Amiga® RKM Libraries: 26 Exec Interrupts

 Introduction            Software Interrupts     Function Reference 
 Servicing Interrupts    Disabling Interrupts 

26 Exec Interrupts / Introduction

Exec manages the decoding, dispatching, and sharing of all system
interrupts.  This includes control of hardware interrupts, software
interrupts, task-relative interrupts (see the discussion of exceptions in
the "Exec Tasks" chapter), and interrupt disabling and enabling.  In
addition, Exec supports a more extended prioritization of interrupts than
that provided in the 68000.

The proper operation of multitasking depends heavily on the consistent
management of the interrupt system.  Task activities are often driven by
intersystem communication that is originated by various interrupts.

 Sequence of Events During an Interrupt 
 Interrupt Priorities 
 Nonmaskable Interrupt 

26 / Introduction / Sequence of Events During an Interrupt

Before useful interrupt handling code can be executed, a considerable
amount of hardware and software activity must occur.  Each interrupt must
propagate through several hardware and software interfaces before
application code is finally dispatched:

  * A hardware device decides to cause an interrupt and sends a signal to
    the interrupt control portions of the 4703 (Paula) custom chip.

  * The 4703 interrupt control logic notices this new signal and performs
    two primary operations.  First, it records that the interrupt has
    been requested by setting a flag bit in the INTREQ register.  Second,
    it examines the INTENA register to determine whether the
    corresponding interrupt and the interrupt master are enabled.  If
    both are enabled, the 4703 generates an interrupt request by placing
    the priority level of the request onto the three 68000 interrupt
    control input lines (IPL0, IPL1, IPL2).

  * These three signals correspond to seven interrupt priority levels in
    the 68000.  If the priority of the new interrupt is greater than the
    current processor priority, an interrupt sequence is initiated. The
    priority level of the new interrupt is used to index into the top
    seven words of the processor address space.  The odd byte (a vector
    number) of the indexed word is fetched and then shifted left by two
    to create an offset into the processor's auto-vector interrupt table.
    The vector offsets used are in the range of $064 to $07C.  These are
    labeled as interrupt autovectors in the 68000 manual.  The
    auto-vector table appears in low memory on a 68000 system, but its
    location for other 68000 family processors is determined by the
    processor's CPU Vector Base Register (VBR).  VBR can be accessed from
    supervisor mode with the MOVEC instruction.

  * The processor then switches into supervisor mode (if it is not
    already in that mode), and saves copies of the status register and
    program counter (PC) onto the top of the system stack (additional
    information may be saved by processors other than the 68000).  The
    processor priority is then raised to the level of the active

  * From the low memory vector address (calculated in step three above),
    a 32-bit autovector address is fetched and loaded into the program
    counter.  This is an entry point into Exec's interrupt dispatcher.

  * Exec must now further decode the interrupt by examining the INTREQ
    and INTENA 4703 chip registers.  Once the active interrupt has been
    determined, Exec indexes into an ExecBase array to fetch the
    interrupt's handler entry point and handler data pointer addresses.

  * Exec now turns control over to the interrupt handler by calling it as
    if it were a subroutine.  This handler may deal with the interrupt
    directly or may propagate control further by invoking interrupt
    server chain processing.

You can see from the above discussion that the interrupt autovectors
should never be altered by the user.  If you wish to provide your own
system interrupt handler, you must use the Exec SetIntVector() function.
You should not change the contents of any autovector location.

Task multiplexing usually occurs as the result of an interrupt.  When an
interrupt has finished and the processor is about to return to user mode,
Exec determines whether task-scheduling attention is required.  If a task
was signaled during interrupt processing, the task scheduler will be
invoked.  Because Exec uses preemptive task scheduling, it can be said
that the interrupt subsystem is the heart of task multiplexing.  If, for
some reason, interrupts do not occur, a task might execute forever because
it cannot be forced to relinquish the CPU.

