## Chapter 24 – The System Variables ### Overview Certain locations in memory are set aside for specific uses by the system. There are a few routines (used to keep the paging in order), and some locations called system variables (or SYSVARS). You can use **PEEK** and **DPEEK** to read them, in order to find out various things about the system, and on some of them you can usefully change with **POKE** and **DPOKE**. They are listed here with their uses. The area occupied by SYSVARS spans the addresses **23296** (**5B00h**) to **23733** (**5CB5h**) (inclusive) and a subset of them are used in 48K BASIC – addresses **23552** (**5C00h**) to **23733** (**5CB5h**). Note that in 48K mode, there is a buffer area between **23296** (**5B00h**) and **23552** (**5C00h**) which was used for controlling the printer. This was quite a popular location for small machine code programs on the old 48K Spectrum and if any of these routines are tried in *NextBASIC*, the computer will invariably crash. It's advisable that any 48K BASIC program that uses **PEEK**, **POKE** and **USR** to either be run in 48K BASIC mode (although it can be entered in *NextBASIC* mode and transferred using the **SPECTRUM** command) or examined thoroughly and converted so it won't use any of these commands. Alternatively, any machine code routine embedded within it should be moved to a safer area. ### System Variables The system variables listed below, all have unique names, but do not confuse them with *NextBASIC* variables. The computer will not recognize the names as referring to system variables, and they are given solely as mnemonics to be human-readable. The abbreviations in column 1 of the table that follows 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\ **R** Routine entry point. Not a variable. The number in column 1 is the number of bytes in the variable. For two bytes, the first one is the least significant byte; the reverse of what you might expect. So to **POKE** a value *v* to a two byte variable at address *n*, use **DPOKE** instead, as it does the conversion for you. Otherwise you'll need to enter the following for SYSVAR *n* value *v*: ``` POKE n,v-256*INT (v/256) POKE n+1,INT (v/256) ``` and to peek its value, either use **DPEEK** or the expression: ``` PEEK n+256*PEEK (n+1) ```

Notes	Address		Name	Description
	Hex	Dec
R16	5B00	23296	SWAP	Paging subroutine
R17	5B10	23312	STOO	Paging subroutine. Entered with interrupts already disabled and AF, BC on the stack.
R9	5B21	23329	YOUNGER	Paging subroutine
R16	5B2A	23338	REGNUOY	Paging subroutine
R24	5B3A	23354	ONERR	Paging subroutine
X2	5B52	23378	OLDHL	Temporary register store while switching ROMs
X2	5B54	23380	OLDBC	Temporary register store while switching ROMs
X2	5B56	23382	OLDAF	Temporary register store while switching ROMs
X1	5B58	23384	CACHEBNK	8K Bank ID holding cached program data
N1	5859	23385		Reserved for system use

