Basic Blitz: A Surprisingly Addictive VIC-20 Remake

The game Blitz was written and self-published by Simon Taylor for the unexpanded VIC-20 in 1981, then later sold to Mastertronic.

https://www.eurogamer.net/lost-and-found-blitz

I always thought it looked like a game that must have been written in Basic, but I never got around to testing that until the beginning of 2025.

So, here's my version in all its glorious 64 lines of code!

Mine seems to be based on the later Mastertronics' version, because my plane is just one graphic character instead of 2 or 3 and my buildings are multicoloured instead of just black. Multi-coloured buildings adds to the fun, given most actual buildings are grey.

Also, mine doesn't speed up during each flight; it does get faster per level while the number of buildings it generates also increases by 2. My current high score is 533. Game control is pretty simple: you just press 'v' to drop a bomb as the ship flies across the screen. Only one bomb can be dropped at a time.

Design

Enough of the gameplay, let's discuss the software design. The outline of the game is pretty simple:

• Line 5 reserves memory for the graphics characters then calls a subroutine at line 9000 to generate them.
• Line 7 defines a function to simplify random number generation.
• Line 8 is a bit of debug, see later.
• Line 9 resets the high score. So, this only happens once.
• Line 10 starts a game with a width of 5 (so 5x2=10 buildings are generated) and a delay of 100 between each frame.
• Lines 30 to 60 are the main loop of the game. It really is that tiny. The loop terminates when the  plane lands or hits a building. Within that the plane is drawn (by displaying it in its next position then erasing the previous position to avoid flicker).
• Bomb handling is done in lines 45 to 50, but the explosion is handled in lines 200-300.
• End of game is handled in lines 66 to 80 including displaying "Landed" or "Crashed", updating the high score and handling the user wanting to quit.
• Line 99 resets the graphics characters back to the normal character set so that you can carry on editing it.
• The subroutine at line 100 performs the equivalent of a PRINT AT.
• The subroutine at lines 200 to 250 handle a bomb hitting a building (a random number of floors are destroyed).
• The subroutine at lines 8000 to 8070 generate a new level based on W, the width of the cityscape.
• The subroutine at lines 9000 to 9010 generates the graphics characters and sets the sound level to 5.
• The data from lines 9012 to 9090 are the graphics characters themselves, in the sequence: 'blank', 'solid square', 3x building types, 2x roofs, plane, grass.
• The subroutine from lines 9500 to 9520 wait for a key to be released, then pressed, returning the key in A$.

Graphics

Because VIC-20 graphics are weird, programmers end up with bespoke graphics routines, so it's always worth discussing them. Firstly, VIC-20 graphics are tile-based, somewhat like the Nintendo Entertainment System. Video memory contains character codes between 0 and 255, and each character code points to an 8x8 bit pattern at CharacterMemoryBaseAddress+(CharCode*8). Usefully, the base address for the character bit patterns (and the video base address too) can be set by poking 36869. That base address can be set to RAM (which gives the programmer 256 tiles to play with), ROM (which is the default and provides caps+graphics or a caps+lowercase+some graphics option) or can be made to straddle both (which gives the programmer up to 128 tiles to play with + an upper case character set). This is the case even though the user defined graphics (UDGs) have addresses below 8192 while the ROM tiles are above 32768, because of the way the 14-bit VIC-chip's address space is mapped to the VIC-20's, full 16-bit address space.

In practical, unexpanded VIC-20 applications, programmers will want to use as few UDGs as possible to maximise program space while retaining much of the conventional character patterns. In Basic Blitz we therefore set the graphics to straddle mode (value 0xf, giving a CharacterMemoryBaseAddress of 0x1c00) which means characters 0..127 are in RAM and 128..255 are in ROM.

Intuitively, you might imagine that you'd want to start using tile 0 first, but that would waste of most of the tile space, so in fact we always count the UDGs we need backwards from tile 63, because tiles 64 to 127 overlap with video memory itself by default (and are therefore unusable!). Also, because the VIC-20 ROM characters aren't in ASCII order, and amazingly enough don't include the filled-in graphics character I have to provide that. When Basic Blitz is run, it first shows the entire usable character set.

I added this as a bit of debug, because I initially wasn't sure the ROM characters would print out OK. Also, I then made it print Hello in red to test both my PRINT AT subroutine and embedded colour control codes.

