What is a good website that explains how 8-bit video games work?

Question

I'm doing a research project, and I can't seem to find one. Uggh.

Answer 1

I did a bit of 8-bit game programming in the early 1980s on the UK BBC Micro, for my own amusement, creating a pacman type game. It was a fairly typical machine, from the same era as the commodore 64.

The processor was very slow (2MHz) and RAM was very limited for program and screen (32K total). People had to use loads of tricks and all the special hardware features of the machine.

The people at the Atari retro-gaming site (2nd link) should be able to tell you where to look for info on 8 bit game programming. Use the contact link at the bottom of the page for their email addresses.

They may be able to tell you how to contact some of the old atari game programmers like Doug Neubauer who wrote star raiders.

Here's a rough list of the concepts to investigate in detail, via websites, people and books :-

Ask them what a 'sprite routine' is.

Actually, if you google for 'sprite routine' it should turn up lots of 8-bit game programming sites and related terms. Its the key routine for drawing and moving little characters like space invaders or pacman on screen whilst using the minimum of processing time. It was essential to have a fast one for the game to run at all.

When using a TV as a monitor at 50 frames-per-second (UK), there was only 20mS to move all the sprites. If you had 20 sprites, as in space invaders, you just moved a few each frame, such as one column of invaders.

An interrupt routine each 20mS frame period, at the time of the TV Vertical Sync, made game-play decisions. The TV picture is drawn by a graphics chip reading the pixels in screen RAM, looking up colors in a palette and using the TV electron beam to draw 625 horizontal lines of RBG dots. There is a Horizontal Sync pulse every line. It took 16mS out of the 20ms frame to draw the picture

A lot of code was run during the 4mS electron beam flyback period, when the TV electron beam is going back from the bottom of the screen to the top. This stops flicker due to drawing things during the 16mS (out of the 20mS frame period) that the electron beam is illuminating the phosper dots on screen, during which time objects may be in the process of being re-drawn in screen RAM. You don't want to draw half an old object and half a new one.

Some machines had graphics-hardware double-buffering of screen memory. You can draw screen 1 while screen 2 is displayed, then switch screens.

Collisions between objects could be done in software, but some machine had hardware sprite-drawing graphics chips and hardware collision detection, triggering an interrupt service routine to make further game-play decisions.

Many games (such as defender) re-programmed the graphics chip every frame period in the frame sync interrupt service routine to scroll the screen vertically or horizontally, so that the whole picture, 10K Bytes or so of memory, could be moved without using CPU time to copy blocks of memory. The frame sync interrupt service routine just had to draw new objects to fill in the blank column or row left at the top/bottom or left/right of the screen by the scrolling.

Also ask about routines to draw diagonal lines rapidly, for vector graphics games like 'star wars' on many machines and 'elite' on the UK BBC Microcomputer.

Ask them about synchronising sprite updates to the TV vertical sync and also 'electron beam dodging' (dividing the 20 millisecond time for a TV frame into about 5 x 4ms bits and moving sprites when the electron beam isn't on top of the sprite, to avoid the top half of a moving sprite being slightly out of line with the bottom half). It means you have 4 x 4mS periods (16ms) to draw sprites instead of just one 4mS flyback period. 'electron beam dodging' is my name for it, so other programmers may call it something else and you may not find it on google. I found it by disassembling the code of a game.

Also ask about CPU speeds, sound effects, music, joysticks, control keys and the tiny amount of ROM and program-RAM and screen-RAM memory, loading programs from audio tape, copy protection, etc.

Some games changed the colors dynamically in the 2, 4 or 16 color palette to give the appearance of more colors. The colors were changed at a certain time after the Vertical Sync by starting a hardware timer to cause a timer interrupt so many mS down the screen e.g. 10mS after V Sync for half-way down the screen. The game 'elite' on the BBC Micro even switched from 2 color B&W high-res for the spacecraft and space view, to 4 color low res for the control panel at the bottom of the screen.

You need books to fill in the detail in all of the above and show you example code.

You also might look for old programming books on eBay, by searching for the name of the machine + programming. There are loads, quite cheap.

e.g. atari programming or commodore 64 programming

To see some of the games, you can download modern re-creations of space invaders etc or download the MAME emulator that runs copies of the orignal ROM memory chips, so that the games look exactly like the original, including power-up screen memory tests, etc.

Send me an email for any more questions.

Answer 2

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