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<concept id="GUID-49379616-C235-598D-AE43-668998AD072B" xml:lang="en"><title>Process,

Thread, Stack and Memory Attributes</title><shortdesc>Reference for users of the debug monitor tool to the attributes

of Kernel objects and memory structure. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<ul>

<li id="GUID-98737E3D-1A08-53ED-89C5-C0398E02C509"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-4D7F6A1F-3120-5547-B027-344C08A01D83">Process and thread priorities</xref>  </p> </li>

<li id="GUID-F08BDD05-90C3-544E-8C41-788A556F799F"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-DDC45F0A-6B62-5EE6-9E42-D21F07BCF4B4">Thread state summary</xref>  </p> </li>

<li id="GUID-5C39B7CB-D8B1-5DD4-BD62-76595A3B2EB4"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-C6296C07-220E-5EB8-9E12-F7D63AB4B181">Thread and process exit information summary</xref>  </p> </li>

<li id="GUID-4F26D9D7-D335-5332-899D-E4DA6D8F4819"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-68CF9014-4922-56B9-9E53-AC5697F60E8E"> Critical threads and processes</xref>  </p> </li>

<li id="GUID-528E13A3-7E35-5153-87D6-C25CD3E90B6E"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-6530D514-A6CC-5AE9-AA97-C223B2189A09">Kernel calls and thread context</xref>  </p> </li>

<li id="GUID-4D0AC539-A105-5D26-896C-E32A2863213B"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-5A573FEB-A274-5C0F-A6B6-87D5BAD8A21C">Stacks</xref>  </p> </li>

<li id="GUID-3A45E908-3D72-5871-9DEB-454F75A084BE"><p> <xref href="GUID-49379616-C235-598D-AE43-668998AD072B.dita#GUID-49379616-C235-598D-AE43-668998AD072B/GUID-E3F102A4-D187-5BC9-BE97-9F8447A5DB45">Virtual memory and run addresses</xref>  </p> </li>

</ul>

<section id="GUID-4D7F6A1F-3120-5547-B027-344C08A01D83"><title>Process and

thread priorities</title> <p>Internally the scheduler always deals with nanokernel

threads, <codeph>NThread</codeph> objects, and their associated priority between

0 (lowest) and 63 (highest). In general, a thread with a higher priority that

is ready to run will always run in preference to threads with a lower priority.

The only exception is where a higher priority thread waits on a nanokernel

fast mutex held by a lower priority thread. In this case, the higher priority

thread will yield to the lower priority thread holding the mutex. </p> <p>A

Symbian platform thread, a <codeph>DThread</codeph> object, has an embedded <codeph>NThread</codeph>,

which enables it to be scheduled by the nanokernel. </p> <p>There are two

ways of setting a priority for Symbian platform thread: </p> <ul>

<li id="GUID-8568544D-49F4-5D52-8E38-28A09058FB04"><p>using the two-level

priority scheme </p> </li>

<li id="GUID-F3D2BAF8-9C95-59C3-A50E-150A7BDA4EE7"><p>using an absolute priority. </p> </li>

</ul> <p><b>The

two level priority scheme</b> </p> <p>In this scheme, a Symbian platform thread

priority is relative to the priority of its owning process. By default, Symbian

platform threads inherit the priority of their owning process when they are

created. This priority can be raised or lowered relative to the process priority

- this just sets the thread’s priority to the process priority plus or minus

a specified priority weighting. If the priority of the process is changed,

the priority of its threads will change relative to other threads in the system

but will remain the same relative to each other. </p> <p>The default priority

of a process is <codeph>EPriorityForgeround</codeph>, which is an absolute

priority of 350. Threads by default are created with relative priority <codeph>EPriorityNormal</codeph> which

sets them to the same priority as the owning process. The window server lowers

the priority of background UI processes to <codeph>EPriorityBackground</codeph> (250). </p> <p>The

NULL thread, also known as the idle thread, runs at priority 0, and means

that it will only run when there are no other threads ready to run. </p> <p>Symbian

platform thread priorities map onto <codeph>NThread</codeph> priorities in

the range 1 to 31 as shown in the table below. </p> <table id="GUID-E61C3B76-B47D-5B9B-A469-A369D5AB4AFC">