Notes	Address		Name	Description
	Hex	Dec
X2	5B5A	23386	RETADDR	Return address in ROM 1
X1	5B5C	23388	BANKM	Copy of last byte output to I/O port 7FFDh (32765). See Chapter 23 – The Memory. This byte must be kept up to date with the last value output to the port if interrupts are enabled
X1	5B5D	23389	RAMRST	RST 8 instruction. Used by ROM 1 to report old errors to ROM 3
N1	5B5E	23390	RAMERR	Error number passed from ROM 1 to ROM 3. Also used by SAVE/LOAD as temporary drive store
1	5B5F	23391	INKL	INK colour for LoRes
1	5B60	23392	INK2	INK colour for Layer 2
1	5B61	23393	ATTRULA	Attributes for standard mode
1	5B62	23394	ATTRHR	Attributes for HiRes (only paper colour in bits 5:3 is used)
1	5B63	23395	ATTRHC	Attributes for HiColour
1	5B64	23396	INKMASK	Softcopy of EnhancedULA InkMask (or 0)
N1	5B65	23397	LSBANK	Temporary bank number in LOAD/SAVE and other operations
1	5B66	23398	FLAGS3	Various flags. Bits 0, 1, 6 and 7 unlikely to be useful. Bit 2 is set when tokens are to be expanded on printing. Bit 3 is set if print output is RS232. The default (at reset) is Centronics. Bit 4 is set if a disk interface is present. Bit 5 is set if drive B: is present
X1	5B67	23399	BANK678	Copy of last byte output to I/O port 1FFDh (8189). This port is used to control the +3 extended RAM and ROM switching (bits 0..2 – if bit 0 is 0 then bit 2 controls the 'vertical' ROM switch 02 and 13), the disk motor (bit 3) and Centronics strobe (bit 4). This byte must be kept up to date with the last value output to the port if interrupts are enabled
X1	5B68	23400	FLAGN	Flags for the NextZXOS system
1	5B69	23401	MAXBNK	Maximum available RAM bank
X2	5B6A	23402	OLDSP	Old SP (stack pointer) when TSTACK is in use
X2	5B6C	23404	SYNRET	Return address for ONERR
5	5B6E	23406	LASTV	Last value printed by calculator
1	5B73	23411	TILEBNKL	Tiles bank for LoRes
1	5B74	23412	TILEML	Tilemap bank for LoRes
1	5B75	23413	TILEBNK2	Tiles bank for Layer2
1	5B76	23414	TILEM2	Tilemap bank for Layer2
X1	5B77	23415	NXTBNK	Bank containing NXTLIN
X1	5B78	23416	DATABNK	Bank containing DATADD
1	5B79	23417	LODDRV	Holds 'T' if LOAD, VERIFY, MERGE are from tape, otherwise holds 'A', 'B' or 'M'
1	5B7A	23418	SAVDRV	Holds 'T' if SAVE is to tape, otherwise holds 'A', 'B' or 'M'
N1	5B7B	23419	L2SOFT	Softcopy of Layer 2 port
2	5B7C	23420	TILEWL	Width of LoRes tilemap
2	5B7E	23422	TILEW2	Width of Layer 2 tilemap
2	5B80	23424	TILEOFFL	Offset in bank for LoRes tilemap
2	5B82	23426	TILEOFF2	Offset in bank for Layer 2 tilemap
2	5B84	23428	COORDSX	X Coordinate of last point plotted (Layer 1/2)
2	5B86	23430	COORDSY	Y Coordinate of last point plotted (Layer 1/2)
1	5B88	23432	PAPERL	PAPER colour for LoRes mode
1	5B89	23433	PAPER2	PAPER colour for Layer 2 mode
Nx	5B8A	23434	TMPVARS	Base of temporary system variables (space shared with bottom of TSTACK)

Notes	Address		Name	Description
	Hex	Dec
X117	5BFF	23551	TSTACK	Temporary stack grows down from here. Used when RAM bank 7 is switched in at top of memory while executing the editor or calling NextZXOS. It may safely go down to 5B8Ah if necessary. This guarantees at least 117 bytes of stack when NextBASIC calls NextZXOS
N8	5C00	23552	KSTATE	Used in reading the keyboard
N1	5C08	23560	LASTK	Stores newly pressed key
1	5C09	23561	REPDEL	Time (in 50ths of a second) that a key must be held down before it repeats. This starts off at 35, but you can POKE in other values
1	5C0A	23562	REPPER	Delay (in 50ths of a second) between successive repeats of a key held down – initially 5)
X2	5C0B	23563	RETVARS	Address of local variables on return stack
N1	5C0D	23565	K_DATA	Stores 2nd byte of colour controls entered from keyboard
N2	5C0E	23566	TVDATA	Stores bytes of colour, AT and TAB controls going to TV
X38	5C10	23568	STRMS	Addresses of channels attached to streams
2	5C36	23606	CHARS	256 less than address of character set (which starts with space and carries on until ©). Normally in ROM, but you can set up your own in RAM and make CHARS point to it
1	5C38	23608	RASP	Length of warning buzz
1	5C39	23609	PIP	Length of keyboard click
1	5C3A	23610	ERRNR	1 less than the report code. Starts off at 255 (for -1) so PEEK 23610 gives 255
X1	5C3B	23611	FLAGS	Various flags to control the NextBASIC system
X1	5C3C	23612	TVFLAG	Flags associated with the TV
X2	5C3D	23613	ERRSP	Address of item on machine stack to be used as error return
N2	5C3F	23615		Reserved for system use
N1	5C41	23617	MODE	Specifies K, L, C, E or G cursor.
2	5C42	23618	NEWPPC	Line to be jumped to
N1	5C44	23620		Reserved for system use
2	5C45	23621	PPC	Line number of statement currently being executed
1	5C47	23623	SUBPPC	Number within line of statement currently being executed
1	5C48	23624	BORDCR	Border colour multiplied by 8; also contains the attributes normally used for the lower half of the screen
2	5C49	23625	E_PPC	Number of current line (with program cursor)
X2	5C4B	23627	VARS	Address of variables
N2	5C4D	23629	DEST	Address of variable in assignment
X2	5C4F	23631	CHANS	Address of channel data
X2	5C51	23633	CURCHL	Address of information currently being used for input and output
X2	5C53	23635	PROG	Address of NextBASIC program
X2	5C55	23637	NXTLIN	Address of next line in program
X2	5C57	23639	DATADD	Address of terminator of last DATA item
X2	5C59	23641	E_LINE	Address of command being typed in
2	5C5B	23643	K_CUR	Address of cursor
X2	5C5D	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	5C5F	23647	X_PTR	Address of the character after the ? marker
X2	5C61	23649	WORKSP	Address of temporary work space
X2	5C63	23651	STKBOT	Address of bottom of calculator stack
X2	5C65	23653	STKEND	Address of start of spare space
N1	5C67	23655	BREG	Calculator's B register

