Part 25
The system variables

Subjects covered...

        POKE, PEEK

The bytes in memory from 5B00h (23296) to 5CB6h (23734) are set aside 
for specific uses by the system. There are a few routine (used to keep 
the paging in order), and some locations called system variables. You 
can peek these to find out various things about the system, and some 
of them can be usefully poked. They are listed here with their uses.

There is quite a difference, as you might expect, between the system
variables' area in 48 BASIC mode and in +3 BASIC mode. In 48 BASIC
mode, all the variables and routines below 5C00h (23552) do not exist;
instead there is a buffer between 5B00h (23296) and 5C00h (23552)
which is 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 +3 BASIC, the computer will
invariably crash. Any old program that uses PEEK, POKE and USR is
therefore a safer bet if it is run in 48 BASIC mode (although it can
be entered in +3 BASIC mode and transferred using the SPECTRUM
command). If there is a chance that a program might inadvertently
address the added I/O ports of the +3, then 'OUT 32765,48' will set
bit 5 in port 7FFDh to disable further use of the added ROM/RAM
switching.

Although system variables have names, you should not confuse them with
the words and names used in BASIC. The computer will not recognise the
names as referring to system variables; they are given solely as
mnemonics for humans.

The abbreviations in column 1 of the table have the following 
meanings:

X - The variables should not be poked because the system might crash.

N - Poking the variables 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 or 
routine. For a two-byte word, the first byte is the least significant 
- the reverse of what you might expect. So, to poke a value 'v' into 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...

        PRINT PEEK n+256* PEEK (n+1)


NOTES   ADDRESS         NAME    CONTENTS
        HEX (DECIMAL)
--------------------------------------------------------------------

