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Plan for video generation

A project log for Kobold - retro TTL computer

A 16 bit computer with 20 address bits and video display, from just a few TTL and memory chips. Instructions resemble 68000 / PDP-11.

roelhroelh 04/26/2019 at 19:050 Comments

Trying to have the hardware simple, and keeping in mind that 'large' SRAMs are easy to obtain, the choice was made to have just a single basic video mode.

That is, 640 x 480 pixels, with 256 colors per pixel. The pixel time for VGA is 40nSec.

The hardware needs assistance from software in order to operate correctly.

The pixel counter will increment every 80nS, so the video RAM will deliver two new pixel values every 80nS. The 8 bit color values go to the 'first pixel' and 'second pixel' registers. During the next 80nS, each of the outputs of these registers will be enabled for 40 nS.

When the pixel counter has reached a certain value, an interrupt will be given to the CPU. In the interrupt, the CPU will:

The sequence of actions might be a little bit different than listed here. 

The actions might be done by microcode, and the 16 bit processor can deliver 16 bit at a time, so this will be fast enough for most operations. If there are applications (games) that need higher speed, resolution could be dropped to 320 x 240 (TBD to be determined). [ edit: Sprites were added to the video system, see here]

Since the starting point of a line is under software control, it will be easy to do fast vertical scrolling. A clear screen will also go fast, because only a single cleared line has to be present, and all other lines can point to the same cleared line.

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