Notes	Address		Name	Description
	Hex	Dec
N2	5C68	23656	MEM	Address of area used for calculator's memory (usually MEMBOT, but not always)
1	5C6A	23658	FLAGS2	More flags. (Bit 3 set when CAPS SHIFT or CAPS LOCK is on)
X1	5C6B	23659	DF_SZ	The number of lines (including one blank line) in the lower part of the screen
N2	5C6C	23660		Reserved for system use
2	5C6E	23662	OLDPPC	Line number to which CONTINUE jumps
1	5C70	23664	OSPPC	Number within line of statement to which CONTINUE jumps
N1	5C71	23665	FLAGX	Various flags
N2	5C72	23666	STRLEN	Length of string type destination in assignment
N2	5C74	23668	T_ADDR	Address of next item in syntax table (very unlikely to be useful)
2	5C76	23670	SEED	The seed for RND. This is the variable that is set by RANDOMIZE
3	5C78	23672	FRAMES	3 byte (least significant byte first), frame counter incremented every 20ms
2	5C7B	23675	UDG	Address of first user-defined graphic. You can change this, for instance, to save space by having fewer user-defined characters
1	5C7D	23677	COORDS	X-coordinate of last point plotted
1	5C7E	23678		Y-coordinate of last point plotted
X1	5C7F	23679	GMODE	Graphical layer/mode flags
X1	5C80	23680	PRCC	Full address of next position for LPRINT to print at (in ZX Printer buffer). Legal values 5B00 – 5B1F[^p274-1]
1	5C81	23681	STIMEOUT	Screensaver control
2	5C82	23682	ECHO_E	33-column number and 24 line number (in lower half) of end of input buffer
2	5C84	23684	DF_CC	Address in display file of PRINT position
2	5C86	23686	DF_CCL	Like DF_CC for lower part of screen
X1	5C88	23688	S_POSN	33-column number for PRINT position
X1	5C89	23689		24-line number for PRINT position
X2	5C8A	23690	SPOSNL	Like S_POSN for lower part
1	5C8C	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	5C8D	23693	ATTR_P	Permanent current colours, etc., (as set up by colour statements)
1	5C8E	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	5C8F	23695	ATTR_T	Temporary current colours, etc., (as set up by colour items)
N1	5C90	23696	MASK_T	Like MASK_P, but temporary
1	5C91	23697	P_FLAG	More flags
N30	5C92	23698	MEMBOT	Calculator's memory area – used to store numbers that cannot conveniently be put on the calculator stack
2	5CB0	23728		Not used
2	5CB2	23730	RAMTOP	Address of last byte of NextBASIC system area.
2	5CB4	23732	P_RAMT	Address of last byte of physical RAM.

[^p274-1]: Not used in 128K mode or when certain peripherals are attached