Apple II
Technical Notes
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                                                  Developer Technical Support


Apple IIGS
#18:    Do-It-Yourself SCC Interrupts

Revised by:    Mike Askins, Matt Deatherage & Jim Mensch        November 1988
Written by:    Mike Askins                                          June 1987

This Technical Note describes how to get the IIGS to pass control to you for 
SCC interrupt handling when you are not using the serial firmware.
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The Apple IIGS serial firmware is a robust environment for almost every serial 
programming application; however, there may be times when you must go around 
the firmware to handle things yourself (despite popular belief, these times 
are the exception rather than the rule).  If you want to handle SCC interrupts 
on the IIGS without using the serial firmware, there are two procedures you 
may follow.

The first method calls your interrupt handling routine instead of the built-in 
serial firmware when an interrupt is generated.  This routine is always the 
second one called when an interrupt occurs (AppleTalk is first) and should 
give your routine plenty of time to handle the interrupt.  You must confirm 
that the serial port was the source of the interrupt.  If so, perform your 
task, clear the carry bit and perform an RTL.  If not, set the carry bit and 
perform an RTL.  You enter your routine in full-native mode with an eight-bit 
accumulator and registers, and you must exit the same way.  Your routine must 
also preserve the data bank register.  Implementing this method is easy.  To 
do so, simply use GetVector and SetVector in the Miscellaneous Tools to save 
the current vector (always a good idea so you can replace it when your 
interrupt handler is removed) and replace it with a vector to your routine.  
The reference number for the SCC interrupt vector is $0009.

The second method calls your interrupt handler last.  It simply tells one of 
the serial ports not to claim a generated interrupt.  Control then passes 
through the entire interrupt handling chain to the user interrupt vector at 
$3FE and $3FF.  The procedure for implementing this method follows:

1.    Set your bank $00 handling address at the usual place in $3FE and 
      $3FF.
2.    Set a system interrupt flag byte, SerFlag.  The ROM version 
      determines the method of finding the address of SerFlag.  In ROM 
      2.0 and later, you can get the address with a call to the 
      Miscellaneous Tools GetAddr using a reference number of $000E.  
      Although you should not need to write a special case for earlier 
      ROM versions, you can do so if you wish; the address of SerFlag in 
      ROM 1.0 is $E10104.  Refer to the Apple II Miscellaneous Technical 
      Note #7 for information on identifying Apple IIGS ROM versions.

      Once you have the correct address of SerFlag, preserve the byte's current 
      value, then turn on the bits in the byte which reflect the port from
      which you will be handling interrupts.  The bits for the different ports
      are as follows:

          Port 1:    ORA    #%00111000
          Port 2:    ORA    #%00000111

      When you are finished handling interrupts from the chosen port (i.e.,
      when you terminate), you should restore the byte to its original value.


Further Reference
o    Apple IIGS Toolbox Reference Manual, Volume 1
o    Apple IIGS Firmware Reference Manual
o    Apple II Miscellaneous Technical Note #7, Apple II Family Identification