Sinclair BASIC tokenized file

Sinclair BASIC is a dialect of the BASIC programming language created by by Nine Tiles Networks Ltd and used in the 8-bit home computers from Sinclair Research and Timex Sinclair.

The original 4KB version was developed for the Sinclair ZX80, followed by an 8KB version for the ZX81 and 16 KB version for ZX Spectrum.

Some unusual features of the Sinclair BASIC:

• There were keys on the keyboard for each BASIC keyword. For example, pressing P caused the entire command PRINT to appear. Some commands needed multiple keypresses to enter, For example, BEEP was keyed by pressing CAPS SHIFT plus SYMBOL SHIFT to access extended mode, keeping SYMBOL SHIFT held down and pressing Z.
• When programs where SAVEd, the file written to disk or tape contained all of BASIC's internal state information, including the values of any defined basic variables, as well as the BASIC tokens.

BASIC File Layout

On a ZX81 , a saved BASIC file is a snapshot of the computer memory from memory location 16393 through to the end of the variable table. There is no header.

16436||FRAMES||Counts the frames displayed on the television. Bit 15 is 1. Bits 0 to 14 are decremented for each frame set to the television. This can be used for timing, but PAUSE also uses it. PAUSE resets to 0 bit 15, & puts in bits 0 to 14 the length of the pause. When these have been counted down to zero, the pause stops. If the pause stops because of a key depression, bit 15 is set to 1 again.

Address	Name	Description
16393	VERSN	0 Identifies ZX81 BASIC in saved programs.
16394	E_PPC	Number of current line (with program cursor).
16396	D_FILE	Pointer to the start of the 'Display file', i.e. what is being displayed on screen
16398	DF_CC	Address of PRINT position in display file. Can be poked so that PRINT output is sent elsewhere.
16400	VARS	Pointer to start of BASIC Variable table
16402	DEST	Address of variable in assignment.
16404	E_LINE	Pointer to line currently being entered
16406	CH_ADD	Address of the next character to be interpreted: the character after the argument of PEEK, or the NEWLINE at the end of a POKE statement.
16408	X_PTR	Address of the character preceding the marker.
16410	STKBOT	pointer to start (bottom) of stack
16412	STKEND	pointer to end (top) of stack
16414	BERG	Calculator's b register.
16415	MEM	Address of area used for calculator's memory. (Usually MEMBOT, but not always.)
16417		not used
16418	DF_SZ	The number of lines (including one blank line) in the lower part of the screen.
16419	S_TOP	The number of the top program line in automatic listings.
16421	LAST_K	Shows which keys pressed.
16423		Debounce status of keyboard.
16424	MARGIN	Number of blank lines above or below picture: 55 in Britain, 31 in America.
16425	NXTLIN	Address of next program line to be executed.
16427	OLDPPC	Line number of which CONT jumps.
16429	FLAGX	Various flags.
16430	STRLEN	Length of string type destination in assignment.
16432	T_ADDR	Address of next item in syntax table (very unlikely to be useful).
16434	SEED	The seed for RND. This is the variable that is set by RAND.
16438	COORDS	x-coordinate of last point PLOTted.
16439		y-coordinate of last point PLOTted.
16440	PR_CC	Less significant byte of address of next position for LPRINT to print as (in PRBUFF).
16441	S_POSN	Column number for PRINT position.
16442		Line number for PRINT position.
16443	CDFLAG	Various flags. Bit 7 is on (1) during compute & display mode.
16444	PRBUFF	Printer buffer (33rd character is NEWLINE).
16477	MEMBOT	Calculator's memory area; used to store numbers that cannot conveniently be put on the calculator stack.
16507		not used
16509		First BASIC line.

Each BASIC line is stored as:

 2 byte line number (in big-endian format)
 2 byte length of text including NEWLINE (in little endian format, length "excludes" the line number and length, i.e. to skip between lines you add "length of text" +4 bytes.
 text (BASIC tokens)
 NEWLINE (0x76)

When a numeric constant is included in the text of a BASIC line, an ASCII string displaying the constant value will be inserted, followed by the token 0x7E, and the next 5 bytes are the value of the constant in floating point format.

Following the last BASIC line comes the VARIABLEs table.

     #first byte of name implies type:
     #011nnnnn = single letter variable name, followed by 5 byte number
     #101nnnnn = multi letter variable name (last letter has high bit set), followed by 5 byte number
     #100nnnnn = array of numbers: 
     #111nnnnnn = control variable of a for-next loop
     #010nnnnn = single letter variable name - 0x20, 2 byte string length, text of string
     #110nnnnn = array of characters

References

[]http://www.worldofspectrum.org/ZX81BasicProgramming/ ZX81 BASIC Programming]