\N2\C2\LN\LF\CT\F6\C3 3/4 OF ALL DEMOS DONT WORK ON ALL AMIGAS\C2\LF\LN\LF\F7\FU\C1by Poison/DEVILS\LF\LF

\F8The Amiga development goes forward. Now faster processors and
new kickstart are very common. Too bad that many coders can
develop themself to program demos that work on most machines.
This is an article which will give you some hints to \C2how to
prevent incompatility:\LF\LF

Operating system v2.0\C1 appeared some months ago, but a lot of demos
still not work, because the majority of coders are lazy, and have
a mentality that consists of making their demos work on the most
current machine (for now) the A500, and not thinking about the
\C2problems that owners of expanded machines will have. \C1Sometimes a
demo which needs 1 MB of memory only looks for the A500 memory
expansion (adress $C00000) without recognizing the 1 MB chip
expansion of A500+ and A2000 (at adress $80000) or extra fast
memory (extensions beginning at $200000).\LF\LF

Sometimes a demo in two (or more) disks is able to load from an
extra floppy drive but only seeks for df1:, and on A2000 (there's
probably more of these machines than you think), the first
external drive is df2:, physical floppy unit TWO, don't ask me
why, but in majority of cases df2:  is not recognized. It's not
dramatic for a demo, but \C2it proves how the land of compatibily is
far!\C1  But all coders don't code "compatible" 'cause they're lazy,
a lot of them never heard about df2:, or stuff like that.\LF\LF

And the problems of compatibility doesn't appear only on A2000,
there's also a lot of productions that crash on big processors
like 68030 & Amigas with "PC bridgeboard" (PC emulator) cards...
\C3It's time to write one time for all what a coder should and
shouldn't do to make his demos work on all machines :\LF\LF


\C2\CTMEMORY\LF\FU\C1\LF
\C3Always test all memory expansion adresses.\C1 You can use your own
routine (you know, putting a byte at expansion adress then testing
if the byte was written and looking if it wasn't written at adress
$0) but I don't think it's a good idea, here's why :\LF\LF

PC Bridgeboard cards need a 128 KB ram to exchange data between PC
side and Amiga one, this memory is not available for exec library
(AllocMem etc.), but it's in Amiga adressing space. The problem
is that on an A2000B, with only 1MB chip memory, this 128 KB ram
is at adress $200000!!  If you test by your own routine the
memory here, the byte will be written ok because there \C2IS\C1 memory,
but your program will think there's here an expansion card of 512
KB or more (because it's the same address!) and in fact only 128
KB are here. All this speech was to advice you to use exec's
routine TypeOfMem(a1= expansion_adress), this function needs of
course the adress of mem to test (in a1), and then returns in d0 :
NULL (0) if there's no mem here (according to exec, so no more
problems with bridgeboard cards), and a value different of 0
(special values!  -see later) if you can use the mem here.
\C3Example of use in his little proggy :\LF\F9\LF

\C1 * TypeOfMem's offset in exec\LF\LF
 TypeOfMem  equ -534\LF\LF

\C1 move.l  4.w,a6        \C2* exec lib\LF
\C1 move.l  #$200000,a4   \C2* test mem\LF
\C1 move.l  a4,a1         \C2* at $200000\LF
\C1 jsr     TypeOfMem(a6) \C2* call func\LF
\C1 tst.l   d0            \C2* d0<>0\LF
\C1 bne     memfound      \C2* =mem found\LF
\C1 move.l  #$C00000,a4   \C2* test mem\LF
\C1 move.l  a4,a1         \C2* at $C00000\LF
\C1 jsr     TypeOfMem(a6) \LF
\C1 tst.l   d0            \C2* idem\LF
\C1 bne     memfound\LF
 ...\LF
\C1 memfound:             \C2* if mem found\LF
\C1 move.l  a4,extramem   \C2* save addr\LF
\C1 ...\LF\LF


\F8Not very complex, and \C2you must do it, if you want
total mem expansion compatibility\C1. Another useful
thing in this method is that TypeOfMem returns you
the... type of memory (you couldn't guess it eh
?) at the adress requested (in d0). So bit 1 of
d0 will be set if expansion is CHIP memory, and
idem for bit 2 if expansion is FAST mem. Of
course these two bits can't be set in the same
time!  (bit 0 of d0 will normally be set, it
indicates that memory is PUBLIC, you don't have to
worry about that). Call TypeOfMem before
destroying exec, of course!  (it's very easy to
get the extra ram adress if there's any, save it
somewhere and then send system to hell). That's
for the memory.\LF\LF

\CT\C2<TURN PAGE>\XX