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Sinclair BASIC (taking its name from innovator Sir Clive Sinclair) is a dialect of the BASIC programming language used in the 8-bit home computers from Sinclair Research and Timex Sinclair. The Sinclair BASIC interpreter was made by Nine Tiles Networks Ltd.
Originally developed in 1979 to fit in the 4 kB ROM of the ZX80, it was initially an incomplete implementation of the 1978 ANSI minimal BASIC standard and evolved through the 8 kB ROM ZX81 and TS1000 to be an almost complete version in the 16 kB ROM ZX Spectrum.
Original Sinclair BASIC
On the Spectrum there are 86 reserved words in Sinclair BASIC, denoting commands (of which there were 50), functions (31), and other keywords (5). They were entered with the ZX Spectrum's sometimes obscure context-based keyword entry system, whereby BEEP (for example), was keyed by pressing CAPS plus SYMBOL SHIFT, followed by SYMBOL SHIFT plus Z. More common commands were entered using a single keystroke, for example pressing P caused the entire command PRINT to appear.
- ABS, ACS, AND1, ASN, AT, ATN, ATTR, BEEP, BIN, BORDER, BRIGHT7, CAT2, CHR$</u>3, CIRCLE, CLEAR, CLOSE#2, CLS, CODE3, CONTINUE, COPY, COS, DATA, DEF FN, DIM, DRAW, ERASE2, EXP, FLASH7, FN, FOR, FORMAT2, GO SUB, GO TO, IF4, IN, INK7, INKEY$3, INPUT, INT, INVERSE7, LEN, LET5, LINE, LIST, LLIST, LN, LOAD, LPRINT, MERGE, MOVE2, NEW, NEXT, NOT1, OPEN#2, OR1, OUT, OVER7, PAPER7, PAUSE, PEEK, PI, PLOT, POINT, POKE, PRINT, RANDOMIZE, READ, REM, RESTORE, RETURN, RND, RUN, SAVE, SCREEN$3, SGN, SIN, SQR, STEP, STR$3, TAB, TAN, THEN 4, TO, USR, VAL3, 6, VAL$3, VERIFY
Commands found exclusively on the ZX81 and its clones, the TS1000 and TS1500 are:
- FAST, SCROLL, SLOW, UNPLOT, GOSUB, GOTO (vs the Spectrum's functionally identical GO SUB, GO TO)
On the ZX Spectrum each reserved word was assigned a character code between 165 and 255 in the latter half of the system character set, and expanded by referencing a token table held in ROM. As a result, any reserved word in a program listing occupied just one byte of memory, a significant saving over traditional letter-by-letter storage. This also meant that the BASIC interpreter could quickly determine any command or function by evaluating a single byte.
The 128k Spectrum models—the ZX Spectrum 128, +2, +3, +2A, and +2B—introduced a conventional letter-by-letter BASIC input system, and two new commands, neither of which was present in or recognised by the machine's legacy version of 48k BASIC:
- PLAY (which operated the 128k models' AY-3-8910 music chip)
- SPECTRUM (which switched the 128k Spectrum into a 48k Spectrum compatibility mode)
- The AND, NOT, and OR functions are logical operators.
- CAT, ERASE, FORMAT and MOVE were originally designed to be used with peripherals, but at the launch of ZX Spectrum, they had not been completely implemented, such that their use generated an error message (Invalid Stream). Later with the aid of the ZX Interface 1 shadow ROM, they were used for the ZX Microdrive. (The shadow ROM was paged when the BASIC interpreter detected a syntax error, which is why most ZX Microdrive commands use a "*").
- String variable names must consist of only one alphabetical character.
Thus, LET A=5, LET Apples=5, and LET A$="Hello" are all good, while LET APPLES$="Fruit" is not.
- Unlike most other BASIC dialects, Sinclair Basic did not include the ELSE operator in the IF–THEN(–ELSE) clause.
Thus, instead of
10 IF V=5 THEN GO TO 50 ELSE GO TO 100
10 IF V=5 THEN GO TO 50
20 GO TO 100
- LET is compulsory (i.e., LET A=1 but never A=1). This practice is also different from most other BASIC dialects.
- The VAL function does not just evaluate numbers, but also evaluates full expressions. For example, PRINT VAL(A$) will output 14 when given an A$ of "3*3+4+COS(0)".
- INK, PAPER, FLASH, BRIGHT, OVER and INVERSE set attributes for outputting text and graphics to the screen. They can be used either as commands, to apply to all subsequent output until set again, or within a PRINT statement, to apply only from that point until the end of the statement.
- Machine code could be executed using the USR function, the value provided being the start address of the machine code to execute and the return value being the contents of the BC register pair (unlike most other Z-80 based computers that returned the value of the HL register pair). thus:
LET A=USR 30000
would jump to and begin executing the machne code starting at memory address 30000, and on succesful completion would store the value of the BC register pair into the variable A which can then be used by the programmer.
Unlike the LEFT$(), MID$() and RIGHT$() functions used in the ubiquitous Microsoft BASIC dialects for home computers, parts of strings in Sinclair BASIC are accessed in a manner similar to arrays. For example, A$ (5 TO 10) will give a substring starting with the 5th and ending with the 10th character of A$. As with modern programming languages such as Python, it was therefore possible to replace the LEFT$() and RIGHT$() commands simply by omitting the left or right array position respectively; for instance A$ ( TO 5) is equivalent to LEFT$(5).
The Timex BASIC dialect, used on the Spectrum-compatible TS2068, but not the TC2048, which used the ordinary Sinclair BASIC, has the following six keywords as well as the ordinary Sinclair BASIC ones:
- DELETE deletes BASIC program lines. SHIFT + 0 with the K cursor produces the command DELETE
- FREE is a function that gives the amount of free RAM. PRINT FREE will show how much RAM is free
- ON ERR is an error detection function mostly used as ON ERR GO TO or ON ERR GO SUB
- RESET has no use on the basic machine. It was intended to reset peripherals
- SOUND commands the AY-3-8192 sound ship
- STICK is a function that gives the position of the internal joystick (Timex Sinclair 2090)
- Beta BASIC, written by Dr. Andy Wright, was originally a BASIC extension, but ended up as a full-fledged interpreter.
- SuperBASIC, a much more advanced BASIC dialect, introduced with the Sinclair QL personal computer, with some similarities to the earlier Sinclair BASICs.
- Vickers, Steven (1982). Sinclar ZX Spectrum BASIC Programming. Sinclair Research.
- Ardley, Neil (1984). Sinclair ZX Spectrum+ User Guide. Dorling Kindersley in association with Sinclair Research. ISBN 0-86318-080-9.
- Sinclair ZX Spectrum BASIC Programming — The original 1982 manual by Steven Vickers (referenced above)
- Sinclair ZX81 Basic Programming — also by Vickers
- The History of Sinclair BASIC — By Andrew Owen, whose Sinclair BASIC documents are hosted by the World of Spectrum website