                    Table 26-1: Interrupts by Priority

    Hardware    Pseodo-
    Priority    Priority   Description                Label   Type
    --------    --------   -----------                -----   ----
             |     1    Serial transmit buffer empty  TBE      H
       1 ----|     2    disk block complete           DSKBLK   H
             |     3    software interrupt            SOFTINT  H
       2 ----|     4    external INT2 & CIAA          PORTS    S
             |     5    graphics coprocessor          COPER    S
       3 ----|     6    vertical blank interval       VERTB    S
             |     7    blitter finished              BLIT     H
             |     8    audio channel 2               AUD2     H
             |     9    audio channel 0               AUD0     H
       4 ----|
             |     10   audio channel 3               AUD3     H
             |     11   audio channel 1               AUD1     H
             |     12   Serial receive buffer full    RBF      H
       5 ----|
             |     13   disk sync pattern found       DSKSYNC  H
             |     14   external INT6 & CIAB          EXTER    S
       6 ----|
             |     15   special (master enable)       INTEN    -
       7 ----|____ --   non-maskable interrupt        NMI      S

26 / Introduction / Interrupt Priorities

Interrupts are prioritized in hardware and software.  The 68000 CPU
priority at which an interrupt executes is determined strictly by
hardware.  In addition to this, the software imposes a finer level of
pseudo-priorities on interrupts with the same CPU priority. These
pseudo-priorities determine the order in which simultaneous interrupts of
the same CPU priority are processed.  Multiple interrupts with the same
CPU priority but a different pseudo-priority will not interrupt one
another.  Interrupts are serviced by either an exclusive handler or by
server chains to which many servers may be attached, as shown in the Type
field of the table.  The table above summarizes all interrupts by

The 8520s (also called CIAs) are Amiga peripheral interface adapter chips
that generate the INT2 and INT6 interrupts.  For more information about
them, see the Amiga Hardware Reference Manual.

As described in the Motorola 68000 programmer's manual, interrupts may
nest only in the direction of higher priority.  Because of the
time-critical nature of many interrupts on the Amiga, the CPU priority
level must never be changed by user or system code. When the system is
running in user mode (multitasking), the CPU priority level must remain
set at zero.  When an interrupt occurs, the CPU priority is raised to the
level appropriate for that interrupt. Lowering the CPU priority would
permit unlimited interrupt recursion on the system stack and would
"short-circuit" the interrupt-priority scheme.

Because it is dangerous on the Amiga to hold off interrupts for any period
of time, higher-level interrupt code must perform its business and exit
promptly.  If it is necessary to perform a time-consuming operation as the
result of a high-priority interrupt, the operation should be deferred
either by posting a software interrupt or by signalling a task.  In this
way, interrupt response time is kept to a minimum.  Software interrupts
are described in a later section.

26 / Introduction / Nonmaskable Interrupt

The 68000 provides a nonmaskable interrupt (NMI) of CPU priority 7.
Although this interrupt cannot be generated by the Amiga hardware itself,
it can be generated on the expansion bus by external hardware.  Because
this interrupt does not pass through the 4703 interrupt controller
circuitry, it is capable of violating system code critical sections.  In
particular, it short-circuits the DISABLE mutual-exclusion mechanism.
Code that uses NMI must not assume that it can access system data

26 Exec Interrupts / Servicing Interrupts

Interrupts are serviced on the Amiga through the use of interrupt handlers
and servers.  An interrupt handler is a system routine that exclusively
handles all processing related to a particular 4703 interrupt.  An
interrupt server is one of possibly many system routines that are invoked
as the result of a single 4703 interrupt.  Interrupt servers provide a
means of interrupt sharing. This concept is useful for general-purpose
interrupts such as vertical blanking.

At system start, Exec designates certain interrupts as handlers and others
as server chains.  The PORTS, COPER, VERTB, EXTER, and NMI interrupts are
initialized as server chains.  Therefore, each of these may execute
multiple interrupt routines per each interrupt.  All other interrupts are
designated as handlers and are always used exclusively.

