Description: Apple II: Interrupt Handling (2/97)
Header: Apple II: Interrupt Handling (2/97)

 Article Created: 21 September 1984
 Article Reviewed/Updated: 28 February 1997

 TOPIC -----------------------------------------------------------

 This article describes interrupt handling routines in the Apple II.

 DISCUSSION ------------------------------------------------------

 The user should be familiar with the 6502 interrupt requirements as defined
 in the Synertek Programming and Hardware manuals. This article applies to the
 interrupt request (IRQ). The use of the non-maskable interrupt (NMI) in a
 disk system is not recommended. The data and programs on the disk may be
 destroyed if an NMI occurs while the Apple is writing data to the disk. The
 DOS disables IRQ during critical code making it relatively safe to use.

 HARDWARE
 --------

 For interrupts, the Apple peripheral slots have defined three pins:

 IRQ, Interrupt ReQuest          pin 30
 INT IN, daisy chain in          pin 28
 INT OUT, daisy chain out        pin 23

 The daisy chain structure allows an interface card to disable the next higher
 number card from requesting an interrupt. Slot 0 has the highest priority and
 slot 7 has the least.

                                     SLOT

               0            1            2                7
             +--+         +--+         +--+             +--+
             |  |         |  |         |  |             |  |
             |  |         |  |         |  |             |  |
  IRQ -------+--+---------+--+---------+--+--  - - -  --+--+
             |  |         |  |         |  |             |  |
  INT IN     |  |O       I|  |O       I|  |O           I|  |
  and OUT ---|  +---------+  +---------+  +--  - - -  --+  |
             |  |         |  |         |  |             |  |
             |  |         |  |         |  |             |  |
             +--+         +--+         +--+             +--+

 The system was designed so that if INT IN for a slot is low then that slot
 may not generate an interrupt. Each slot should pull INT OUT low when it is
 requesting an interrupt to disable lower priority slots. Cards that don't use
 IRQ should wire INT IN and INT OUT together so that any higher priorty slots
 can still disable cards in lower priority slots. This priorty system fails if
 there is an empty slot between any two interrupting cards.

 The hardware logic required to generate INT OUT is INT IN anded with the
 active low interrupt signal from the peripheral device. IRQ is INT IN anded
 with the active high interrupt signal from the peripheral device.

 SOFTWARE
 --------

 There are two ways to cause the 6502 to follow the IRQ vector. A logical zero
 on the IRQ pin of the 6502 while the IRQ flag of the processor is cleared, or
 executing a BREAK instruction in a program:

 First, the Apple monitor determines whether a BREAK or an IRQ has occured. In
 the Auto start ROM, this routine is at $FA40 and in the old monitor it's at
 $FA86. This routine stores the 6502 accumulator at location $45 and retrieves
 the processor status flags. A BREAK drops into the monitor with the address
 of the BREAK operation code + 2 and a dump of the 6502 registers. The Auto-
 Start ROM has the option of jumping to a user's routine after a break. Both
 monitor ROMs jump to the address contained in memory at $3FE and $3FF after
 an IRQ.

 Interrupt Request
 -----------------

 The user must have the address of his interrupt handler stored in $3FE and
 $3FF before the first interrupt is generated.

 Caution: The accumulator does not contain valid data when it is vectored to
 $3fe and $3FF.

 The accumulator must be restored from location $45 before the return from
 interupt instruction, (RTI) is executed. The user must also be careful to
 leave the other registers as they were when an interrupt occured.

 Interrupts and BASIC
 --------------------

 If the user is careful to restore all the 6502 registers and not disturb
 BASIC's memory locations in the interrupt handling routine, the interrupt
 will be transparent to BASIC. Be very careful of page 0 locations. Save and
 restore any information on the stack when you're not sure.

 Applesoft and Integer BASIC both use the 6502 stack extensively in keeping
 track of GOSUBs and FOR-NEXT loops. This makes it difficult to have an
 interrupt modify BASIC program execution. To do this the easiest way, program
 the interrupt routine to set a flag byte when an event occurs and then
 program the BASIC program to PEEK that flag byte's address and respond when
 the flag byte is set.

 You may use the Applesoft ONERR GOTO statement to modify the execution of an
 Applesoft program when the interrupt occurs. The following machine language
 routine causes an 'error' condition in Applesoft.

 10 POKE 800,162: POKE 801,100: POKE 802,104: POKE 803,104
 20 POKE 804,104: POKE 805,76 : POKE 806,233: POKE 807,242
 30 POKE 1022,0 : POKE 1023,8
 40 ONERR GOTO 1000
 50 PRINT "NO INTERRUPT"
 60 GOTO 50
 1000 IF PEEK (222) <> 100 THEN END
 1010 PRINT "INTERRUPT!!!"
 1020 RESUME

 The POKEs set the IRQ vector to generate an error number 100 when an
 interrupt occurs. The Applesoft onerr routine can check decimal location 222
 and if it doesn't equal 100 then you have a normal Applesoft or DOS error.
 Treat the IRQ generated error like any other Applesoft error. RESUME and the
 routine on page 82 of the Applesoft reference manual will work normally.
 Please do a search on ON ERR GOTO for more information.

 Interrupts and DOS
 ------------------

 The interrupt checking routine in the monitor saves the 6502 accumulator at
 location $45 while it checks for a break. Unfortunately, DOS also uses
 location $45 as temporary storage while DOS parses the numeric parts of its
 commands. This can result in range errors or reading the wrong record, slot,
 or drive if an interrupt occurs during parsing. There is no way around this
 problem at this time. To use interrupts with DOS in the safest way, disable
 IRQ when doing any disk access.


 Article Change History:
 28 Feb 1997 - Reviewed for technical accuracy, revised formatting.
 Copyright 1984-97, Apple Computer, Inc.
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