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Tips &Tidbits

By Scott T Boyd, Editor

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