The Fred Fish Collection 1.5/ffcollection-1-5-1992-11.iso/ff_progs/prog-asm/examples.lzh/EXAMPLES/AUDIODEV/AUDIO.DOC

Text File | 1991-08-16 | 6.4 KB | 98 lines

  This is a direct replacement for KickStart's audio device. This replacement
is both smaller and faster than Commodore's official release. When KickStart
is booted, the audio.device is loaded into an area (of the Amiga 1000) which
is thereafter write-protected. It is not possible to overwrite, or delete
the functions of the device (aside from creating a custom KickStart disc). On
the Amiga 2000 and 500, KickStart is already in system ROM and no part of
it (including the audio device) can be altered.
  Exec automatically creates the device's function table and Base structure
during the boot procedure, and links its node into the list of devices
thereby making its existance "known" to the system. Unfortunately, the
replacement device cannot be opened as "audio.device" while the original
exists (even if the version number is different). We could call the device
something besides "audio.device", but then applications would have to be
rewritten to open this new device explicitly. By finding some way of fooling
Exec into thinking that the replacement is the original, all applications
will be directed to this replacement automatically. Calling the function
RemLibrary to remove the original would seem to be the answer. For some
bizarre reason, Exec's OpenDevice "knows" that the audio device's functions
can be found in the protected memory even after the original is removed from
Exec's list of devices. The result is that instead of looking for the
(replacement) device in the devs drawer of the bootdisk (as it should),
Exec always creates a function table for the original functions in the pro-
tected memory. Net result, you can't get rid of the fucker using the standard
(i.e. the official, documented) procedure.
  We need to open the original device, and replace each one of its vectors
in the function table (Open, Close, Expunge, BeginIO, and AbortIO) with the
replacement's vectors (via SetFunction). Furthermore, we need to replace the
assembly language "audio block done" handlers as they are also tighter than
the originals. Finally, we must not allow this new device to be expunged
(i.e. removed when no more tasks have it open). If another task tried to
open the audio device after it had been expunged, Exec would construct the
original device all over again. Stupid? Of course! Many of the lame-brained
bugs in the Amiga operating system are due to the fact that too many C
programmers were allowed to write the operating system. Aside from the speed
penalties of not using assembly language, much of the operating system was
compiled with the Amiga C compiler which had a fatal bug as demonstated in
the following code. This code was extracted directly from the version 33.4
audio device which is what is on KickStart 1.2.

.1 move.l  a2,-(a7)
   bsr.s   someFunction
   clr.b   $1e(a2)
.2 tst.b   $1f(a2) ;test some value for 0
.3 addq.l  #8,a7   ;Oooppps! Someone didn't realize that this affects the Z flag
.4 beq.s   .5      ;Uh oh. We aren't branching as a result of the test at .2

  At label .1, the contents of register a2 are pushed onto the stack. This
is how C code passes values between functions. A call is made to someFunction.
Now, when returning from someFunction, the stack needs to be re-adjusted
because of that previous push. The Amiga C Compiler always adjusts for the
stack only before a branch needs to be taken, or before an rts. In this way,
the stack can be readjusted for several calls in one instruction instead of
after each individual call. This is perfectly good if the adjustment is made
using the lea instruction {as in lea  12(a7),a7}. Unfortunately, whoever
wrote the C compiler also used the addq instruction to make adjustments to
the stack. This instruction affects ALL the status register flags. Note that
this instruction is sandwiched inbetween the tst instruction and the beq.
Obviously, the stack isn't ever going to be zero when we add 8 to it. Conse-
quently, the branch on equal to .5 WILL NEVER BE TAKEN. Since a large part
of the amiga operating system is written in C (particularly Intuition) and
compiled with this compiler, and this original code is still part of the
1.2 operating system, one can understand why mysterious GURU crashes still
occur. Don't necessarily blame the application software.
  Also, the audio device's Expunge routine seems to be completely illogical.
Exec is only allowed to Expunge when the device's OpenCnt (# of tasks which
have it open) is NOT 0! If you open the device and call RemLibrary, Exec
will indeed remove it from under your nose. Imagine that a task has the
audio device open, the amount of free memory is low, and some task makes an
allocation request for a block larger than currently available. Exec will
snake through its list of devices, calling the Expunge routine of each in an
attempt to remove those that are no longer being used. A device's Expunge
routine is supposed to fail (i.e. return 0) if the device is being used, but
in this case, Exec is allowed to remove the audio device and use its memory.
Now what happens when that poor task who thinks that he has the audio device
open tries to call a device function that may have been overwritten? BANG!!!
Something ugly is probably going to happen though its anyone's guess exactly
what. By the way, the audio device's (version 33.4) Expunge routine is
written in C.
  I undertook to rewrite the device in assembly for 2 reasons:
1). A realtime function (i.e. audio output) should be written in the most
    efficient language available. (That's assembly, not C).
2). Someone had to correct the bugs in the C code.
  In order to replace the original device after KickStart is loaded, I wrote
a program called NewAudio. Since this program needs to be executed only
once, a good place to execute it is in the startup-sequence (probably last
after all the dust has settled). This program installs the new functions and
handlers, and assures that the device cannot be expunged under any circumstances.
This results in approximately 3K being dedicated to a permanent audio
device. I think that this is worth having a faster device (most all programs
that feature sound use the audio device instead of manipulating the hardware
directly), and one that won't cause a possible system crash.
  Alternately, after booting the computer, you can double click on the
NewAudio icon. Just be sure that you do this as soon as Workbench comes up
(i.e. before you run any program that uses the audio device). You need only
do this once.

   JEFF GLATT     dissidents