 Interrupt Data Structure    Interrupt Handlers 
 Environment                 Interrupt Servers 

26 / Servicing Interrupts / Interrupt Data Structure

Interrupt handlers and servers are defined by the Exec Interrupt
structure.  This structure specifies an interrupt routine entry point and
data pointer.  The C definition of this structure is as follows:

    struct Interrupt
        struct Node is_Node;
        APTR        is_Data;
        VOID      (*is_Code)();

Once this structure has been properly initialized, it can be used for
either a handler or a server.

26 / Servicing Interrupts / Environment

Interrupts execute in an environment different from that of tasks. All
interrupts execute in supervisor mode and utilize the single system stack.
This stack is large enough to handle extreme cases of nested interrupts
(of higher priorities).  Interrupt processing has no effect on task stack

All interrupt processing code, both handlers and servers, is invoked as
assembly code subroutines.  Normal assembly code register conventions
dictate that the D0, D1, A0 and A1 registers be free for scratch use.  In
the case of an interrupt handler, some of these registers also contain
data that may be useful to the handler code.  See the section on handlers

Because interrupt processing executes outside the context of most system
activities, certain data structures will not be self-consistent and must
be considered off limits for all practical purposes.  This happens because
certain system operations are not atomic in nature and might be
interrupted only after executing part of an important instruction
sequence.  For example, memory allocation and deallocation routines do not
disable interrupts.  This results in the possibility of interrupting a
memory-related routine.  In such a case, a memory linked list may be
inconsistent during and interrupt. Therefore, interrupt routines must not
use any memory allocation or deallocation functions.

In addition, interrupts may not call any system function which might
allocate memory, wait, manipulate unprotected lists, or modify
ExecBase->ThisTask data (for example Forbid(), Permit(), and mathieee
libraries).  In practice, this means that very few system calls may be
used within interrupt code. The following functions may generally be used
safely within interrupts:

    Alert(), Disable(), Enable(), Signal(), Cause(),
    GetMsg(), PutMsg(), ReplyMsg(), FindPort(), FindTask()

and if you are manipulating your own List structures while in an interrupt:

    AddHead(), AddTail(), RemHead(), RemTail(), FindName()

In addition, certain devices (notably the timer device) specifically allow
limited use of SendIO() and BeginIO() within interrupts.

26 / Servicing Interrupts / Interrupt Handlers

As described above, an interrupt handler is a system routine that
exclusively handles all processing related to a particular 4703 interrupt.
There can only be one handler per 4703 interrupt.  Every interrupt handler
consists of an Interrupt structure (as defined above) and a single
assembly code routine.  Optionally, a data structure pointer may also be
provided.  This is particularly useful for ROM-resident interrupt code.

An interrupt handler is passed control as if it were a subroutine of Exec.
Once the handler has finished its business, it must return to Exec by
executing an RTS (return from subroutine) instruction rather than an RTE
(return from exception) instruction. Interrupt handlers should be kept
very short to minimize service-time overhead and thus minimize the
possibilities of interrupt overruns. As described above, an interrupt
handler has the normal scratch registers at its disposal.  In addition, A5
and A6 are free for use.  These registers are saved by Exec as part of the
interrupt initiation cycle.

For the sake of efficiency, Exec passes certain register parameters to the
handler (see the list below).  These register values may be utilized to
trim a few microseconds off the execution time of a handler.  All of the
following registers (D0/D1/A0/A1/A5/A6) may be used as scratch registers
by an interrupt handler, and need not be restored prior to returning.

    Don't Make Assumptions About Registers.
    Interrupt servers have different register usage rules (see the
    "Interrupt Servers" section).

 Interrupt Handler Register Usage 

26 / / Interrupt Handlers / Interrupt Handler Register Usage

Here are the register conventions for interrupt handlers.

 D0 Contains no valid information.

 D1 Contains the 4703 INTENAR and INTREQR registers values AND'ed
    together.  This results in an indication of which interrupts are
    enabled and active.

