Editing Macintosh Toolbox (section)

=== 68000 Implementation ===
The Toolbox uses the 68000's illegal instruction exception to implement system calls through '''A-line traps'''—opcodes from $A000-$AFFF that trigger exception vector 10 ($28):

'''Trap Word Format:'''
<pre>
Bits 15-12: 1010 (A-line identifier)
Bit 11:     0=OS trap, 1=Toolbox trap
Bit 10:     Auto-pop flag (Toolbox only)
Bit 9:      Reserved (later: extended trap)
Bits 8-0:   Trap number (0-511 or 0-1023)
</pre>

'''Dispatch Process:'''
# CPU encounters A-line instruction
# Exception 10 triggers, saving PC and SR
# Dispatch routine at ($28) examines trap word
# For Toolbox traps: index into dispatch table at $E00
# For OS traps: index into dispatch table at $400
# Jump to routine address from table
# Routine executes and returns via RTS/RTD

'''Register-Based Parameters:'''
Despite Pascal conventions, many traps use registers for speed:
* '''D0-D2''': Scratch/parameters/results
* '''A0-A1''': Pointers/handles
* '''A2-A4''': Preserved across calls
* '''A5''': QuickDraw globals pointer
* '''A6''': Stack frame (when used)
* '''A7''': Stack pointer

'''Common Trap Examples:'''
* '''$A876''' (_NewPtr): Allocate pointer, size in D0, result in A0
* '''$A322''' (_NewHandle): Allocate handle, size in D0, result in A0  
* '''$A8FE''' (_NewWindow): Parameters on stack, result in A0
* '''$A873''' (_SetPort): GrafPort in A0, no result