The Macintosh Plus or any Macintosh with the 128K ROM's has a built-in
debugger, a very limited tool compared to Macbug or any other advanced
debugger.
Here are the commands:
G - The Go command with no parameter it will start execution at PC.
You can also have execution started anywhere by typing 'G XXXXXX',
where XXXXXX is the address you want it to start at.
DM XXXXXX - will display memory values starting at XXXXXX.
SM XXXXXX XXXX - will set memory location XXXXXX equal to value XXXX.
Note : You can include a string of values and they will
go into sequentially higher memory locations.
PC XXXXXX - will set the current program counter to XXXXXX.
NOTE : Issuing the PC command without a value prints the
current PC Value.
Ax - prints out the current value of Address register x.
Dx - prints out the current value of data register x.
Ax XXXXXX - sets Address register x to value XXXXXX.
Dx XXXXXX - sets Data register x to value XXXXXX.
Here are a few tricks:
To determine which ROM version you are using via the mini-monitor:
1. Press the interrupt switch on the programmer's switch to enter
the built-in mini-monitor.
2. Type DM 400000. If the third byte value displayed is EA, it means
the revised ROMs; if it's EE, it means the original 128K ROMs.
3. Type G to return.
To return to the shell, type 'SM 15000 A9 F4', then 'G 15000'. This will
usually get you back to the Finder.