BBC BASIC Programmers' Reference

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=====Interpreter internal variables=====

//by Richard Russell, June 2009//\\ \\  The table below lists the addresses and purposes of a number of memory locations used internally by the //BBC BASIC for Windows// interpreter. Only very rarely will a BASIC programmer be interested in these values, and only experienced programmers with a good understanding of the inner workings of the interpreter should attempt to use them. It will hardly ever be required to write to these locations, and doing so without a full understanding of the consequences is likely to crash the interpreter or make it unstable.\\ \\  The addresses shown in the first column are appropriate when accessed directly from BASIC, but they cannot be used from assembler code. In the latter case the locations must be accessed indirectly, for example instead of **!408** use **[^!408]**.\\ \\ 

^ Address\\ ^ Purpose\\ ^
| !108\\ | Base pointer to linked list of variables etc. starting with **A**\\ |
| !112\\ | Base pointer to linked list of variables etc. starting with **B**\\ |
| !116\\ | Base pointer to linked list of variables etc. starting with **C**\\ |
| !120\\ | Base pointer to linked list of variables etc. starting with **D**\\ |
| !124\\ | Base pointer to linked list of variables etc. starting with **E**\\ |
| !128\\ | Base pointer to linked list of variables etc. starting with **F**\\ |
| !132\\ | Base pointer to linked list of variables etc. starting with **G**\\ |
| !136\\ | Base pointer to linked list of variables etc. starting with **H**\\ |
| !140\\ | Base pointer to linked list of variables etc. starting with **I**\\ |
| !144\\ | Base pointer to linked list of variables etc. starting with **J**\\ |
| !148\\ | Base pointer to linked list of variables etc. starting with **K**\\ |
| !152\\ | Base pointer to linked list of variables etc. starting with **L**\\ |
| !156\\ | Base pointer to linked list of variables etc. starting with **M**\\ |
| !160\\ | Base pointer to linked list of variables etc. starting with **N**\\ |
| !164\\ | Base pointer to linked list of variables etc. starting with **O**\\ |
| !168\\ | Base pointer to linked list of variables etc. starting with **P**\\ |
| !172\\ | Base pointer to linked list of variables etc. starting with **Q**\\ |
| !176\\ | Base pointer to linked list of variables etc. starting with **R**\\ |
| !180\\ | Base pointer to linked list of variables etc. starting with **S**\\ |
| !184\\ | Base pointer to linked list of variables etc. starting with **T**\\ |
| !188\\ | Base pointer to linked list of variables etc. starting with **U**\\ |
| !192\\ | Base pointer to linked list of variables etc. starting with **V**\\ |
| !196\\ | Base pointer to linked list of variables etc. starting with **W**\\ |
| !200\\ | Base pointer to linked list of variables etc. starting with **X**\\ |
| !204\\ | Base pointer to linked list of variables etc. starting with **Y**\\ |
| !208\\ | Base pointer to linked list of variables etc. starting with **Z**\\ |
| !212\\ | Base pointer to linked list of variables etc. starting with **_**\\ |
| !216\\ | Base pointer to linked list of variables etc. starting with **`**\\ |
| !220\\ | Base pointer to linked list of variables etc. starting with **a**\\ |
| !224\\ | Base pointer to linked list of variables etc. starting with **b**\\ |
| !228\\ | Base pointer to linked list of variables etc. starting with **c**\\ |
| !232\\ | Base pointer to linked list of variables etc. starting with **d**\\ |
| !236\\ | Base pointer to linked list of variables etc. starting with **e**\\ |
| !240\\ | Base pointer to linked list of variables etc. starting with **f**\\ |
| !244\\ | Base pointer to linked list of variables etc. starting with **g**\\ |
| !248\\ | Base pointer to linked list of variables etc. starting with **h**\\ |
| !252\\ | Base pointer to linked list of variables etc. starting with **i**\\ |
