eh?

If you'd like information on Port 4 you could mail me, but the only use it has is if you're trying to change the screen resolution on the ti86 (eg to run ti83(+) programs which use a smaller screen size). Alternatively get hold of a copy of Dan Eble's guide to ti85 ports, Port 4 is the same on both calculators except that the first three bits must be %010????? on the ti86 to stop the calculator freezing.