How is the 320x200x16 graphics mode mapped in the IBM PCjr?

Question

I've read that the PCjr supports a 320x200 pixel mode with 16 colors. But what I can't seem to find is how much memory that takes, how it's organized (chars?) or where it's located in memory.

I'm sure my searching skills are just rusty but I can't seem to find real info on it.

Can this mode be addressed in a similar method as Mode 13h? (linear)

General information on how to program this mode would be appreciated.

Thanks

Answer 1

The PCjr and Tandy 1000 both support graphics modes which take each byte of display memory which display each byte as two pixels, the first of which takes its color from the four upper bits and the second of which takes its color from the four lower bits.

The 160x200 16-color mode maps memory like the CGA's graphic modes. The 320x200 mode divides the screen into 4-pixel-high stripes, somewhat like the 2-pixel stripes of CGA graphics modes, where the bottom three stripes of each group each use memory 8192 bytes past the previous one. This mode requires reserving a 32768-byte region of memory for use by the display controller (normally the system only reserves 16384 bytes). Once that memory is reserved, the Tandy 1000 will allow all of it to be accessed at either its actual location or 0xB800:0; the PCjr, however, will only allow the first 16384 bytes to be accessed at 0xB800:0, so some programs for the Tandy 1000 that are written to use the 0xB800:0 mapping will leave two scan lines out of each quartet blank if run on a PCjr (IIRC, Tandy's Christmas demo would do that).