Graphics characters can easily be printed, because they're the normal characters '6, '7', 8', 9', ':', ';', '<', '=', '>', '?'. Normal text can be displayed, but you have to force 'inverse' characters which is achieved by preceding each print statement with <ctrl>+9 and ending with a true character <ctrl>+0.

Colours

Colours on a VIC-20 are strangely limited. There's a block of colour attribute memory, one location for each video byte, but each one is only 4 bits, which means you can only select an INK colour for on pixels. The PAPER colour is global, defined by bits 4..7 of 36879. The VIC-20 partially gets around this by normally making characters 128 to 255 inverse characters, but also by defining bit 3 of 36879 as normal or inverse mode.

The upshot though is that with the ROM character sets you can choose a common PAPER colour with any INK, or the common PAPER colour as INK, with any INK colour as PAPER. But when you select the character set to straddle RAM and ROM, you can only choose any INK colour + the common PAPER colour.

Hence in Basic Blitz, the background is white (as that seems most useful) and I have to define a UDG just so that I can get a filled in green character for grass with a building on top.

Sound

BASIC Blitz, sound is pretty simple. The initialisation routine switches audio on to level 5 (POKE 36878, 5); and leaves it there. There are 3 voice channels, which are individually switched on if bit 7 is set. In practical terms, each voice has a range of about 2 octaves, the first one having values from 128 to 65; then the next octave from 64 to 33. Beyond 32, the frequency ratio between each note is 1.03 to 1.06, close to that of a semitone 1.059 making most note intervals unusably out of tune.

The plane makes a drone sound using the lowest pitch audio channel (address 36874) OR'd with the bottom 4 bits of the jiffy clock at PEEK(162).

The bomb uses the high octave channel (at 36876) just generating an ascending tone. If the bomb hits a building it's silenced and the noise channel with a fixed low pitch of 129. The important thing, finally is to turn off all the sounds when they're done, by poking the channels with 0.

Playing The Game

You can run this VIC-20 Javascript emulator and type in the code (if the keyboard mapping allows it):

https://nippur72.github.io/vic20-emu/

I've found this emulator is better than the Dawson one for .prg files. Here's how to load the .prg on a desktop/laptop. First download the BasicBlitz.prg from my Google Drive. Then drag the file from wherever you downloaded it from to the emulator in the browser. It will automatically load and run!

However, it's also useful to be able to type in code directly for editing, debugging and other stuff.

The keyboard on my MacBook M4 doesn't map correctly to VIC-20 keys, because the emulator does a straight translation from character codes to VIC-20 keys rather than from key codes. This means that pressing Shift+':' gives you ';' on this emulator rather than '[' as marked on a VIC-20 keyboard.

Mostly this makes typing easier, but the VIC-20 uses a number of embedded attribute key combinations. Basic Blitz doesn't use many, here's how to type what it does use, it isn't easy!

In Chrome, you need enable console mode, by typing function key F12. Then tap on the Console tab. In Safari, you need to choose Safari:Settings... then Select the 'Advanced tab'; and click on "Show features for web developers" at the bottom. Then the "Develop" menu appears on the menu bar and you can then choose Develop:Show JavaScript Console.

So far so good. Now, you can type most of the text as normal, but whenever you need to type a special code, type pasteChar(theCode) in the console followed by Enter (e.g. pasteChar(147) for the clear screen code). Here are the codes you'll need:

• Inverse 'R' => 18. This is for Reverse text, which ends with inverse nearly underline => 146.
• Inverse '£' (Red) => 28.
• Inverse '┓' (Black) => 144.
• Inverse 'S' => 19 (this is the home code).
• Inverse heart => 147 (this is the clear screen code).
• Inverse up-arrow => 30 (this is green).
• Inverse 'Q' and inverse '|'can be typed directly just using the down cursor and left cursor respectively.
• The codes in line 8045 are more colour codes used for the buildings. They are 144 (Black), 28 (Red), 159 (Inverse filled diagonal=cyan), 156 (checkered-black character=purple), 30 (inverse up arrow = green), 31 (inverse left arrow=blue), 158 (inverse 'π' = yellow).

Conclusion

The original VIC-20 Blitz program, though derivative in its own way, is so simple it could have been written in BASIC, as this version proves. The arcane design of the VIC-20 hardware and its lousy BASIC implementation means there's a lot of subtle complexity even in a simple game. Finally, although there are many emulators for the VIC-20, both the Javascript implementations I know of have limitations and bugs which make distributing this game and/or modifying it non-trivial.