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 carry names, but do not confuse them with the variables used by the BASIC. You cannot use the names in a BASIC program; they are simply mnemonics that are used to make it easier to refer to the variables.
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)
and to peek its value, use the expression
PEEK n + 256*PEEK (n+1)
| Notes | Address | Name | Contents |
| 1 | 16384 | 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 W n W 14 gives one of the usual reports, 15 W n W 34 or 99 W n W 127 gives a nonstandard report, and 35 W n W 98 is likely to mess up the display file. |
| X1 | 16385 | FLAGS | Various flags to control the BASIC system. |
| X2 | 16386 | ERR_SP | Address of first item on machine stack (after GOSUB returns). |
| 2 | 16388 | 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). |
| N1 | 16390 | MODE | Specifies K, L, F or G cursor. |
| N2 | 16391 | PPC | Line number of statement currently being executed. Poking this has no lasting effect except in the last line of the program. |
| S1 | 16393 | VERSN | 0 identifies 8K ROM in saved programs. |
| S2 | 16394 | E_PPC | Number of current line (with program cursor). |
| SX2 | 16396 | D_FILE | See Chapter 27. |
| S2 | 16398 | DF_CC | Address of PRINT position in display file. Can be poked so that PRINT output is sent elsewhere. |
| SX2 | 16400 | VARS | See Chapter 27. |
| SN2 | 16402 | DEST | Address of variable in assignment. |
| SX2 | 16404 | E_LINE | See Chapter 27. |
| SX2 | 16406 | CH_ADD | Address of the next character to be interpreted: the character after the argument of PEEK, or the ENTER at the end of a POKE statement. |
| S2 | 16408 | X_PTR | Address of the character preceding the marker. |
| SX2 | 16410 | STKBOT | } |
| } See Chapter 27. | |||
| SX2 | 16412 | STKEND | } |
| SN1 | 16414 | BREG | Calculator's b register. |
| SN2 | 16415 | MEM | Address of area used for calculator's memory. (Usually MEMBOT but not always.) |
| S1 | 16417 | not used | |
| SX1 | 16418 | DF_SZ | The number of lines (including one blank line) in the lower part of the screen. |
| S2 | 16419 | S_TOP | The number of the top program line in automatic listings. |
| SN2 | 16421 | LAST_K | Shows which keys pressed |
| SN1 | 16423 | Debounce status of keyboard. | |
| SN1 | 16424 | MARGIN | Number of blank lines above or below picture - 31. |
| SX2 | 16425 | NXTLIN | Address of next program line to be executed. |
| S2 | 16427 | OLDPPC | Line number to which CONT jumps. |
| SN1 | 16429 | FLAGX | Various flags. |
| SN2 | 16430 | STRLEN | Length of string type designation in assignment. |
| SN2 | 16432 | T-ADDR | Address of next item in syntax table (very unlikely to be useful). |
| S2 | 16434 | SEED | The seed for RND. This is the variable that is set by RAND. |
| S2 | 16436 | FRAMES | Counts the frames displayed on the television. Bit 15 is 1. Bits 0 to 14 are decremented for each frame sent to the television. This can be used for timing, but PAUSE also uses it. PAUSE resets bit 15 to 0 and 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 | COORDS | x-coordinate of last pointed PLOTted. |
| S1 | 16439 | y-coordinate of last pointed PLOTted. | |
| S1 | 16440 | PR_CC | Less significant byte of address of next position for LPRINT to print at (in PRBUFF). |
| SX1 | 16441 | S_POSN | Column number for PRINT position. |
| SX1 | 16442 | Line number for PRINT position. | |
| S1 | 16443 | CDFLAG | Various flags. Bit 7 is on (1) during compute and display mode. |
| S33 | 16444 | PRBUFF | Printer buffer (33rd character is ENTER). |
| SN30 | 16477 | MEMBOT | Calculator's memory area; used to store numbers that cannot conveniently be put on the calculator stack. |
| S2 | 16507 | not used |
Source: ZX81 BASIC Programming by Steven Vickers, (c) Sinclair Research Limited.
Many thanks to W. John Guineau and his trusty scanner.