ultima_patch.s

  org $300
  clc
  xce
  rep #$30   ; full 16 bit registers
  pea #$000B ; push literal "000B" on stack
  ldx #$2303 ; set toolcall
  jsl E10000 ; call tool "03" = Misc. Tools, function "23" = IntSource
  
* We have just made a call to the IIgs toolbox routine called IntSource.
* IntSource enables or disables interrupts based on the bit pattern we 
* pushed on the stack.  (000B)  This is called the srcRefNum
* To see what a srcRefNum of 000B indicates we have to look at Toolbox
* Reference Volume I.  I won't put the whole table here, but I will
* tell you that 000B is adbDisable, which disables ADB Data Interrupts.
* That matches what we were told about this patch disabling an interrupt
* to avoid conflict with Mockingboard/Phasor playback. 
* Let's continue.

  pea #$0004 ; push literal "0004" on the stack
  pea #$0000 ; push literal "0000" on the stack
  pea #$0300 ; push literal "0300" on the stack
  ldx #$1003 ; set toolcal
  jsl E10000 ; call tool "03" = Misc. Tools, function "10" = SetVector

* This makes a second tool call to the SetVector routine.
* While it looks like we push 3 parameters on the stack, it's actually 2.
*   parameter 1 (word) vectorRefNum      0004 = Interrupt Vector (makes sense)
*   parameter 2 (long)    vectorPtr  00000300 = Pointer to the handler

  sec
  xce
  sep #$30  ; turn off 16-bit
  rts       ; so we can safely return to BASIC