<tgroup cols="8"><colspec colname="col0"/><colspec colname="col1"/><colspec colname="col2"/><colspec colname="col3"/><colspec colname="col4"/><colspec colname="col5"/><colspec colname="col6"/><colspec colname="col7"/>

<tbody>

<row>

<entry><p> <b>Thread priority</b>  </p> </entry>

<entry><p>Idle </p> </entry>

<entry><p>Much Less </p> </entry>

<entry><p>Less </p> </entry>

<entry><p>Normal </p> </entry>

<entry><p>More </p> </entry>

<entry><p>Much More </p> </entry>

<entry><p>Real Time </p> </entry>

</row>

<row>

<entry><p> <b>Process priority</b>  </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

<entry><p> </p> </entry>

</row>

<row>

<entry><p>Low </p> </entry>

<entry><p>1</p> </entry>

<entry><p>1</p> </entry>

<entry><p>2</p> </entry>

<entry><p>3</p> </entry>

<entry><p>4</p> </entry>

<entry><p>5</p> </entry>

<entry><p>22 </p> </entry>

</row>

<row>

<entry><p>Background </p> </entry>

<entry><p>3</p> </entry>

<entry><p>5</p> </entry>

<entry><p>6</p> </entry>

<entry><p>7</p> </entry>

<entry><p>8</p> </entry>

<entry><p>9</p> </entry>

<entry><p>22 </p> </entry>

</row>

<row>

<entry><p>Foreground </p> </entry>

<entry><p>3</p> </entry>

<entry><p>10 </p> </entry>

<entry><p>11 </p> </entry>

<entry><p>12 </p> </entry>

<entry><p>13 </p> </entry>

<entry><p>14 </p> </entry>

<entry><p>22 </p> </entry>

</row>

<row>

<entry><p>High </p> </entry>

<entry><p>3</p> </entry>

<entry><p>17 </p> </entry>

<entry><p>18 </p> </entry>

<entry><p>19 </p> </entry>

<entry><p>20 </p> </entry>

<entry><p>22 </p> </entry>

<entry><p>23 </p> </entry>

</row>

<row>

<entry><p>SystemServer1 </p> </entry>

<entry><p>9</p> </entry>

<entry><p>15 </p> </entry>

<entry><p>16 </p> </entry>

<entry><p>21 </p> </entry>

<entry><p>23 </p> </entry>

<entry><p>25 </p> </entry>

<entry><p>28 </p> </entry>

</row>

<row>

<entry><p>SystemServer2 </p> </entry>

<entry><p>9</p> </entry>

<entry><p>15 </p> </entry>

<entry><p>16 </p> </entry>

<entry><p>21 </p> </entry>

<entry><p>23 </p> </entry>

<entry><p>25 </p> </entry>

<entry><p>28 </p> </entry>

</row>

<row>

<entry><p>SystemServer3 </p> </entry>

<entry><p>9</p> </entry>

<entry><p>15 </p> </entry>

<entry><p>16 </p> </entry>

<entry><p>21 </p> </entry>

<entry><p>23 </p> </entry>

<entry><p>25 </p> </entry>

<entry><p>28 </p> </entry>

</row>

<row>

<entry><p>RealTimeServer </p> </entry>

<entry><p>18 </p> </entry>

<entry><p>26 </p> </entry>

<entry><p>27 </p> </entry>

<entry><p>28 </p> </entry>

<entry><p>29 </p> </entry>

<entry><p>30 </p> </entry>

<entry><p>31 </p> </entry>

</row>

</tbody>

</tgroup>

</table> <p>where: </p> <ul>

<li id="GUID-4315CEE2-2717-5E8F-A8F7-885C621A95B6"><p>the process priority

values are defined by the internal Symbian platform enum <codeph>TProcPriority</codeph>,

defined in <filepath>...\e32\include\kernel\kern_priv.h</filepath>. The symbols

in the table correspond to the symbols in the enum. </p> </li>

<li id="GUID-89D0D7FA-4B17-5753-898A-26C1B20EE59B"><p>the thread priority

values are defined by the internal Symbian platform enum <codeph>TThrdPriority</codeph>,