 A0 Points to the base address of the Amiga custom chips.  This
    information is useful for performing indexed instruction access to
    the chip registers.

 A1 Points to the data area specified by the is_Data field of the
    Interrupt structure.  Because this pointer is always fetched
    (regardless of whether you use it), it is to your advantage to make
    some use of it.

 A5 Is used as a vector to your interrupt code.

 A6 Points to the Exec library base (SysBase).  You may use this register
    to call Exec functions or set it up as a base register to access your
    own library or device.

Interrupt handlers are established by passing the Exec function
SetIntVector(), your initialized Interrupt structure, and the 4703
interrupt bit number of interest.  The parameters for this function are as

    SetIntVector(ULONG intNumber, struct Interrupt *interrupt)

The first argument is the bit number for which this interrupt server is to
respond (example INTB_VERTB).  The possible bits for interrupts are
defined in <hardware/intbits.h>.  The second argument is the address of an
interrupt server node as described earlier in this chapter.  Keep in mind
that certain interrupts are established as server chains and should not be
accessed as handlers.

The following example demonstrates initialization and installation of an
assembler interrupt handler.  See the "Resources" chapter for more
information on allocating resources, and the "Serial Device" chapter in
the Amiga ROM Kernel Reference Manual: Devices for the more common method
of serial communications.


The assembler interrupt handler code, RBFHandler, reads the complete word
of serial input data from the serial hardware and then separates the
character and flag bytes into separate buffers. When the buffers are full,
the handler signals the main process causing main to print the character
buffer contents, remove the handler, and exit.


    The data structure containing the signal to use, task address
    pointer, and buffers is allocated and initialized in main(), and
    passed to the handler via the is_Data pointer of the Interrupt

26 / Servicing Interrupts / Interrupt Servers

As mentioned above, an interrupt server is one of possibly many system
interrupt routines that are invoked as the result of a single 4703
interrupt. Interrupt servers provide an essential mechanism for interrupt

Interrupt servers must be used for PORTS, COPER, VERTB, EXTER, or NMI
interrupts. For these interrupts, all servers are linked together in a
chain. Every server in the chain will be called in turn as long as the
previous server returned with the processor's Z (zero) flag set.  If you
determine that an interrupt was specifically for your server, you should
return with the processor's Z flag cleared (non-zero condition) so that
the remaining servers on the chain will be skipped.

    Use The Z Flag.
    VERTB (vertical blank) servers should always return with the Z (zero)
    flag set.  The processor Z flag is used rather than the normal
    function convention of returning a result in D0 because it may be
    tested more quickly by Exec upon the server's return.

The easiest way to set the condition code register is to do an immediate
move to the D0 register as follows:

           MOVEQ   #0,D0

           MOVEQ   #1,D0

The same Exec Interrupt structure used for handlers is also used for
servers.  Also, like interrupt handlers, servers must terminate their code
with an RTS instruction.

Interrupt servers are called in priority order.  The priority of a server
is specified in its is_Node.ln_Pri field. Higher-priority servers are
called earlier than lower-priority servers.  Adding and removing interrupt
servers from a particular chain is accomplished with the Exec
AddIntServer() and RemIntServer() functions.  These functions require you
to specify both the 4703 interrupt number and a properly initialized
Interrupt structure.

Servers have different register values passed than handlers do.  A server
cannot count on the D0, D1, A0, or A6 registers containing any useful
information.  However, the highest priority system vertical blank server
currently expects to receive a pointer to the custom chips A0.  Therefore,
if you install a vertical blank server at priority 10 or greater, you must
place custom ($DFF000) in A0 before exiting.  Other than that, a server is
free to use D0-D1 and A0-A1/A5-A6 as scratch.

 Interrupt Server Register Usage 

26 / / Interrupt Servers / Interrupt Server Register Usage

 D0 Scratch.

 D1 Scratch.

 A0 Scratch except in certain cases (see note above).

