Section 2 - Commodore Computers

164 COMPUTE!
164 COMPUTE!
May.1982,I,,"e24
May, Issue 24
dynamically from BASIC via POKEs, etc. Note
dynamically from BASIC via POKEs, etc. Note
that Easykey. Easykey, as given here, reserves over 450 bytes
for soft strings. Note also the addresses of the
labels "STABLE" and "STRINGS." lGS." I Iff you assemble
your ownn copy of Easykey, be sure and note the
addresses of these two labels and convert them to
decimal if you intend to use dynamic soft keys.
decimal if you intend to use d )'namic soft keys.
...if.if you control these seven
pOinters, pointers, you control BASIC...
Program 2. 2, thee BASK' BASIC program, is a sample
which will allow )'UU you to redefine all 26 soft keys to
any string )'o u like and th en save the res ulting
any string you like and then save the resulting
definitions in a disk file e for later use. Study the
technique, and you )'ou should be able to produce an)' any
kind of soft keys you might want. The program
kind of soft ke)'s yo u might wa nt. The program
fragmcnt (Program 3) will allow the reloading of
predefin ed sets produced b)' the previous program.
fragment (Program 3) will allow the reloading of
predefined sets produced by the previous program.
Inside Atari Afari BASIC: Part 4
This month we will feature a short discussion of
the various "tallies" "tables" used by b)' Atari BASK' IC and how
to find them. thcm. Some of this material is well covered
by b)' some of the articles in COMPUTE!'* COMPUTE!'.I First Book Bool< of
Atari, Alm-;, so if you YO ll arc are too lOO impatient to wait for next
month )'ou ca n run out and bu)' the book. Next
month you can run out and buy the book. Next
monthm we will begin to use the information we
discover this month to "fool" BASIG IC into letting lelling us
do things it was never designed for. We begin:
do things it was ncver des igned for. Wc begin:
When an Atari BASK; IC program is SAVEd to
disk oro r cassette, there are only 14 bytes of zero
page written wrillen out along with the main tables and
program. TheseT 14 bytes consist of seven two-byte two-b),tc
pointers {in (in the traditional low-byte, high-byte
form) which tell BASIC where everything ever),thing is in the
particular program being SAVEd (or later being
LOADed). All the other important zero page locations
(and there are arc over ovcr 50 of them) are arc regenerated
and/orjecalculated and/Ol',recalcu lated by b)' BASIC anytime you )'ou
type t)' NEWN E\'" or SAVE (or, for r some locations, RUN,
,
GOTO, GOTO. etc.). The implication is that, if you )'ou control
thesc seven poilll.ers, )'ou control BASIC so let's
these seven pointers, you control BASK! so let's
examine e their names and functions. fUllctions. Table 1I givesg a
summary thereof.
e reof',
TheT first thing you )'ou may note about this table is
that some of the locations (indicated by asterisks)
have duplicate labels. If you examine the mnemonic
have duplicate labels. If you examine the mnemonic
meanings, you will probabl)' probably see why: the pointer
can mean different things in different contexts.
can mean different things in different contexts.
Forr example, the space pointed to b)' by location n $80
(decimal 128) is used for different purposes, de
(decimal 128) is used fo r different purposes, de­
pendingg on whether BASIC is currentlyy working
on entering a new line (it uses OUTBUFF) or
executing an expression within a program (whenn it
uses ARGOPS).
Youu might also note that I provided a list of
more than seven pointers. The locations $8EE and
$90 are not SA VEd and reLOADed because the),
$90 are not SAVEd and reLOADed because they
are always dependent onn the currcnt current state of the
program (i.e., whether it is RUNning, whether it
has executed a DIM statement, etc.). The), They are
included here for completeness: aside from the
zero page locations (and the $600 page locations
with BASIC A + ), these pointers completely ), definee
BASIC'S IC's usage of the Atari computer's memory r),
space. So now let's go into detail about what each
of these pointers is used for.
Table I: BASIC's Critical Zero-Page Pointers
Table 1: BASIC'S Critical Zero-Page Pointers
Location .Mnemonic Which means:
Hex
80 80
Decimal 128
128*
*
Label,
Label:
LOMEN ARGOPS
pointer to LOw MEMory limit
A ARGument/OPeratorStack
RCume ntlOPc 80 128* · OUTBUFF syntax OUTput BUFFer
82
84 86 88 8A 8C BC
8E BE
8E BE 130 132 134
136* 136· 138 140 142
142*
2 ·
VNTP VNTD
VVTP STMTAB STMCUR
STARP ENDSTAR RUNSTK
Variable Name Table Pointer
Variable Name Table Dummy end
Variable Value Table Pointer
STateMentTABIe TABle
CURrentSTateMent pointerr
STring/ARray Pointer
END D STringl STring/ARray A space
RUNtime STacK pointer
90 144 TOPRSTK TOP of Run lime STacKspace
90 144 TOPRSTK TOP of Runtime STacK space
90 144*
· MEMTOP pointer MEMorv ory TOP limit
We already noted that ARGOPS is used in
We already noted that ARGOPS is used in
expression evaluation. ation. That is, whenever BASIC
sees any an)' kind of expression n to be evaluated ated [e.g.,
,
3*A A + B or SIN(30) or 2"(LOG(4/EXP(Y*Z*3))-I/
2' (LOG(4/EXP(Y*Z'3))- 11
(Z"2.5 (Z' +ATN(Z)) + or even 1.25], it must put intermediate
results and/or operatorso ono n a "stack."
"
ARGOPS points to a 256 byte area reserved for
both the argument stack and the e operator stack.
(What actually ), happens in Execute Executc Expression is
extremely extremel), complex and far beyond be)'ond the scope of
this article.) anicle.) Since expression evaluation and program
entry cannot occur at allhe the same time, liln OUT-
BUFF shares this same 256 byte space. When a
B FF shares this sa me 256 b)'te spacc. When a
program line is entered, , BASIC C checks it for syntax
and converts it to internal tokens, pl ac in g these
and converts it to internal tokens, placing these
tokens temporarily y into this 256 byte buffer (before
moving them into the appropriate place in the