TI-86 Port 4 Documentation
By Mr.Z (RANDY) 11/26/2003
Updated by Mr.Z 09/03/2004

Port 4 on the TI-86 is poorly documented, so I shall present what I have found.
It is similar to the TI-85 port 4, & is as such write only.

7 ?
6 1=ROM enabled
5 ?
4\ 0 80 columns 0 96 columns 1 128 columns 1 160 columns
3/ 0            1            0             1
2\ 0 4x interrupt 0 2x interrupt 1 4/3x interrupt 1 normal interrupt
1/ 0    speed     1    speed     0     speed      1      speed
0 ?

I think bit 0 is RAM disable, but I have yet to find a way to test my theory. I do
know that setting it from a program in RAM will freeze the calculator.

I have not verified that setting bit 2 & clearing bit 1 produces exactly 4/3x
interrupt timing, but the amount of duplicated graphics makes that seem likely.

Bits 3 & 4 select the number of pixels per line. The 96-column mode is apparently
defective (the last 16 pixels of each row are right-aligned, while the rest of the
row is left-aligned). I presume that the 160 column mode may be related to the
TI-89, if it in fact uses the same screen driver.

Bit 7 may be unused, but since bit 5 is between 2 used bits, it must serve some
purpose not immediately obvious (i.e., my calculator did not crash or start acting
strangely with it set or cleared).

Bit 6 seems quite pointless, as the ROM would obviously never clear it, & programs
in RAM can just ignore the ROM if they do not need to use it.