 A1 Points to the data area specified by the is_Data field of the
    Interrupt structure.  Because this pointer is always fetched
    (regardless of whether you use it), it is to your advantage to make
    some use of it (scratch).

 A5 Points to your interrupt code (scratch).

 A6 Scratch.

In a server chain, the interrupt is cleared automatically by the system.
Having a server clear its interrupt is not recommended and not necessary
(clearing could cause the loss of an interrupt on PORTS or EXTER).

Here is an example of a program to install and remove a low-priority
vertical blank interrupt server:


This is the assembler VertBServer installed by the C example:


26 Exec Interrupts / Software Interrupts

Exec provides a means of generating software interrupts. Software
interrupts execute at a priority higher than that of tasks but lower than
that of hardware interrupts, so they are often used to defer hardware
interrupt processing to a lower priority.  Software interrupts use the
same Interrupt data structure as hardware interrupts.  As described above,
this structure contains pointers to both interrupt code and data, and
should be initialized as node type NT_INTERRUPT (not NT_SOFTINT which is
an internal Exec flag).

A software interrupt is usually activated with the Cause() function.  If
this function is called from a task, the task will be interrupted and the
software interrupt will occur.  If it is called from a hardware interrupt,
the software interrupt will not be processed until the system exits from
its last hardware interrupt.  If a software interrupt occurs from within
another software interrupt, it is not processed until the current one is
completed.  However, individual software interrupts do not nest, and will
not be caused if already running as a software interrupt.

    Don't Trash A6!
    Software interrupts execute in an environment almost identical to
    that of hardware interrupts, and the same restrictions on allowable
    system function calls (as described earlier) apply to both.  Note
    however that, unlike other interrupts, software interrupts must
    preserve A6.

Software interrupts are prioritized.  Unlike interrupt servers, software
interrupts have only five allowable priority levels:  -32, -16, 0, +16,
and +32.  The priority should be put into the ln_Pri field prior to
calling Cause().

Software interrupts can also be generated by message arrival at a
PA_SOFTINT message port.  The applications of this technique are limited
since it is not permissible, with most devices, to send IO requests from
within interrupt code.  However, the timer.device does allow such
interactions, so a self-perpetuating PA_SOFTINT timer port can provide an
application with quite consistent timing under varying multitasking loads.
The following example demonstrates use of a software interrupt and a
PA_SOFTINT port.  See the "Exec Messages and Ports" chapter for more
information about messages and ports.


26 Exec Interrupts / Disabling Interrupts

As mentioned in the "Exec Tasks" chapter, it is sometimes necessary to
disable interrupts when examining or modifying certain shared system data
structures.  However, for proper system operation, interrupts should never
be disabled unless absolutely necessary, and never for more than 250
microseconds.  Interrupt disabling is controlled with the Disable() and
Enable() functions. Although assembler DISABLE and ENABLE macros are
provided, we strongly suggest that you use the system functions rather
than the macros for upwards compatibility and smaller code size.

In some system code, there are nested disabled sections.  Such code
requires that interrupts be disabled with the first Disable() and not
re-enabled until the last Enable().  The system Enable() and Disable()
functions are designed to permit this sort of nesting.

Disable() increments a counter to track how many levels of disable have
been issued.  Only 126 levels of nesting are permitted. Enable()
decrements the counter, and reenables interrupts when the last disable
level has been exited.

26 Exec Interrupts / Function Reference

The following chart gives a brief description of the Exec functions that
control interrupts.  See the Amiga ROM Kernel Reference Manual: Includes
and Autodocs for details about each call.

                   Table 26-2: Exec Interrupt Functions
 |                                                                        |
 | Interrupt Function                Description                          |
 |  AddIntServer()  Add an interrupt server to a system server chain.     |
 |         Cause()  Cause a software interrupt.                           |
 |       Disable()  Disable interrupt processing.                         |
 |        Enable()  Restart system interrupt processing.                  |
 |  RemIntServer()  Remove an interrupt server from a system server chain.|
 |  SetIntVector()  Set a new handler for a system interrupt vector.      |

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