R16     5B00h (23296)   SWAP    Paging subroutine.
R17     5B10h (23312)   STOO    Paging subroutine. Entered with 
                                interrupts already disabled and AF, BC 
                                on the stack.
R9      5B21h (23329)   YOUNGER Paging subroutine.
R16     5B2Ah (23338)   REGNUOY Paging subroutine.
R24     5B3Ah (23354)   ONERR   Paging subroutine.
X2      5B52h (23378)   OLDHL   Temporary register store while 
                                switching ROMs.
X2      5B54h (23380)   OLDBC   Temporary register store while 
                                switching ROMs.
X2      5B56h (23382)   OLDAF   Temporary register store while 
                                switching ROMs.
N2      5B58h (23384)   TARGET  Subroutine address in ROM 3.
X2      5B5Ah (23386)   RETADDR Return address in ROM 1.
X1      5B5Ch (23388)   BANKM   Copy of last byte output to I/O port 
                                7FFDh (32765). This port is used to 
                                control the RAM paging (bits 0...2), 
                                the 'horizontal' ROM switch (0<->1 and 
                                2<->3 - bit 4), screen selection (bit 
                                3) and added I/O disabling (bit 5). 
                                This byte must be kept up to date with 
                                the last value output to the port if 
                                interrupts are enabled.
X1      5B5Dh (23389)   RAMRST  RST 8 instruction. Used by ROM 1 to 
                                report old errors to ROM 3.
N1      5B5Eh (23390)   RAMERR  Error number passed from ROM 1 to ROM 
                                3. Also used by SAVE/LOAD as temporary 
                                drive store.
2       5B5Fh (23391)   BAUD    RS232 bit period in T states/26. Set 
                                by FORMAT LINE.
N2      5B61h (23393)   SERFL   Second-character-received-flag, and 
                                data.
N1      5B63h (23395)   COL     Current column from 1 to width.
1       5B64h (23396)   WIDTH   Paper column width. Defaults to 80.
1       5B65h (23397)   TVPARS  Number of inline parameters expected 
                                by RS232.
1       5B66h (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	5B67h (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 0<->2 and 1<->3), 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.
N1	5B68h (23400)	XLOC	Holds X location when using the
				unexpanded COPY command.
N1	5B69h (23401)	YLOC	Holds Y location when using the
				unexpanded COPY command.
X2	5B6Ah (23402)	OLDSP	Old SP (stack pointer) when TSTACK is
				in use.
X2	5B6Ch (23404)	SYNRET	Return address for ONERR.
5	5B6Eh (23406)	LASTV	Last value printed by calculator.
2	5B73h (23411)	RCLINE	Current line being renumbered.
2	5B75h (23413)	RCSTART	Starting line number for
				renumbering. The default value is 10.
2	5B77h (23415)	RCSTEP	Incremental value for renumbering. The
				default is 10.
1	5B79h (23417)	LODDRV	Holds 'T' if LOAD, VERIFY, MERGE are
				from tape, otherwise holds 'A', 'B'
				or 'M'.
1	5B7Ah (23418)	SAVDRV	Holds 'T' if SAVE is to tape,
				otherwise holds 'A', 'B' or 'M'.
1	5B7Bh (23419)	DUMPLF	Holds the number of 1/216ths user for
				line feeds in 'COPY EXP'. This is
				normally set to 9. If problems are
				experienced fitting a dump onto a
				sheet of A4 paper, POKE this
				location with 8. This will reduce the
				size of the dump and improve the
				aspect ratio slightly. (The quality of
				the dump will be marginally degraded,
				however.)
N8	5B7Ch (23420)	STRIP1	Stripe one bitmap.
N8	5B84h (23428)	STRIP2	Stripe two bitmap. This extends to
				5B8Bh (23436).
X115	5BFFh (23551)	TSTACK	Temporary stack grows down from
				here. Used when RAM page 7 is switched
				in at top of memory (while executing
				the editor or calling +3DOS). it may
				safely go down to 5B8Ch (and across
				STRIP1 and STRIP2 if necessary). This
				guarantees at least 115 bytes of stack
				when BASIC calls +3DOS.
N8	5C00h (23552)	KSTATE	Used in reading the keyboard.
N1	5C08h (23560)	LASTK	Stores newly pressed key.
1	5C09h (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	5C0Ah (23562)	REPPER	Delay (in 50ths of a second) between
				successive repeats of a key held down
				- initially 5.
N2	5C0Bh (23565)	DEFADD	Address of arguments of user defined
				function (if one is being evaluated),
				otherwise 0.
N1	5C0Dh (23566)	TVDATA	Stores bytes of colour, AT and TAB
				controls going to TV.
X38	5C10h (23568)	STRMS	Addresses of channels attached to
				streams.
2	5C36h (23606)	CHARS	256 less than address of character set
				(which starts with space and carries
				on to (C)). Normally in ROM, but you
