The System Variables - Sinclair ZX81 User Manual

Chapter 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, & some of them can be usefully poked. They are listed
here with their uses.
These are called system variables, & 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, & 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
1 16384
X1 16385
X2 16386
2 16388
N1 16390
N2 16391
S1 16393
S2 16394
SX2 16396
S2 16398
SX2 16400
SN2 16402
SX2 16404
SX2 16406
S2 16408
SX2 16410
SX2 16412
SN1 16414
Name Contents
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
127 gives a non-standard report, and 35
the display file.
FLAGS Various flags to control the BASIC system.
ERR_SP Address of first item on machine stack (after GOSUB returns).
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.
MODE Specified K, L, F or G cursor.
PPC Line number of statement currently being executed. Poking this has no
lasting efect except in the last line of the program.
VERSN 0 Identifies ZX81 BASIC in saved programs.
E_PPC Number of current line (with program cursor).
D_FILE See chapter 27.
DF_CC Address of PRINT position in display file. Can be poked so that PRINT
output is sent elsewhere.
VARS See capter 27.
DEST Address of variable in assignment.
E_LINE See chapter 27.
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.
X_PTR Address of the character preceding the
STKBOT See chapter 27.
STKEND See chapter 27.
BERG Calculator's b register.
n 14 gives one of the usual reports, 15 n 34 or 99
n 98 is liable to mess up
marker.
n