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<concept id="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC" xml:lang="en"><title>Interrupt

Dispatcher</title><shortdesc>An interrupt is a condition that causes the CPU to suspend normal

execution, enter interrupt handling state and jump to a section of code called

an interrupt handler. The ASSP/variant part of the base port must implement

an interrupt dispatcher to manage interrupts. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>An interrupt source is a hardware device or software action that can force

the CPU to enter interrupt handling state. </p>

<p>Typically, a number of interrupt sources are monitored by an interrupt

controller. This is hardware that generates a single interrupt notification

to the CPU, and provides information about which interrupts are pending, i.e.

which interrupts require action to be taken. </p>

<section id="GUID-1FAA26F5-BFB2-55A0-977C-1538EBF3C82A"><title>ISR</title> <p>An

interrupt service routine, or ISR, is code that deals with a pending interrupt.

The Symbian platform kernel responds to an interrupt notification by calling

an ISR for each pending interrupt. The process of calling ISRs is called interrupt

dispatch. </p> <p>The ISR is a single bare function. It is not a class member. </p> <p>Each

ISR takes a single 32-bit parameter that is, typically, a pointer to an owning

class, although it can be any value that is appropriate. The parameter is

defined as a <xref href="GUID-6D079976-9119-31FA-8E21-C3B815F94648.dita"><apiname>TAny</apiname></xref> * type, so a cast is almost always necessary. </p> <p>ISRs

are usually kept in an ISR table. </p> </section>

<section id="GUID-8E58F4C9-0290-55E0-A4FD-B6C2361BE205"><title>Interrupt ID</title> <p>An

interrupt source is identified by number, defined as a <xref href="GUID-7A2A43EC-6125-3BFE-834B-23C37F7B40D5.dita"><apiname>TInt</apiname></xref> type.

Typically, the ASSP layer defines this number for each interrupt in a header

file and exports it so that it can be included and used by device drivers. </p> <p>Where

the ASSP layer is split into a common layer and a variant (device specific)

layer, then the variant layer may also define its own set of interrupt IDs. </p> <p>This

number is usually referred to as the interrupt ID. </p> </section>

<section id="GUID-A8C9C079-D043-5A5F-9F08-CD8656F6702A"><title>Binding and

unbinding</title> <p>Only one ISR can be associated with each possible interrupt

source. Making this association is known as binding. ISRs can be bound and

unbound during normal operation, but only one ISR can be bound to an interrupt

source at any one time. </p> <p>A device driver binds an ISR by calling <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita#GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3/GUID-4E3CB472-3525-32F8-9BC4-8ECFEE931E7B"><apiname>Interrupt::Bind()</apiname></xref>,

passing the interrupt source ID; similarly, the device driver can unbind the

ISR by calling <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita#GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3/GUID-CCC9A397-608C-3EAF-830F-A59800C2E8E5"><apiname>Interrupt::Unbind()</apiname></xref>, also passing the <xref href="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC.dita#GUID-76A30EC4-4B99-5471-9E80-F853C91485BC/GUID-8E58F4C9-0290-55E0-A4FD-B6C2361BE205">interrupt

ID</xref>. </p> </section>

<section id="GUID-DDA62ABB-9CC6-44DC-B08D-FEE5AC505858"><title>Dispatching interrupts</title> <p>At its simplest, this is

the process of deciding which interrupts are pending and calling the ISR for

each. </p> <p>The following pseudo code shows the general principle: </p> <codeblock id="GUID-9C971C66-BB26-5A07-9373-3542B95A16FD" xml:space="preserve">

    {

       FOREVER

              {

        Get next pending interrupt;

        if None

            {

               return;

            }

        call ISR for the pending interrupt;

        }

    }

</codeblock> <p>In practice the dispatcher may have to do some more work to

communicate with the interrupt controller hardware. </p> </section>

<section id="GUID-9026A4AC-57AF-545D-887C-AF43E0B37EDC"><title>Chained interrupts</title> <p>A

system may have multiple interrupt controllers to handle a large number of

interrupt sources. These are usually prioritised by connecting the interrupt

output of a lower-priority controller to an interrupt input of a higher-priority

controller. This is called chaining. </p> <fig id="GUID-49264B94-DF6D-5F11-8815-D42CDBF94E39">

