BBC BASIC Programmers' Reference

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).