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Concept
Locomotive BASIC
Summary
Locomotive Basic is a proprietary dialect of the BASIC programming language written by Locomotive Software on the Amstrad CPC (where it was built-in on ROM) and the later Locomotive BASIC-2 as a GEM application on the Amstrad PC1512 and 1640. It was the main descendant of Mallard BASIC, the interpreter for CP/M supplied with the Amstrad PCW.
There are two versions of Locomotive BASIC: 1.0 which only came with the CPC model 464, and 1.1 which shipped with all other versions. BASIC 1.1 was also shipped with the Amstrad CPC Plus series machines, as part of the included game cartridge.
Development was based on existing work recently undertaken writing Mallard BASIC for Acorn Computers Z80 addon for the BBC Micro. It is reported to have taken around 12 weeks to enhance the existing code, and was "very influenced" by BBC BASIC, though adding additional functions to do things that would have required assembly language on the BBC.
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, and PAPER in all versions; plus FILL in v1.1), even allowing the creation of multiple screens, windows, and the like, although the color system and palette handling was awkward. 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 one-off call respectively to the BASIC line number of the user's choice.
Also, when compared to other home computers of the time, the Amstrad via Locomotive BASIC granted a relatively high level of control over the CPC sound chip, an AY-3-8912 with 3 melodic channels and 1 noise channel. The same chip was also used on late-model ZX Spectrums, as well as the Atari ST and MSX computers, but none of those had such a complete built-in SOUND command.
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