Nov 94 Tips
Volume Number: 10
Issue Number: 11
Column Tag: Tips &Tidbits
Tips &Tidbits 
By Scott T Boyd, Editor
Note: Source code files accompanying article are located on MacTech CD-ROM orsource code disks.
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Tip Of The Month
Trapping Non-Trap Vector Calls
The Debugger has certain limitations in its ability to trace A-Line traps and
such. As an example, suppose we’re interested in breaking on calls to CacheFlush. It’s
a trap vector which is usually referenced by a JSR ([xx]) instead of the usual A-Line
Trap vector. Because it doesn’t go through the normal A-line dispatcher, we can’t
break on references to it with the Trap Intercept mechanism.
Here’s some example code that creates a dummy procedure in an application, and
an initialization proc that patches the low-memory trap vector so that it points to the
dummy procedure. I modify the dummy procedure to make it a JMP to the original
value of the low memory vector [Remember, this is for debugging, not for shipping
code. Shipping code should generally not modify code it’s about to execute - Ed stb].
Finally, the doPatch proc undoes the patch and restores the system to its previous
state.
To use this, set a breakpoint at my_doPatch_Proc. When you drop into your
debugger, look at the stack to see who’s calling. After some looking, you can automate
the process by observing where the interesting return addresses are on the stack with
an action clause (such as the one shown here) that would list them in the -Notes-
window:
?ra := (ra7)^; { return addr is contents of A7 }
{ In the next line we check PC for an address in the Quadra 900 ROM }
if ?pc = 40887824 then ?ra := (ra7+#46+#28)^-2; { # = decimal }
writeln(?ra:ProcPtr); { display the address as a proc name + offset }
resume;
/* 1 */
procedure my_doPatch_Proc; { make it at least 6 bytes long }
begin
end;
procedure doPatch(doit:Boolean);
CONST jCacheFlush = $6f4;
type jmpL = record opc:integer; addr:Longint; end;
VAR
q:^jmpL;
p:^Longint;
begin
p := pointer(jCacheFlush); {$06F4 is the low-mem global
jCacheFlush}
q := @my_doPatch_Proc;
if doit then begin
with q^ do begin
opc := $4EF9; {JMP.L}
addr := p^;
end;
p^ := Ord(q);
end
else begin
p^ := q^.addr; { undo patch }
end;
end;
- Steve Jasik, Menlo Park, CA
Even Better Data Gathering
The September issue’s tip-of-the-month recommended a way to log standard
information during crashes using the EveryTime MacsBug macro. This technique is
particularly useful for beta sites to report bugs, but there’s an even easier way. Many
versions of MacsBug already have a built-in macro called StdLog that does almost the
same thing. Beta-sites can just use stdlog to supply you with information and it doesn’t
require them to muck with their “Debugger Prefs” file.
- Harold Ekstrom, Walnut Creek, CA