defined in <filepath>...\e32\include\kernel\kern_priv.h</filepath>. The symbols

in the table correspond to the symbols in the enum. </p> </li>

</ul> <p><b>Absolute

priority scheme</b> </p> <p>It is possible to set an absolute priority that

is not relative to the process priority; it is not affected by changes in

the process priority. </p> </section>

<section id="GUID-DDC45F0A-6B62-5EE6-9E42-D21F07BCF4B4"><title>Thread state

summary</title> <p>This is a brief summary about nanokernel thread states

and Symbian platform thread states. </p> <p><b>Nanokernel

thread states</b> </p> <p>The state of a nanokernel thread is referred to

as the NState (or N-state). This is to disambiguate it from any other state,

such as the state of a Symbian platform thread (referred to as the MState

or M-state). </p> <p>The states of a nanokernel thread are defined by the

values of the <xref href="GUID-379D9320-AC3C-3206-8A5D-EE6E5983EBDC.dita#GUID-379D9320-AC3C-3206-8A5D-EE6E5983EBDC/GUID-2184305D-18EF-3322-9276-20F8A4253455"><apiname>NThreadBase::NThreadState</apiname></xref> enumeration. </p> <p><b>Symbian platform thread states</b> </p> <p>The state of a Symbian platform

thread is referred to as the MState (or M_state). This is in addition to the

nanokernel N-state, and tracks threads waiting on Symbian platform synchronization

objects. The <codeph>DThread</codeph> class representing a Symbian platform

thread is internal to Symbian, but the following table defines its possible

states. The values in the left-hand column are the enumerators of the internal

enumeration <codeph>DThread::TThreadState</codeph>. </p> <table id="GUID-C6ACF17D-2E18-5FCB-9383-BAE57D8F420F">

<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

<tbody>

<row>

<entry><p> <codeph>ECreated</codeph>  </p> </entry>

<entry><p>The initial state of all Symbian platform threads. It is a transient

state; the thread starts in this state when the <codeph>DThread</codeph> object

is created, and stays in that state until it is ready to be resumed, typically

when DLL linkage and memory allocation is complete. At this point, the state

will change to <codeph>EReady</codeph>. </p> </entry>

</row>

<row>

<entry><p> <codeph>EDead</codeph>  </p> </entry>

<entry><p>This is the final state of a Symbian platform thread. A thread enters

this state when it reaches the end of its exit handler, just before the nanokernel

terminates it. In effect, the thread has exited but has not yet been deleted. </p> </entry>

</row>

<row>

<entry><p> <codeph>EReady</codeph>  </p> </entry>

<entry><p>This indicates that the thread is not waiting on, or attached to

any Symbian platform kernel wait object. It does not necessarily imply that

the thread is actually ready to run - this is indicated by the N-state. For

example, a thread that is explicitly suspended or waiting on a nanokernel

wait object (generally a fast semaphore) still has a READY M-state provided

that it is not attached to any Symbian platform wait object. </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitSemaphore</codeph>  </p> </entry>

<entry><p>This indicates that the thread is currently blocked waiting for

a Symbian platform semaphore, and is enqueued on the semaphore’s wait queue.

The thread’s <codeph>DThread::iWaitObj</codeph> field points to the semaphore. </p> <p>For

example, this is the case if the thread calls <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita#GUID-C197C9A7-EA05-3F24-9854-542E984C612D/GUID-50223158-D05D-33FE-A3DD-FFA9E2F464FF"><apiname>User::WaitForRequest()</apiname></xref> or <xref href="GUID-AED27A76-3645-3A04-B80D-10473D9C5A27.dita#GUID-AED27A76-3645-3A04-B80D-10473D9C5A27/GUID-68F18434-4758-33BA-9959-1F92A50651A2"><apiname>RSemaphore::Wait()</apiname></xref>  </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitSemaphoreSuspended</codeph>  </p> </entry>

<entry><p>This indicates that the thread has been explicitly suspended after

blocking on a Symbian platform semaphore, and is enqueued on the semaphore’s

suspended queue. The thread’s <codeph>DThread::iWaitObj</codeph> field points

to the semaphore. </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitMutex</codeph>  </p> </entry>

<entry><p>This indicates that the thread is currently blocked waiting for

a Symbian platform mutex, and is enqueued on the mutex wait queue. The thread’s <codeph>DThread::iWaitObj</codeph> field

points to the mutex. </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitMutexSuspended</codeph>  </p> </entry>

