28. The system variables

The bytes in memory from 16384 to 16508 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 BASIC. The computer will not recognize the names as referring to system variables, and they are given solely as mnemonics for you humans.

The abbreviations in column 1 have the following meanings:

X The variable should not be poked because the system might crash.
N Poking the variable will have no lasting effect.
S The variable is saved by SAVE.

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)

& to peek its value, use the expression

PEEK n + 256*PEEK (n+1)

Notes	Address		Name	Contents
	Decimal	Hex
1	16384	4000	ERR_NR	1 less than the report code. Starts off at 255 (for - 1), so PEEK 16384, if it works at all, gives 255. POKE 16384,n can be used to force an error halt: 0  n  14 gives one of the usual reports, 15  <= n <= 34 or 99 <= n <= 127 gives a non-standard report, and 35 <= n <= 98 is liable to mess up the display file.
X1	16385	4001	FLAGS	Various flags to control the BASIC system. Bit 0: Suppression of leading space. Bit 1: Control Flag for the printer. Bit 2: Selects K or F mode; or, F or G Bit 6: FP no. or string parameters. Bit 7: Reset during syntax checking.
X2	16386	4002	ERR_SP	Address of first item on machine stack (after GOSUB returns).
2	16388	4004	RAMTOP	Address of first byte above BASIC system area. You can poke this to make NEW reserve space above that area (see chapter 26) or to fool CLS into setting up a minimal display file (chapter 27). Poking RAMTOP has no effect until one of these two is executed.
N1	16390	4006	MODE	Specified K, L, F or G cursor.
N2	16391	4007	PPC	Line number of statement currently being executed. Poking this has no lasting effect except in the last line of the program.
S1	16393	4009	VERSN	0 Identifies ZX81 BASIC in saved programs.
S2	16394	400A	E_PPC	Number of current line (with program cursor).
SX2	16396	400C	D_FILE	See chapter 27.
S2	16398	400E	DF_CC	Address of PRINT position in display file. Can be poked so that PRINT output is sent elsewhere.
SX2	16400	4010	VARS	See chapter 27. Ptr. to variable area.
SN2	16402	4012	DEST	Address of variable in assignment.
SX2	16404	4014	E_LINE	See chapter 27. Ptr. to workspace.
SX2	16406	4016	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.
S2	16408	4018	X_PTR	Address of the character preceding the  marker. Ptr. to syntax error.
SX2	16410	401A	STKBOT	See chapter 27. Ptr. to calculator stack bottom.
SX2	16412	401C	STKEND	See chapter 27. Ptr. to calculator stack end.
SN1	16414	401E	BERG	Calculator's b register. Used for many different counting purposes
SN2	16415	401F	MEM	Address of area used for calculator's memory. (Usually MEMBOT, but not always.)
S1	16417	4021	not used	Unused by ZX BASIC. Or FLAG Y for G007
SX1	16418	4022	DF_SZ	The number of lines (including one blank line) in the lower part of the screen.
S2	16419	4023	S_TOP	The number of the top program line in automatic listings.
SN2	16421	4025	LAST_K	Shows which keys pressed.
SN1	16423	4027	DEBOUNCE	Debounce status of keyboard.
SN1	16424	4028	MARGIN	Number of blank lines above or below picture: 55 in Britain, 31 in America.
SX2	16425	4029	NXTLIN	Address of next program line to be executed.
S2	16427	402B	OLDPPC	Line number of which CONT jumps.
SN1	16429	402D	FLAGX	Various flags. Bit 0: Reset indicates an arrayed variable Bit 1: Reset indicates a given variable exists Bit 5: Set during INPUT mode Bit 7: Set when INPUT is to be numeric
SN2	16430	402E	STRLEN	Length of string type destination in assignment. Length of a string, or a BASIC line
SN2	16432	4030	T_ADDR	Address of next item in syntax table (very unlikely to be useful). Ptr. to 'parameter table. & distinguishes between PLOT & UNPLOT
S2	16434	4032	SEED	The seed for RND. This is the variable that is set by RAND.
S2	16436	4034	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.
S1	16438	4036	COORDS	x-coordinate of last point PLOTted.
S1	16439	4037		y-coordinate of last point PLOTted.
S1	16440	4038	PR_CC	Less significant byte of address of next position for LPRINT to print as (in PRBUFF).
SX1	16441	4039	S_POSN	Column number for PRINT position.
SX1	16442	403A		Line number for PRINT position.
S1	16443	403B	CDFLAG	Various flags. Bit 7 is on (1) during compute & display mode. Bit 6 - the true fast/slow flag Bit 7 - copy of the fast/slow flag. RESET when FAST needed
S33	16444	403C	PRBUFF	Printer buffer (33rd character is NEWLINE).
SN30	16477	405D	MEMBOT	Calculator's memory area; used to store numbers that cannot conveniently be put on the calculator stack.
S2	16507	407B	not used	Or RESTART to G007
	16509	407D	PROGRAM	The BASIC program starts here

Exercises

1. Try this program

10 FOR N=0 TO 21
20 PRINT PEEK (PEEK 16400+256*PEEK 16401+N)
30 NEXT N

This tells you the first 22 bytes of the variables area: try to match up the control variable N with the description in chapter 27.

2. In the program above, change line 20 to

20 PRINT PEEK (16509+N)

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