I'm posting this here so someone like Kerm (hint hint wink wink) can remember to put it on the wiki or something. Also I guess it might be useful to some of you. I'm like 99% sure this stuff works, but as always it might not, so use at your own risk. It originally came from https://github.com/unknownloner/wiki/blob/master/ti84cse/HalfRes.md.

Half-res mode on the CSE

Half res mode is an LCD mode which results in a halved horizontal resolution. This can also be used for double buffering, because one can write to the left side of the screen while displaying the right side or vice versa.

Entering half-res mode takes 4 steps:

1. Enable interlacing
   
2. Enable partial images 1 and 2
   
3. Position the source areas of both partial images on top of each other, and make them 160 pixels in size.
   
4. Set the output destinations of the partial images to be 160 pixels apart.

Enabling Interlacing
To enable interlacing, set bit 10 of LCD register 01h

Code:

To disable interlacing, set LCD register 01h back to 0000h

Code:

Enable partial images
Partial images are enabled by setting bits 12 and 13 of lcd register
07h for partial images 1 and 2 respectively.

Code:

To disable, reset bits 12 and 13

Code:

Set partial image positions
The partial image display positions are controlled by LCD registers
80h and 83h. Their starting lines are controlled by 81h and 84h, and
their ending lines are controlled by 82h and 85h. When using half-res mode, set the display position of image 1 to 0, and set the display position of image 2 to 160.

The start and end lines of both images should be identical. For example, to display the left half of the screen, set the start lines to 0 and the end lines to 159. To display the right half, set the start lines to 160 and the end lines to 319. In other words, to display a section of the screen starting at 'N', set the start lines to N and the end lines to N + 159.

This code will set up the display positions, and then set the start and end lines to display the left half of the screen.

Code:

Optimize time!

Code:

Hahaha very funny zeldaking .

Nice work Unknown, it is important that this information be stored somewhere accessible.

One note is that it might be a good idea to manipulate the bits while preserving the rest of each register. This would require reading the contents of each register and manipulating the bits before outputting again however.

Quick question: By simply changing the start and end positions of image 1 and image 2, does this swap the GRAM addresses? Or do I need to do that in a different manner?

EDIT: Kind of figured it out, seems like this code displays the right half:


Code:


And this code the left half:


Code:


Is there a better way? I am shooting for double buffering; how would I go about updating the LCD if I am using my left side as the buffer?

That's exactly how I've been doing my double buffering, to my knowledge there's no better way.

You might think that the LCD display offset would do it, but the LCD display offset doesn't do anything when you're using full screen partial images like half-res mode does.

You have to switch off between sides you're drawing to. Display the left side while drawing on the right, then swap and display the right while drawing on the left, and so on.

That's also what xLIBC does, it just hides the fact that it's adding an offset to all your draw commands based on which buffer you're currently drawing to

Ah, okay. So it is constantly switching rather than updating a single side. That makes things pretty interesting, but useful! Thanks!