<image href="GUID-0DB79535-E4E6-50BD-852D-B2F177202C9C_d0e365802_href.png" placement="inline"/>

</fig> <p>An interrupt from a lower priority controller will appear as an

interrupt on the highest-priority controller. </p> <p>When the interrupt dispatcher

of the higher-priority controller detects that it is the chained interrupt

that is pending, the usual way of dealing with this is to run a secondary

dispatcher to determine which interrupt on the chained controller is pending. </p> <p>There

may be further levels of chaining before the true source of the interrupt

has been identified. </p> </section>

<section id="GUID-ED6F2F47-7A16-5AE6-8E5B-B2475F6EDEAA"><title>Multiple interrupt

sources and pseudo interrupt sources</title> <p>It is possible that a single

input to an interrupt controller is shared by several interrupt sources. </p> <fig id="GUID-DC96E3A8-9820-5CD4-8020-3B55398388D9">

<image href="GUID-DCBBDFA7-1E6C-5B00-A13E-A25794668E12_d0e365824_href.png" placement="inline"/>

</fig> <p>It appears necessary to bind multiple ISRs to the same interrupt.

However, this is not possible. There are two ways of dealing with this: </p> <ul>

<li id="GUID-0D954444-C2C3-51CC-8E1D-7EB063CDACAA"><p>Maintain a list of all

ISRs that are bound to this single interrupt source, and call all the ISRs

in the list when the interrupt is dispatched. This is most conveniently implemented

by binding a single ISR to the interrupt, which then calls all the real ISRs

bound to this interrupt </p> </li>

<li id="GUID-C5EFE907-26A5-568D-8CF0-DE5E89ED5CBB"><p>Create pseudo interrupts.

These are extra interrupt IDs that do not exist in the interrupt controller,

but represent each of the interrupt sources connected to the single shared

interrupt source. An ISR can then be bound to each pseudo interrupt. The interrupt

dispatcher can then determine which of the sources are actually signalling

and call the appropriate ISR via that pseudo interrupt ID. This is effectively

an implementation of a <xref href="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC.dita#GUID-76A30EC4-4B99-5471-9E80-F853C91485BC/GUID-9026A4AC-57AF-545D-887C-AF43E0B37EDC">chained

interrupt</xref>, and assumes that the interrupt dispatcher can identify which

of the sources is signalling. </p> </li>

</ul> </section>

<section id="GUID-A87DE0F9-2095-5CA6-BE88-3A2EAABB0D33"><title>Interrupts

in the split ASSP/Variant Configuration</title> <p>When a common ASSP extension

is used, a device may have additional peripherals external to the ASSP, and

there needs to be a way of allowing extra interrupt binding and dispatch functions

to be added later by the variant layer. This must be handled by the port as

Symbian OS does not provide any additional API to support this. </p> <p>Device

drivers should be able to use the <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita"><apiname>Interrupt</apiname></xref> class functions

without having to know where the interrupt is actually implemented. This implies

that all requests should go to the core implementation of functions like <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita#GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3/GUID-4E3CB472-3525-32F8-9BC4-8ECFEE931E7B"><apiname>Interrupt::Bind()</apiname></xref>, <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita#GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3/GUID-BB169E6E-D8F9-3762-899D-6DBA4B29CF87"><apiname>Interrupt::Enable()</apiname></xref> etc. </p> <p>To enable the core implementation of these functions to decide

whether an interrupt ID refers to a core interrupt or device specific interrupt,

a common technique is to "tag" the interrupt ID. A simple way is to use positive

numbers to identify core interrupts and negative numbers to identify device

specific interrupts. The ISRs for device specific interrupts are not stored

in the core ISR table, instead the device specific layer provides its own

ISR table. </p> <p>The general pattern for creating the core-device specific

split is that the core derives an implementation from class <xref href="GUID-A83A7C3C-7DC0-3B9C-842F-70FCC751365D.dita"><apiname>Asic</apiname></xref>,

and the device specific part further derives from this core implementation.

