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|Developer||Nine Tiles Networks, Sinclair|
|Platform||ZX80, ZX81, ZX Spectrum|
Sinclair BASIC 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.
Reserved words 
On the ZX Spectrum, there are 86 reserved words in Sinclair BASIC, denoting commands (of which there were 50), functions (31), and other keywords (5):
VAL,[note 7][note 9]
- Other keywords
They are entered via Sinclair's somewhat unorthodox keyword entry system. The most common commands require just a single keystroke; for example, pressing P causes the entire command
BEEP (for example) is keyed by pressing CAPS SHIFT plus SYMBOL SHIFT to access extended mode (later models include an EXTENDED MODE key), keeping SYMBOL SHIFT held down and pressing Z. Keywords are colour-coded on the keyboard to indicate which ⇧ Shift-sequence is required.
Commands found exclusively on the ZX81 and its clones, the TS1000 and TS1500 are
GOTO (vs the Spectrum's functionally identical
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 original Spanish ZX Spectrum 128 included four additional commands in Spanish, one of which was undocumented. These can be translated as:
EDIT(to edit a line number or invoke the full screen string editor)
RENUM(to renumber the program lines)
DELETE(to delete program lines)
WIDTH(to set the column width of the RS232 device, but undocumented as the code was broken)
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$. As with modern programming languages such as Python, it was therefore possible to replace the
RIGHT$() commands simply by omitting the left or right array position respectively; for instance
A$ ( TO 5) is equivalent to
T/S 2000 BASIC 
DELETEdeletes BASIC program line ranges. ⇧ Shift+0 with the
Kcursor produces the command
FREEis a function that gives the amount of free RAM.
PRINT FREEwill show how much RAM is free.
ON ERRis an error detection function mostly used as
ON ERR GO TOor
ON ERR CONT.
RESETcan be used to reset the behaviour of
ON ERR. It was also intended to reset peripherals.
SOUNDcontrols the AY-3-8192 sound chip.
STICKis a function that gives the position of the internal joystick (Timex Sinclair 2090).
Zebra Systems in New York released a cartridge called ZebraOS; a version of T/S 2000 BASIC that used the 512x192 screen mode. Timex of Portugal released a software extension called BASIC64 to allow better Basic programming with the advanced 512x192 graphic mode available only on Timex 2000 series computers. This extension added commands and do a complete memory remap to avoid the system to overwrite the extended screen memory area. Two versions existed: a version for TC2048 and a version for TS/TC2068 because they have different memory map.
Derivatives and successors 
YS MegaBasic, written by Mike Leaman.
INVERSEset 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
- 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
- The control variable of a
FORloop must consist of only one alphabetical character.
- Unlike most other BASIC dialects, Sinclair Basic did not include the
ELSEoperator in the
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
LETis compulsory (i.e.,
LET A=1but never
A=1). This practice is also different from most other BASIC dialects.
- The AND, NOT, and OR functions are logical operators.
- String variable names must consist of only one alphabetical character. Thus,
LET Apples=5, and
LET A$="Hello"are all good, while
LET APPLES$="Fruit"is not.
- Machine code could be executed using the
USRfunction, 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 Z80 based computers that returned the value of the HL register pair). thus:
LET A=USR 30000
would jump to and begin executing the machine code starting at memory address 30000, and on successful completion would store the value of the BC register pair into the variable A which can then be used by the programmer.
VALfunction does not just evaluate numbers, but also evaluates full expressions. For example,
PRINT VAL(A$)will output 14 when given an
VAL$does the same but returns a string.
- Garfield, Simon (2010-02-28). "Sir Clive Sinclair: "I don't use a computer at all"". The Guardian (Guardian Media Group). Retrieved 2011-05-23. "He is keen to credit [...], not least Nine Tiles, the company that made the Basic operating software."
- Spectrum 128 ROM Disassembly - Spanish Spectrum 128
- 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
- Timex Computer World: Basic 64 user manual for Timex Computer 2048