Perverse Code: Deviant Forth

The Forth programming language has a set of functions (or words) called primitives. These are traditionally written in the language of the host machine to keep the system as simple and efficient as possible. Typically the primitives required no more than about five machine instructions each to implement.

Deviant Forth explores the possibility of using Forth to implement some of the primitives. Warning, prolonged exposure to this code may cause permanent damage to your eyes!

As a exercise, let's implement NIP in Forth:

: NIP SWAP DROP ;

Standard stuff so far. Now lets try to implement SWAP and DROP!

: DROP DUP - + ;

: SWAP OVER >R >R DROP R> R> ;

Things are beginning to look ugly. On most systems DROP can be implemented in one machine instruction and SWAP in four. It's also possible to implement DUP, + and OVER in Forth:

: DUP >R R@ R> ;

: + >R DUP DUP - R> - - ;

: OVER >R DUP DUP R@ - DUP >R - R> R> + ;

I'm no Forth purist, but I feel repelled by the abomination I've programmed. Despite this, it's interesting to discover so many Forth words can be implemented with just >R, R>, R@ and -, however unnatural it might be.

As an afront to the very nature of Forth I challenge you to implement ROT using only >R, R>, R@ and -. If you can in less than 50 words I'd love to see your solution, however unpleasant :-)

RobotWar and the Army of Clones

Back in 1981 Silas Warner created a game for MUSE Software which would go on to spawn an entire army of clones. Silas is probably better know as the author of the legendary Castle Wolfenstein. However it was RobotWar which went on to inspire a whole new genre.

Silas developed RobotWar for the PLATO computer system and later ported it to the Apple II for release by MUSE. The game is set at a time in the distant future when war has been declared hazardous to human health. Wars still rage, but the combatants are robots programmed to battle to the death.

RobotWar is played by writing the control program for one of these battle robots. The program controls the movement, radar and gun of the robot. Here's one of the example robots:

;     ROBOT  'SCANNER'
; SEND RADAR PULSE AND FIRE IF AN
; ENEMY IS SPOTTED

SCAN
AIM + 7 TO AIM

CHECK
AIM TO RADAR
IF RADAR > 0 GOTO SCAN
0 - RADAR TO SHOT
GOTO CHECK

Scanner is a simple stationary warrior. The first instruction (at scan) turns the gun 7° clockwise. The next instruction (at check) lines up the radar with the gun. The radar returns a positive number if it detects nothing, otherwise it returns minus the distance to the opponent.

The next line jumps back to scan if the radar returns positive. Otherwise a shell is fired by the penultimate instruction and the next line jumps back to check.

RobotWar earned a strong following in the early 1980's. Computer Gaming World organised annual tournaments and a group of enthusiasts formed the Postal RobotWar Club of America.

RobotWar has inspired a huge number of clones, I'll just mention a select few here:

• CROBOTS by Tom Poindexter (1985) uses a subset of C
• P-Robots by David Malmberg (1988) uses a Pascal subset
• AT-Robots by Ed T Toten (1992) uses assembly language
• Robocode by Mathew Nelson (2001) uses Java

Robocode is currently the most active. More information is available on the RoboWiki.

Please let me know if you think I've missed an important robot programming game or one of historical note. Also I'd love to hear from you if you have any memories of the above.

Five Computers I Longed to Possess in the 1980s

Back in the 1980's I owned a couple of computers, a Commodore 16 and a Spectrum +2. However, this didn't stop me wanting to own a whole range of other machines.

1. Jupiter Ace: I loved this from the moment I read about it. A Z80 based micro with a built in Forth interpreter, what more could I desire? Only about 9000 machines were manufactured and I've been waiting patiently ever since for one to come up on eBay.
2. Sam Coupé: a Sinclair Spectrum clone with 256K memory, a 6MHz Z80, optional internal disk drives and a 25MHz Z80 processor upgrade. Somewhere in the region of 12000 units were sold. A few years ago I picked up one of these at a car boot sale for a fiver! ;-)
3. NeXT Computer: from the moment I saw screenshots and read about this workstation built around Motorola's 68030 I knew I had to have one. After seeing the hefty price tag I knew I never would. Even now these sell for a small fortune on eBay.
4. Dragon 64: the Dragon's 6809 processor looked amazing compared to the 6502 used in most computers of the time. It boasted two stacks, plenty of registers and on chip multiplication. Whenever I wrote code for the 6502 I'd secretly hope to write for the 6809 one day.
5. KITT: a sentient, artificially intelligent computer built by Knight Industries and installed into a Pontiac Firebird Trans Am. Unfortunately my diecast model came without William Daniels' voice and the cool red lights. If KITT is ever on eBay I'll likely sell my house to buy it!

Let me know if you think I've missed off any cool machines :-)