The usual technique is to add a set of virtual functions to the core class

that can be derived by the device specific part. The core can provide default

implementations for these functions that would just return <xref href="GUID-0BEA3647-7888-3612-A2D3-7E27AC405E29.dita"><apiname>KErrArgument</apiname></xref> to

trap illegal ID numbers. This API would need functions equivalent to each

of the functions defined by the <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita"><apiname>Interrupt</apiname></xref> class. </p> <p>As

an example, the core layer for the template reference board defines a class <codeph>TemplateAssp</codeph> that

is derived from <xref href="GUID-A83A7C3C-7DC0-3B9C-842F-70FCC751365D.dita"><apiname>Asic</apiname></xref>. <codeph>TemplateAssp</codeph> defines

the pure virtual functions: <codeph>InterruptBind()</codeph>, <codeph>InterruptUnbind()</codeph>, <codeph>InterruptEnable()</codeph> etc,

all with signatures that are the same for the comparable functions defined

by <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita"><apiname>Interrupt</apiname></xref>, and which are implemented by the <codeph>Template</codeph> class. </p> <fig id="GUID-458C7825-5B35-583C-BDF6-7DCD21DAE670">

<image href="GUID-B7E7E6D6-7824-505C-BA0B-B7861897E78F_d0e365918_href.png" placement="inline"/>

</fig> </section>

<section id="GUID-9D98586F-AD1D-5C50-9AD8-F81D72135382"><title>Spurious interrupts</title> <p>In

the Kernel Architecture 2, it is a convention that unbound interrupts should

be bound to a "spurious" interrupt handler, i.e. an interrupt handler that

faults the system indicating the number of the interrupt. This aids debugging

by identifying interrupts that are enabled without corresponding ISRs. </p> </section>

<section id="GUID-109C6250-DC5B-48EC-B1A0-24E2E9731B38"><title>Interrupt priority</title> <p>The interrupt architecture supports

the concept of adjustable interrupt priorities. Symbian platform defines the <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita#GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3/GUID-FA4CFED7-D694-399C-8F84-FA9FE3C3A171"><apiname>Interrupt::SetPriority()</apiname></xref> function

that can implement this. The function is passed the ID of the interrupt source

to be adjusted together with a priority value. The meaning of the priority

value is hardware and implementation dependent, and is defined by the port. </p> </section>

<section id="GUID-77E83634-BBF6-5190-9434-9FB700547CD0"><title>The ISR table</title> <p>The

Variant must provide a table where each entry defines which <xref href="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC.dita#GUID-76A30EC4-4B99-5471-9E80-F853C91485BC/GUID-1FAA26F5-BFB2-55A0-977C-1538EBF3C82A">ISR</xref> is bound to which interrupt source. The table must have enough

space for entries for each interrupt source that is known to the Variant. </p> <p>When

the Variant is split into an ASSP layer and a Variant layer, the ISR table

is put in the ASSP layer and will not normally include ISRs for the Variant

interrupt sources - these will be handled by separate chained dispatchers

in the Variant layer. </p> <p>Symbian platform provides the <xref href="GUID-2C9B6510-D045-3FA1-AD65-B544E30D34C7.dita"><apiname>SInterruptHandler</apiname></xref> structure,

defined in the header file <filepath>...\e32\include\kernel\arm\assp.h</filepath> to

encapsulate the entry for an ISR. The ISR table is, therefore, just an array

of <xref href="GUID-2C9B6510-D045-3FA1-AD65-B544E30D34C7.dita"><apiname>SInterruptHandler</apiname></xref> items. For example, if a system has

32 possible interrupt sources, then the ISR table would be defined as: </p> <codeblock id="GUID-6C95C2EF-A882-565A-8718-07BF7E4A7AC5" xml:space="preserve">...

