## 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