<entry><p>This indicates that the thread has been explicitly suspended after

blocking on a Symbian platform mutex, and is enqueued on the mutex suspended

queue. The thread’s <codeph>DThread::iWaitObj</codeph> field points to the

mutex. </p> </entry>

</row>

<row>

<entry><p> <codeph>EHoldMutexPending</codeph>  </p> </entry>

<entry><p>This indicates that the thread has been woken up from the EWaitMutex

state but has not yet claimed the mutex. The thread is enqueued on the mutex

pending queue and the thread’s <codeph>DThread::iWaitObj</codeph> field points

to the mutex. </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitCondVar</codeph>  </p> </entry>

<entry><p>This indicates that the thread is waiting on a condition variable. </p> </entry>

</row>

<row>

<entry><p> <codeph>EWaitCondVarSuspended</codeph>  </p> </entry>

<entry><p>This indicates that the thread is suspended while waiting on a condition

variable. </p> </entry>

</row>

</tbody>

</tgroup>

</table> </section>

<section id="GUID-C6296C07-220E-5EB8-9E12-F7D63AB4B181"><title>Thread and

process exit information summary</title> <p>User threads and processes have

“exit information”. When a thread or process terminates the reason for the

termination is found in the exit information. For example, a panic will store

the panic category and reason in the exit information. Exit information has

three parts: the exit type, exit reason and exit category. </p> <p>Exit type

is defined by the <xref href="GUID-0296BFC6-7F7C-3259-AF21-7E9B5C304B24.dita"><apiname>TExitType</apiname></xref> enum. </p> <p>When a thread

or process is created, its exit type is set to 3. An exit type of 3 indicates

that the thread is still active, though not necessarily running. If the thread

terminates for any reason, then the exit type is changed to reflect the cause

of the exit. </p> <p>Once the thread or process has exited, the exit reason

and exit type fields will contain useful information. The contents depends

on the type of exit. </p> <p>Note that if the main thread in a process exits,

then the process will exit with the same exit information as the thread. </p> <p><b>Exit category: Terminate</b> </p> <p>if <xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita#GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5/GUID-CE11A762-AF52-3122-86C8-C2F362AEF764"><apiname>RThread::Terminate()</apiname></xref> or <xref href="GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695.dita#GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695/GUID-3DD41FD4-F389-340F-8E18-8FDEBC953251"><apiname>RProcess::Terminate()</apiname></xref> is

called, then the exit category is <codeph>Terminate</codeph>, and the exit

reason is the value of the <codeph>aReason</codeph> argument passed to these

functions. </p> <p><b>Exit

category: Kill</b> </p> <p>If <xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita#GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5/GUID-1F717486-784A-32B9-A048-EE4F2450F8C8"><apiname>RThread::Kill()</apiname></xref> or <xref href="GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695.dita#GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695/GUID-8E87D3AE-E3F0-34DD-98C6-71B0D0D55FB8"><apiname>RProcess::Kill()</apiname></xref> is

called, then the exit category is <codeph>Kill</codeph>, and the exit reason

is the value of the <codeph>aReason</codeph> argument passed to these functions. </p> <p><b>Exit category: panic</b> </p> <p>If a thread panics, then the exit category

is <codeph>panic</codeph>, and the exit reason is the panic number. For example

a USER-19 panic would give the following exit information: </p> <codeblock id="GUID-ACC94269-E06E-5C7F-ACAF-74764FE21A0F" xml:space="preserve">exit type = 2

exit category = “USER”

exit reason = 19

</codeblock> </section>

<section id="GUID-68CF9014-4922-56B9-9E53-AC5697F60E8E"><title> Critical threads

and processes</title> <p>Marking a thread or process as “system critical”

means that it is an integral and essential part of the system, for example,

the file server. In effect the thread or process is being declared necessary

for correct functioning of the device. If a system critical thread exits or

panics then the device will reboot; during development it will enter the debug

monitor. A thread can be set as process critical, which means that if it panics

the process will be panicked. </p> </section>

<section id="GUID-6530D514-A6CC-5AE9-AA97-C223B2189A09"><title> Kernel calls

and thread context</title> <p>When a user thread makes a call into any kernel

code, the kernel code continues to run in the context of the user thread.