| !256\\ | Base pointer to linked list of variables etc. starting with **j**\\ |
| !260\\ | Base pointer to linked list of variables etc. starting with **k**\\ |
| !264\\ | Base pointer to linked list of variables etc. starting with **l**\\ |
| !268\\ | Base pointer to linked list of variables etc. starting with **m**\\ |
| !272\\ | Base pointer to linked list of variables etc. starting with **n**\\ |
| !276\\ | Base pointer to linked list of variables etc. starting with **o**\\ |
| !280\\ | Base pointer to linked list of variables etc. starting with **p**\\ |
| !284\\ | Base pointer to linked list of variables etc. starting with **q**\\ |
| !288\\ | Base pointer to linked list of variables etc. starting with **r**\\ |
| !292\\ | Base pointer to linked list of variables etc. starting with **s**\\ |
| !296\\ | Base pointer to linked list of variables etc. starting with **t**\\ |
| !300\\ | Base pointer to linked list of variables etc. starting with **u**\\ |
| !304\\ | Base pointer to linked list of variables etc. starting with **v**\\ |
| !308\\ | Base pointer to linked list of variables etc. starting with **w**\\ |
| !312\\ | Base pointer to linked list of variables etc. starting with **x**\\ |
| !316\\ | Base pointer to linked list of variables etc. starting with **y**\\ |
| !320\\ | Base pointer to linked list of variables etc. starting with **z**\\ |
| !324\\ | Base pointer to linked list of functions (**FN**) \\ |
| !328\\ | Base pointer to linked list of procedures (**PROC**)\\ |
| !332\\ | Pointer to string accumulator (65536 bytes)\\ |
| !336\\ | Pointer to string input and temporary buffer (256 bytes)\\ |
| !340\\ | Current value of **PAGE**\\ |
| !344\\ | Current **TRACE** setting\\ |
| !348\\ | Current value of **LOMEM**\\ |
| !352\\ | Pointer to start of heap free space (**END**)\\ |
| !356\\ | Current value of **HIMEM**\\ |
| !360\\ | Pointer to first installed **LIBRARY** (if any)\\ |
| !364\\ | Pointer to NUL-terminated error string (**REPORT$**)\\ |
| !368\\ | Temporary storage for **ON ERROR LOCAL**\\ |
| !372\\ | Pointer to current **ON ERROR** statement (if any)\\ |
| !376\\ | Current **DATA** pointer (if any)\\ |
| !380\\ | Current value of **COUNT**\\ |
| !384\\ | Pointer to (or within) currently-executing statement\\ |
| !388\\ | Pointer to current **ON TIME** statement (if any)\\ |
| !392\\ | Pointer to current **ON CLOSE** statement (if any)\\ |
| !396\\ | Pointer to current **ON MOVE** statement (if any)\\ |
| !400\\ | Pointer to current **ON SYS** statement (if any)\\ |
| !404\\ | Pointer to current **ON MOUSE** statement (if any)\\ |
| !408\\ | Address at which the most recent error was detected (used to find **ERL**) \\ |
| !412\\ | Current 'random' number, bits 0-31 (**RND**)\\ |
| ?416\\ | Current 'random' number, bit 32 (0 or 1)\\ |
| ?417\\ | Current value of **WIDTH**\\ |
| ?418\\ | Current value of **ERR**\\ |
| ?419\\ | Current ***FLOAT** (bits 0,1) ***HEX** (bit 2) ***LOWERCASE** (bit 3) and **OPT** (bits 4-7) setting\\ |
| \\ | \\ |
| !420\\ | Pointer to temporary path/filename buffer (256 bytes)\\ |
| !424\\ | Pointer to ***KEY** string storage (256 bytes)\\ |
| !428\\ | Pointer to keyboard queue (256 bytes)\\ |
| !432\\ | Pointer to event queue (512 bytes)\\ |
| !436\\ | Pointer to current ***KEY** string expansion\\ |
| !440\\ | Pointer to user-defined character storage (2048 bytes)\\ |
| \\ | \\ |
| !536\\ | ***SPOOL/*SPOOLON** channel number (not 64-bit editions)\\ |
| !540\\ | ***EXEC** channel number (not 64-bit editions)\\ |

In 64-bit editions some interpreter internal variables are moved or absent, notably !336 (pointer to temporary buffer), !368 (temporary storage for ON ERROR LOCAL), !424 (pointer to *KEY storage), !432 (pointer to event queue) and !440 (pointer to user-defined characters). These make way for 64-bit pointers ]332 (pointer to string accumulator), ]364 (pointer to error string), ]420 (pointer to path/filename buffer), ]428 (pointer to keyboard queue) and ]436 (pointer to current *KEY expansion).