Apr 94 Tips
Volume Number: 10
Issue Number: 4
Column Tag: Tips & Tidbits
Tips & Tidbits 
Edited by Scott Boyd, Editor
Note: Source code files accompanying article are located on MacTech CD-ROM orsource code disks.
Tip Of The Month
MPW C Navigation Time Saver
MPW has a nice feature for C programmers: Using the tool CMarker, you can
make a list of marks for all the functions in a source file.
Unfortunately, when you open the Mark menu, you can wind up with something
unusable. For example, the list could be so long that it takes too much time to find
what you’re looking for. Moreover, you are often looking for a function because you
are in a piece of code in which you see it being called. It’s right there in front of you.
It would be nice to be able to use that piece of context to get you to the function itself.
The following MPW Shell statement sets up a function key which jumps to a
function when you highlight its name and press control-enter (or any other key you
wish).
/* 1 */
setkey control-enter 'find `catenate "{Active}".§`"{Active}"'
Be sure to get all of the quotes right. MPW cares about the different quotes.
Reading from left-to-right, the quotes are straight-, back-, double-, double-, back-,
double-, double-, and single-quotes. The § character is option-6.
- Manuel Carlos Alcoba
Informacion, Integracion, Inteligencia
Madrid, Spain
This column is your opportunity to spread the word about little bits of
information that you find out about. These tidbits can be programming related or they
can be user tips that are particularly useful to programmers.
MacTech Magazine will pay $25 for every tip used, and $50 for the Tip of the
Month. Or you can take your award in orders or subscriptions.
To submit a tip, send in a letter to the magazine. E-mail is our preferred
method, but feel free to send something via the US Mail. See page two for all addresses.
If you do send snail mail, enclose a printed copy and a disk copy of the letter so that it
does not have to be retyped.
INLINE ASSEMBLY IN PASCAL
Knowing how to write inline assembly within your pascal code can sometimes
save you time and trouble. For example, I use an Inline routine that returns the
current stack address for debugging purposes. The routine looks like this:
/* 2 */
INTERFACE
FUNCTION GetStackTop: LongInt; INLINE
$2E8F;
My code calls it just as if it were a regular function:
VAR
stackAddress: LongInt;
...
stackAddress := GetStackTop;
When used properly, Inline assembly provides considerable speed improvements
and allows you to enhance your Pascal code in ways not possible with straight Pascal.
Define your inline routines in the INTERFACE part of a file. In my example, the
hexadecimal number $2E8F represents the instruction MOVE.L A7,(A7). The compiler
places this instruction directly into the code. This reduces the number of instructions
needed to write and compile this routine in assembly language and also avoids calling
overhead for a function. No calling overhead simply means no JSR instructions are
generated in the code. When I was learning how to write inline assembly, the biggest
obstacle was lack of documentation on how to obtain the hexadecimal format (e.g.
$2E8F) of assembly language instructions. You can laboriously try to compose the
hexadecimal instruction from some 68000 assembly books, but this usually takes
several minutes, is error prone, and requires that you know exactly what you are
doing. There is a much easier way to obtain the hexadecimal format of any assembly
language instruction, provided you have MPW assembler. This is what you need to do:
Suppose you want to find the hexadecimal format of the MOVE.L A7,(A7)
instruction.
1) In MPW, create a new file called Example.a and enter the text:
/* 3 */
RoutineName PROC
MOVE.L A7,(A7)
END
2) Create a ‘.o’ file with the MPW ASM tool. In this case I call the file Example.a and
the ASM tool creates an Example.a.o file:
/* 4 */
Asm Example.a
3) Now disassemble the Example.a.o file with the dumpObj MPW tool. The dumpObj
takes whatever is inside a compiled file and disassembles it back into assembly
language instructions. In this case it disassembles the file Example.a.o into a file
called Example.o.a:
/* 5 */
dumpobj 'Example.a.o' > 'Example.o.a'
4) The disassembled file contains the line
/* 6 */
00000000: 2E8F '..' MOVE.L A7,(A7)
The second number on that line, “i.e. 2E8F”, is the hexadecimal format of the
assembly instruction.
From now on, anytime you need a hexadecimal format of an instruction, just add
the instruction to Example.a file and execute the ASM and dumpObj MPW tools. After
you do this a couple of times you will realize this is a simple repetitious process. This
usually takes less than a minute and is much easier and quicker than finding the
instruction from a book.
- Marek Hajek
Champaign, Illinois
Another hex-for-opcode trick
Finding the hex for an instruction can be useful in a number of situations. One
situation I’ve found myself in is sitting in Macsbug on a machine and needing to change
a single instruction. For example, suppose Igot a branch condition backwards, and
want to change the sense of the conditional branch from BLE to BGT. I’d like to change
the instruction with a SM (set memory) Macsbug command, but first I have to figure
out the value to set. I could guess (don’t laugh, I’ve done it before, and even got it right
after just a few guesses on a couple of occasions), but a better way is to let someone do
all the guessing (or table lookups) for me. The fastest tool I’ve found is TMONPro on
some handy nearby machine. Here’s how:
1) Open up an assembly window. Set its address to PlayMem. That’s a chunk of
memory that TMON sets aside when it loads. It’s there for you to play with.
2) Select the first line. It probably reads something like ORI.B #$00,D0.
Regardless, select the whole line.
3) Now type in the instruction you want. It replaces the selection. Now press
return or enter.
4) TMONPro assembles your instruction right then and there.
5) Open a memory window and point it at PlayMem. An easy way to do that is to
command-click on PlayMem’s address over on the left-hand side of the assembly
window. The memory window now shows the hex values for the instruction you
just typed in.
Of course, you might also just use it on the machine you’re debugging. That’s
even faster because now you can simply type over the line of assembly, press enter,
and the code changes in place.
- Ned Flanders
Springfield
Playing a sound asynchronously and continuously
When allocating your channel, pass a callbackproc. When playing the sound using
bufferCmd, immediately play a callbackCmd as well. Your callback will be called when
there is about 50ms of sound left to play, and can queue a new bufferCmd in the same
channel (even though it's at interrupt time).
- Jon W{tte
Sweden
[When asked about the { in his name, Jon replied :
“For reference, it's option-u a, or a-umlaut - ä. However, I'm trying to push
the brace as the next character to be included in the Swedish naming character set :-)”
]