Adding video - decisions

A project log for Big Daddy 68k

Making a games console from the ground-up

SimonHSimonH 07/16/2017 at 10:170 Comments

It's time to add video-out to the circuit...but how! There are a lot of different options but - considering I've never done anything like this before - the simpler the better.

There are three main choices:

  1. Have a framebuffer that the CPU can write into, which gets scanned out to the display by another piece of hardware. This is a bitmap, held in memory where each location's value represents the colour or luminosity for a particular pixel on the display.
    • Pros: complex images can be drawn with ease as the CPU can build up a picture independently of the display hardware. The CPU can take as long as it likes.
    • Cons: requires memory. If it's from the main pool of memory then a way needs to be made to arbitrate CPU/scan-out access. If it's an extra pool of memory, how does the CPU write into it?
  2. Have a one-line virtual framebuffer, like the Atari 2600. This design does not require any memory (or no more than one pixel's memory) and the CPU directly writes each pixel's value from registers straight to the TV.
    • Pros: requires no memory, simple circuit.
    • Cons: requires all the CPU time just to make the picture. Free time only available in the hblank or vblank. Hard to make complex images due to the limited time per pixel.
  3. Have an abstract command-based image generator. For example, you can buy LCD panels which have a simple GPU included. You write the "draw pixel at" or "draw circle at" commands to this GPU and it gets on and does it. This GPU manages the connection to the LCD itself.
    • Pros: requires no memory (in our circuit), simple circuit, complex images possible, high colour and high resolution possible. No need to worry about the actual video interface.
    • Cons: low performance...unless you just want to draw loads of lines and circles on the screen. Less fun - nothing to learn!

So I have decided to go with #1. Having a framebuffer. Lots of stuff to learn, the most flexibility and the most CPU time free. We are trying to make a simple games console so getting useful graphics out onto a TV is a must!

Next time: more decisions.