Locomotive BASIC

From Wikipedia, the free encyclopedia

Locomotive Basic is a proprietary dialect of the BASIC programming language written by Locomotive Software used only on the Amstrad CPC (where it was built-in on ROM). It was the main ancestor of Mallard BASIC, the interpreter for CP/M supplied with the Amstrad PCW and later the Amstrad-designed and built ZX Spectrum +3.

Contents 1 Version history 2 Features 3 Contemporary rivals 4 Drawbacks

[edit] Version history

It was published in two versions: 1.0 which only came with the CPC model 464, and 1.1 which shipped with all other versions. A special update—or rather, a ROM extension—was available on the Amstrad CPC Plus series machines, which added specialised BASIC commands for taking advantage of the extra capabilities of those machines.

[edit] Features

It was a rather simple but powerful BASIC implementation by the standards of the day, featuring dedicated commands for handling graphics (such as DRAW, PLOT, INK, PAPER, and FILL in v1.1), even allowing the creation of multiple screens, windows, and the like, although the color system and palette handling was awkward. Interestingly enough, a table giving the numeric codes for the 27 system colors was printed over the built-in 3" disk drive casing on the 664 and later machines. Simple as it was, it did stand out however among other BASICs of the time by offering a timer-based software interrupt mechanism using the EVERY or AFTER commands; this offered a timed repeating or once-off call respectively to the BASIC line number of your choice.

Also, it granted an almost full control over the CPC sound chip, an AY-3-8912 with 3 melodic channels and 1 noise channel (which was also used on the ZX Spectrum, Atari ST and MSX computers, with none of them having such a complete built-in SOUND command). Everything, from selecting a particular channel or a combination of channels, setting envelopes, volume, pitch, noise, and so on could be done with a single SOUND command, with up to 7 parameters. The only thing that could not be done with BASIC was perhaps playing back digital sampled sounds, like in the game RoboCop.

Disk, tape, and file management were managed by BASIC itself, and were usually good enough for simple file management, with commands such as GET, PUT, ERASE, SAVE, MERGE, RUN, CAT, LOAD etc. In fact, during those years, the BASIC supplied as standard with every low-cost home computer also acted as a more or less simple operating system.

Also available were some special commands for memory allocation and handling, like MEMORY and a parametric LOAD command, allowing, for example, to load a file containing "raw" picture data into video memory, causing it to be displayed, with a couple of BASIC instructions. Adding the right memory address(es) as parameter to the commands LOAD or SAVE you could handle raw uncompressed 17 KB screen pictures at ease. CALLing another address you've got a forced system reset (call 0), the famous "Press Any Key" (call &bb18) or for eliminating flicker in animation by allowing you to synchronize with the monitor's raster scan via "sync frame-flyback" (call &bd19); this was given its own dedicated command in Basic 1.1 - FRAME. With PEEK and POKE, you've got a nice interface for assembly language.

[edit] Contemporary rivals

Locomotive BASIC compared to the Commodore 64's BASIC (Commodore BASIC), which had no dedicated commands for graphics or sound, allowed doing pretty much anything that was within the standard capabilities of the machine. This was not unimportant, as some other machines of the era using full graphics or sound was limited to assembler programmers. MSX, Spectrum and some others offered a similar, more or less complete command set for their sound and graphics capabilities. The only things going clearly beyond BASIC capabilities were the overscan modes used in games and demos, weird 27-color graphics modes, digital sound playback, and smooth scrolling.

Unlike Sinclair BASIC or Commodore 64 BASIC, which had various keyboard command shortcuts or specialized keys for choosing symbols or colors, Locomotive BASIC keywords were typed in full and the interpreter parsed, recognized and tokenised them. However, there were abbreviations like "?" for "PRINT" and a few shortcuts. Programs could be saved onto cassette tape or floppy disk and retrieved as binary or ASCII files.

[edit] Drawbacks

Drawbacks of Locomotive basic included the lack of structured programming, something it shared with almost all other common BASICs of its time except BBC BASIC on the BBC Micro.

Locomotive BASIC used and required line numbers and interpreted code ran at rather slow speed, especially when plotting graphics. This could be rectified by compiling the code, but compilers - though available - were a paid-for optional extra, not included as standard with the machine. However, there were several games (even commercial games) for the CPC written in Locomotive BASIC, as well as business applications or utilities written entirely in BASIC.