This applies to device driver code. </p> <p>The stack is swapped to a kernel-side

stack and the permissions of the thread are increased to kernel privilege,

but otherwise the user thread is still running. Each thread has a small kernel

stack used to handle kernel calls – it would be dangerous to continue using

the normal thread stack in case it overflows. Some calls are handled in this

state, others – typically device drivers – will post a message to a kernel

side thread to carry out the request. </p> </section>

<section id="GUID-5A573FEB-A274-5C0F-A6B6-87D5BAD8A21C"><title>Stacks</title> <p>When

a process is created, a chunk is allocated to hold the process executable's <codeph>.data</codeph> section

(initialised data) and <codeph>.bss</codeph> section (zero filled data). Sufficient

space (default 2Mb) is also reserved as user-side stack space for threads

that run in that process. </p> <p>By default, each thread is allocated 8k

of user-side stack space. A guard of 8k is also allocated. </p> <p>The stack

area follows the <codeph>.data</codeph> and <codeph>.bss</codeph> sections,

and each thread's user side stack follows. On ARM processors the stack is

descending, so that as items are added to the stack, the stack pointer is

decremented. This means that if the stack overflows, the stack pointer points

into the guard area and causes a processor exception, with the result that

the kernel panics the thread. </p> <fig id="GUID-C87444AA-A8DC-5A5C-B2E6-B15FA69B6CE1">

<image href="GUID-DD0DA06D-4180-54F1-8807-A7BF31D6A1F1_d0e297526_href.png" placement="inline"/>

</fig> <p>Return addresses are stored by pushing them on to the stack so at

any point you can trace through the stack looking at the saved return addresses

to see the chain of function calls up to the present function. </p> <p>The

size of the user-side stack space has an indirect effect on the number of

threads that a process can have. There are other factors involved, but this

is an important one. The limit is a consequence of the fact that a process

can have a maximum of 16 chunks. This means that if threads within a process

can share a heap (allocated from a single chunk), then it is possible to have

a maximum of 128 threads per process [2Mb/(8K + 8K)]. More threads may be

possible if you allow only 4K of stack per thread. </p> <p>Apart from the

kernel stack attached to each thread, the kernel also maintains stacks that

are used during processing of interrupts, exceptions and certain CPU states.

Interrupts and exceptions can occur at any time, with the system in any state,

and it would be dangerous to allow them to use the current stack which may

not even be valid or may overflow and panic the kernel. The kernel stacks

are guaranteed to be large enough for all interrupt and exception processing. </p> </section>

<section id="GUID-E3F102A4-D187-5BC9-BE97-9F8447A5DB45"><title>Virtual memory

and run addresses</title> <p>Symbian platform devices have an MMU which is

used to map the addresses seen by running code to real addresses of memory

and I/O. The MMU in effect creates a virtual memory map, allowing scattered

blocks of RAM to appear contiguous, or for a section of memory to appear at

different addresses in different processes, or not at all. </p> <p>Symbian

platform uses the MMU to provide memory protection between processes, to allow

sharing of memory, efficient allocation of RAM and to make all processes “see”

the same memory layout. Three different memory models are supported by Symbian

platform on ARM CPUs: </p> <ul>

<li id="GUID-456A3D12-E1E0-5289-B5EB-2E3A47F07684"><p>moving model: this is

the model familiar from EKA1 where processes are moved to a run-address in

low memory when executing and moved back to a home-address in high memory

when not running. </p> </li>

<li id="GUID-3A5BCC68-7BAC-58B8-9727-54A880A7CED3"><p>direct model: this is

used when the CPU does not have an MMU, or is emulating a system without an

MMU. Not normally used, but occasionally useful for development boards </p> </li>

<li id="GUID-7CB07322-B33E-5723-975C-6D2442B924F9"><p>multiple model: only

supported in ARM architecture V6 and above, each process has its own set of

MMU tables. A context switch changes the current MMU table to the new thread’s

table, instead of moving memory about in a single table as with moving model. </p> </li>

</ul> <p><b>Fixed

processes</b> </p> <p>For ARM architectures with a virtually-tagged cache,

fixed processes avoid the need to flush the cache on context switches by keeping

all the code and data at a fixed address. This implies that there can only

ever be one instance of each fixed process because the data chunk address

cannot be changed. </p> <p>Important servers such as the file server and window

server are fixed. </p> <p>There is no limit to the number of fixed processes

that can be supported. The kernel will attempt to use ARM domains for fixed

process protection, but there are a limited number of domains so when they

are exhausted normal MMU techniques will be used. Domains are slightly faster

in a context switch but this is negligible compared to the real purpose of

the fixed process in avoiding the cache flush. </p> </section>