				can set up your down in RAM and make
				CHARS point to it.
1	5C38h (23608)	RASP	Length of warning buzz.
1	5C39h (23609)	PIP	Length of keyboard click.
1	5C3Ah (23610)	ERRNR	1 less than the report code. Starts
				off at 255 (for -1) so 'PEEK 23610'
				gives 255.
X1	5C3Bh (23611)	FLAGS	Various flags to control the BASIC
				system.
X1	5C3Ch (23612)	TVFLAG	Flags associated with the TV.
X2	5C3Dh (23613)	ERRSP	Address of item on machine stack to be
				used as error return.
N2	5C3Fh (23615)	LISTSP	Address of return address from
				automatic listing.
N1	5C41h (23617)	MODE	Specifies 'K', 'L', 'C', 'E' or 'G'
				cursor.
2	5C42h (23618)	NEWPPC	Line to be jumped to.
1	5C44h (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	5C45h (23621)	PPC	Line number of statement currently
				being executed.
1	5C47h (23623)	SUBPPC	Number within line of statement
				currently being executed.
1	5C48h (23624)	BORDCR	Border colour multiplied by 8; also
				contains the attributes normally used
				for the lower half of the screen.
2	5C49h (23625)	E PPC	Number of current line (with program
				cursor).
X2	5C4Bh (23627)	VARS	Address of variables.
N2	5C4Dh (23629)	DEST	Address of variable in assignment.
X2	5C4Fh (23631)	CHANS	Address of channel data.
X2	5C51h (23633)	CURCHL	Address of information currently being
				used for input and output.
X2	5C53h (23635)	PROG	Address of BASIC program.
X2	5C57h (23637)	NXTLIN	Address of next line in program.
X2	5C57h (23639)	DATADD	Address of terminator of last DATA
				item.
X2	5C59h (23641)	E LINE	Address of command being typed in.
2	5C5Bh (23643)	K CUR	Address of cursor.
X2	5C5Dh (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	5C5Fh (23647)	X PTR	Address of the character after the []
				marker.
X2	5C61h (23649)	WORKSP	Address of temporary work space.
X2	5C63h (23651)	STKBOT	Address of bottom of calculator stack.
X2	5C65h (23653)	STKEND	Address of start of spare space.
N1	5C67h (23655)	BREG	Calculator's B register.
N2	5C68h (23656)	MEM	Address of area used for calculator's
				memory (usually MEMBOT, but not
				always).
1	5C6Ah (23658)	FLAGS2	More flags. (Bit 3 set when CAPS SHIFT
				or CAPS LOCK is on.)
X1	5C6Bh (23659)	DF SZ	The number of lines (including one
				blank line) in the lower part of the
				screen.
2	5C6Ch (23660)	S TOP	The number of the top program line in
				automatic listings.
2	5C6Eh (23662)	OLDPPC	Line number to which CONTINUE jumps.
1	5C70h (23664)	OSPPC	Number within line of statement to
				which CONTINUE jumps.
N1	5C71h (23665)	FLAGX	Various flags.
N2	5C72h (23666)	STRLEN	Length of string type destination in
				assignment.
N2	5C74h (23668)	T ADDR	Address of next item in syntax table
				(very unlikely to be useful).
2	5C76h (23670)	SEED	The seed for RND. This is the variable
				that is set by RANDOMIZE.
3	5C78h (23672)	FRAMES	3 byte (least significant byte first),
				frame counter incremented every 20ms.
2	5C7Bh (23675)	UDG	Address of first user-defined
				graphic. You can change this, for
				instance, to save space by having
				fewer user-defined characters.
1	5C7Dh (23677)	COORDS	X-coordinate of last point plotted.
1	5C7Eh (23678)		Y-coordinate of last point plotted.
1	5C7Fh (23679)	P POSN	33-column number of printer position.
1	5C80h (23680)	PR CC	Least significant byte of address of
				next position for LPRINT to print at
				(in printer buffer).
1	5C81h (23681)		Not used.
2	5C82h (23682)	ECHO E	33-column number and 24-line number
				(in lower half) of end of input
				buffer.
2	5C84h (23684)	DF CC	Address in display file of PRINT
				position.
2	5C86h (23686)	DF CCL	Like DF CC for lower part of screen.
X1	5C88h (23688)	S POSN	33-column number for PRINT position.
X1	5C89h (23689)		24-line number for PRINT position.
X2	5C8Ah (23690)	SPOSNL	Like S POSN for lower part.
1	5C8Ch (23691)	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	5C8Dh (23692)	ATTR P	Permanent current colours, etc., (as
				set up by colour statements).
1	5C8Eh (23693)	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	5C8Fh (23694)	ATTR T	Temporary current colours, etc., (as
				set up by colour items).
N1	5C90h (23696)	MASK T	Like MASK P, but temporary.
1	5C91h (23697)	P FLAG	More flags.
N30	5C92h (23698)	MEMBOT	Calculator's memory area - used to
				store numbers that cannot conveniently
				be put on the calculator stack.
2	5CB0h (23728)	NMIADD	Holds the address of the users NMI
				service routine.
				NOTE - On previous machines, this did
				not work correctly and these two bytes
				were documented as 'Not used.'
				Programs that user these two bytes for
				passing values may need to be
				modified.
2	5CB2h (23730)	RAMTOP	Address of last byte of BASIC system
				area.
2	5CB4h (23732)	P RAMT	Address of last byte of physical RAM.