Each pixel is 4-bit long ( http://hackspire.unsads.com/wiki/index.php/Memory-mapped_I/O_ports#A4000000_-_Internal_SRAM ).

Cool, thanks for the info. I wasn't sure. I knew the screen had a 4 bit grayscale depth but I wasn't sure if each pixel or the screen had any other property or something else causing the screen memory to take more space. On some Casio monochrome calculators, picture data takes between 2 and 4 bit per pixel.

It actually is possible to change the data size with the Nspire's driver, though there are still 15 colors total available (in the 16-bit color values, only 4 bits are used). You can select 1, 2, 4, 8, or 16 bits per pixel in the buffer. If the bits per pixel is less than 16, the pixel values are used to look up the 16-bit color values from an array contained within the driver. If the bits per pixel is 16, then the pixel values are used as color values (no array involved). Thus, if you want to reduce the buffer size, you can use 1-bit or 2-bit mode (which gives you 2 or 4 colors available)

Though it would be unlikely you would need to do that, remember, the nspire has 32mb of RAM, for TI's bloated OS

Nope, it's 32mb ram. I just checked hackspire.

Though it would be unlikely you would need to do that, remember, the nspire has 32mb of RAM, for TI's bloated OS

Yes, but smaller buffers can increase performance, even if only because the CPU only has 8KB of data cache. Not to mention, there is not as much data to process.