const TInt KInterruptSourceCount = 32;

SInterruptHandler IsrHandlers[KInterruptSourceCount];

...</codeblock> <p>Interrupts are identified in the system by their interrupt

ID number, which is used to index into the ISR table. You are free to allocate

these numbers any way that is convenient for you. </p> <p>On the template

reference board, for example, the ISR table is defined as a static data member

of the <codeph>VariantASSPInterrupt</codeph> class, which is derived from <xref href="GUID-5E593C59-BA22-3B70-AAFB-BFE19E22538A.dita"><apiname>TemplateInterrupt</apiname></xref>.

The class is defined in <filepath>...\template_assp\template_assp_priv.h</filepath>. </p> <codeblock id="GUID-59D7A629-353C-5117-84B7-15CD425481C6" xml:space="preserve">class TemplateInterrupt : public Interrupt

    {

    ... // functions

public:

    static SInterruptHandler Handlers[KNumTemplateInts];

    ...

    };</codeblock> <p>where KNumTemplateInts is defined in the same header

file. </p> <p><b>Factors

that decide the size of the ISR table</b> </p> <p>The number of entries to

be reserved in the ISR table depends on the following factors: </p> <ul>

<li id="GUID-05AE4B19-AA29-56E4-842A-CC65546EFB54"><p>Where the ASSP is targeted

at only a single device, the number of possible interrupts is usually known,

and the table can include an entry for each one. </p> </li>

<li id="GUID-C62B31BD-FAD7-5A76-9E43-2387FD8AFCC8"><p>If any <xref href="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC.dita#GUID-76A30EC4-4B99-5471-9E80-F853C91485BC/GUID-ED6F2F47-7A16-5AE6-8E5B-B2475F6EDEAA">pseudo sources</xref> exist, they should be included in the main table for

efficiency, but note that this is not strictly necessary. </p> </li>

</ul> <p><b>Other

factors affecting the ISR table</b> </p> <p>IRQs and FIQs may need to be distinguished,

although the exact requirement is hardware dependent. Although the table has

one entry for each possible interrupt source, a possible scheme may be to

group IRQs at the start of the table, and FIQs at the end of the table. If

the hardware has separate interrupt controller hardware for IRQs and FIQs

(or at least, different registers) then you will need to arrange the table

so that you can determine from the <xref href="GUID-76A30EC4-4B99-5471-9E80-F853C91485BC.dita#GUID-76A30EC4-4B99-5471-9E80-F853C91485BC/GUID-8E58F4C9-0290-55E0-A4FD-B6C2361BE205">interrupt

ID</xref> whether the interrupt is an IRQ or FIQ. </p> <p>For example: </p> <fig id="GUID-9DD2CC92-A5DB-5C78-A9A6-64402FF04FE2">

<image href="GUID-60949ACD-AAA9-540E-85AE-BB173382D548_d0e366034_href.png" placement="inline"/>

</fig> </section>

<section id="GUID-EACCBDFD-46CD-4D67-B60C-D705867C9116"><title>Location of interrupt handling code</title> <p>Most of the

interrupt dispatching code is implemented in the ASSP layer. This includes

a list of ISRs, code for adding and removing ISRs, enabling and disabling

interrupt sources, and dispatching ISRs. The kernel only provides a pre-amble

and post-amble. </p> <p>The kernel defines, but does not implement, a class

called <xref href="GUID-E7A7083C-97B9-39B9-A147-4A6E314EE3A3.dita"><apiname>Interrupt</apiname></xref> that exports interrupt functionality to

device drivers and other kernel code. The class provides the public API for

using interrupts (but not for dispatching them). The port must provide an

implementation for each function defined by the class. </p> <p>The class is

defined in the header files <filepath>...\e32\include\kernel\arm\assp.h</filepath>,

which is exported to <filepath>...\epoc32\include\kernel\arm</filepath>. </p><p>See

Symbian OS Internals Book, Chapter 6  - Interrupts and Exceptions</p> </section>