Chapter 25: The system variables

BASin

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The bytes in memory from 23552 to 23733 are set aside for specific uses by the system. You can peek them to find out various things about the system, and some of them can be usefully poked. They are listed here with their uses.

These are called system variables, and have names, but do not confuse them with the variables used by the BASIC. The computer will not recognize the names as referring to system variables, and they are given solely as mnemonics for we humans.

Poking system variables in the BASin editor will not always have the desired effect, because there is a layer of abstraction between BASin and the emulated ZX Spectrum memory.

The abbreviations in column 1 have the following meanings:

X	The variables should not be poked because the system might crash.
N	Poking the variable will have no lasting effect.

The number in column 1 is the number of bytes in the variable. For two bytes, the first one is the less significant byte, the reverse of what you might expect. So to poke a value v to a two byte variable at address n, use

POKE n,v-256*INT (v/256)
POKE n+1,INT (v/256)

and to peek its value, use the expression

PEEK n+256*PEEK (n+1)

Notes	Address	Name	Contents
N8	23552	KSTATE	Used in reading the keyboard.
N1	23560	LAST K	Stores newly pressed key.
1	23561	REPDEL	Time (in 50ths of a second, in 60ths of a second in N. America) that a key must be held down before it repeats. This starts off at 35, but you can `POKE` in other values.
1	23562	REPPER	Delay (in 50ths of a second, in 60ths of a second in N. America) between successive repeats of a key held down: initially 5.
N2	23563	DEFADD	Address of arguments of user defined function if one is being evaluated; otherwise 0.
Nl	23565	K DATA	Stores 2nd byte of colour controls entered from keyboard.
N2	23566	TVDATA	Stores bytes of coiour, `AT` and `TAB` controls going to television.
X38	23568	STRMS	Addresses of channels attached to streams.
2	23606	CHARS	256 less than address of character set (which starts with space and carries on to the copyright symbol). Normally in ROM, but you can set up your own in RAM and make CHARS point to it.
1	23608	RASP	Length of warning buzz.
1	23609	PIP	Length of keyboard click.
1	23610	ERR NR	1 less than the report code. Starts off at 255 (for 1) so `PEEK 23610` gives 255.
X1	23611	FLAGS	Various flags to control the BASIC system.
X1	23612	TV FLAG	Flags associated with the television.
X2	23613	ERR SP	Address of item on machine stack to be used as error return.
N2	23615	LIST SP	Address of return address from automatic listing.
N1	23617	MODE	Specifies K, L, C, E or G cursor.
2	23618	NEWPPC	Line to be jumped to.
	23620	NSPPC	Statement number in line to be jumped to. Poking first NEWPPC and then NSPPC forces a jump to a specified statement in a line.
2	23621	PPC	Line number of statement currently being executed.
1	23623	SUBPPC	Number within line of statement being executed.
1	23624	BORDCR	Border colour * 8; also contains the attributes normally used for the lower half of the screen.
2	23625	E PPC	Number of current line (with program cursor).
X2	23627	VARS	Address of variables.
N2	23629	DEST	Address of variable in assignment.
X2	23631	CHANS	Address of channel data.
X2	23633	CURCHL	Address of information currently being used for input and output.
X2	23635	PROG	Address of BASIC program.
X2	23637	NXTLIN	Address of next line in program.
X2	23639	DATADD	Address of terminator of last `DATA` item.
X2	23641	E LINE	Address of command being typed in.
2	23643	K CUR	Address of cursor.
X2	23645	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.
2	23647	X PTR	Address of the character after the `?` syntax error marker in traditional 48K BASIC.
X2	23649	WORKSP	Address of temporary work space.
X2	23651	STKBOT	Address of bottom of calculator stack.
X2	23653	STKEND	Address of start of spare space.
N1	23655	BREG	Calculator's b register.
N2	23656	MEM	Address of area used for calculator's memory. (Usually MEMBOT, but not always.)
	23658	FLAGS2	More flags.
X1	23659	DF SZ	The number of lines (including one blank line) in the lower part of the screen.
2	23660	S TOP	The number of the top program line in automatic listings.
2	23662	OLDPPC	Line number to which `CONTINUE` jumps.
	23664	OSPCC	Number within line of statement to which `CONTINUE` jumps.
N1	23665	FLAGX	Various flags.
N2	23666	STRLEN	Length of string type destination in assignment.
N2	23668	T ADDR	Address of next item in syntax table (very unlikely to be useful).
2	23670	SEED	The seed for `RND`. This is the variable that is set by `RANDOMIZE`.
3	23672	FRAMES	3 byte (least significant first), frame counter. Incremented every 20ms. See Chapter 18.
2	23675	UDG	Address of 1st user defined graphic. You can change this for instance to save space by having fewer user defined graphics.
1	23677	COORDS	x-coordinate of last point plotted.
1	23678		y-coordinate of last point plotted.
1	23679	P POSN	33 column number of printer position.
X2	23680	PR CC	Full address of next position for `LPRINT` to print at (in printer buffer).
2	23682	ECHO E	33 column number and 24 line number (in lower half) of end of input buffer.
2	23684	DF CC	Address in display file of `PRINT` position.
2	23686	DFCCL	Like DF CC for lower part of screen.
X1	23688	S POSN	33 column number for `PRINT` position.
X1	23689		24 line number for `PRINT` position.
X2	23690	SPOSNL	Like S POSN for lower part.
	23692	SCR CT	Counts scrolls: it is always 1 more than the number of scrolls that will be done before stopping with `scroll?` If you keep poking this with a number bigger than 1 (say 255), the screen will scroll on and on without asking you.
1	23693	ATTR P	Permanent current colours, etc (as set up by colour statements).
1	23694	MASK P	Used for transparent colours, etc. Any bit that is 1 shows that the corresponding attribute bit is taken not from ATTR P, but from what is already on the screen.
N1	23695	ATTR T	Temporary current colours, etc (as set up by colour items).
N1	23696	MASK T	Like MASK P, but temporary.
	23697	P FLAG	More flags.
N30	23698	MEMBOT	Calculator's memory area; used to store numbers that cannot conveniently be put on the calculator stack.
2	23728		Used for the NMI in the +3 and +2A, and by the Interface 1.
2	23730	RAMTOP	Address of last byte of BASIC system area.
2	23732	P RAMT	Address of last byte of physical RAM.

This program tells you the first 22 bytes of the variables area:

10 FOR n=0 TO 21
20 PRINT PEEK (PEEK 23627+256*PEEK 23628+n)
30 NEXT n

Try to match up the control variable n with the descriptions above. Now change line 20 to

20 PRINT PEEK (23755+n)

This tells you the first 22 bytes of the program area. Match these up with the program itself.

Chapter 26

Chapter 24

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