Bugs and Tricks

                     BUGS & TRICKS in GFA BASIC

By Corsair of The Giants.

This is an article about stupid BUGS in GFA BASIC. If you want to see a 
little  bit of the power(?) of GFA-BASIC,  please reboot the  intro  of 
this mag. (It's written in it)

There are a lot bugs especially in GFA BASIC 3.6 TT!  Try this in  that 
version (interpreter):  SDPOKE &HFF8241,&H777.  Wham!  Bombs,  no basic 
error, just bombs!
I  advice you to use a lower version of GFA BASIC,  but v3.6 has  extra 
commands. (See Maggie issue V)

Well,  enough  talked  about v3.6,  I shall now give  you  some  useful 
information with some examples about GFA BASIC in general.  Don't blame 
me if I do not talk good English!

The first trick to know is the RESERVE trick.  If you use this command, 
you  can  set  free some memory for using it for  your  own  data  like 
pictures, samples, executing programs etc. Without this command you are 
hopeless! I will give you the old and bad example (don't use this):

RESERVE 30000                 !Reserve all memory exept for 30000 bytes
start%=HIMEM                  !Find start free memory
BLOAD "THING.DAT",start%      !Load data to start%
... Rest of program
RESERVE                       !Set memory back

This  example  works perfectly till the program reaches the  last  line 
(RESERVE) and there appears an error:  ERROR WHILE RESERVE.  GFA  BASIC 
gives  an  error and you have lost all your memory exept  30000  bytes! 
This is the way to prevent it: (use this method!)

mem%=FRE()-30000              !Better way to reserve memory
RESERVE FRE()-mem%            !Reserve it
start%=HIMEM                  !Find start free memory
BLOAD "THING.DAT",start%      !Load data to start%
... Rest of program
RESERVE                       !Wow, no bombs

The  next point is HIMEM.  I'm against it because I've  bad  experience 
with using this command.  The best way is to MALLOC it and later  (when 
leaving  the program) to free it with VOID MFREE(adres%). Something  like 
this:

mem%=FRE()-30000              !Better way to reserve memory
RESERVE FRE()-mem%            !Reserve it
start%=MALLOC(50000)          !"Lock" 50000 bytes memory
BLOAD "THING.DAT",start%      !Load data to start% (max. 50000 bytes)
...  Rest of program
VOID MFREE(start%)                !Set memory free
RESERVE                       

Use always a%=MALLOC(32000) instead of a$=STRING$(32000,0) because when 
a VOID FRE(0) appears (=memory garbage), the pointer  to  a$  could  be 
changed!

Now I will you give my own logbase and physbase routine.  I have  tried 
some variations of this routine but this one works nice:

PROCEDURE init_screens
 RESERVE FRE()-(64512+256)    !Reserve memory (512 bytes waste!)
 screen%=MALLOC(64512)        !Allocate memory
 screen1%=XBIOS(3)            !Save original screen
 screen2%=screen%-(screen% MOD 256)
 screen3%=screen2%+32000
 ' Clean screens:
 scrncln$=STRING$(32000,0)    !I use it only for a while...
 BMOVE V:scrncln$,screen2%    !Clean screen A
 BMOVE V:scrncln$,screen3%    !Clean screen B
 CLR scrncln$                 !Erase scrncln$ (erased after VOID FRE() )
RETURN
PROCEDURE exit_screens
 set_screen(screen1%,screen1%)!Screens back to original (important)
 VOID MFREE(screen%)
 RESERVE
RETURN
PROCEDURE set_screen(log%,phys%)
 VOID XBIOS(5,L:log%,L:phys%,W:-1)
RETURN
PROCEDURE swap_screen
 SWAP screen2%,screen3%
 VOID XBIOS(5,L:screen2%,L:screen3%,W:-1)
RETURN

How to use it:

init_screens                  !Initiale screens
set_screen(screen2%,screen3%) !Work on scr. 2, see scr. 3
PBOX 0,0,100,100              !Wow, a BOX!! (and it's filled)
swap_screen                   !Now we see it
VOID INP(2)                       !Wait for a key
exit_screens                  !Deactivate screens

This was a short example for using this screenrout.  Of course you  can 
do  it  better  but  this is  the  structure.  Never  forget  to  GOSUB 
exit_screens  or  else  you  lose memory and  your  screens  are  still 
activated....

Well,  I stop this article here and hope you can use it.  I'm sure  you 
will  find  more articles in future!  If you have  questions  or  other 
things  (legal!) than you can write to me:

CORSAIR of the (still cool) GIANTS
Wildschutweg 33
1456 AD  Neck
The Netherlands

P.S.: If  people think this text is lame 'coz all is common  knowledge, 
they've  to f*ck off because I try to help people.  If all  the  coders 
learn  from eachother,  you automatically get better  productions.  (Yo 
Bonus, I hope the previous sentence wasn't to rude for the mag)

P.S.  2:  Please use the option to contact me,  I haven't written  this 
text for "the sake of the walls".  Else I'm beginning to feel a  little 
bit angry at the DO-NO-RESPOND-'COZ-IT-COSTS-TOO-MUCH-MONEY-people.  If 
I  receive a few letters (do it!) I will "reveal" some  more  GFA-BASIC 
tricks (vertical rasters? no problem). (I will send them to this mag)