<sect4>

	<sect4>

	  <title>Preparing the Merge Target</title>

	  <title>Preparing the Merge Target</title>

	    earlier, it is very important to never merge changes

	    earlier, it is very important to never merge changes

	    into a sparse working copy.  Always use a full

	    into a sparse working copy.  Always use a full

	    checkout of the branch being merged into.  For instance,

	    checkout of the branch being merged into.  For instance,

      thinking, the missing historical context, the ambiguous

      thinking, the missing historical context, the ambiguous

      standards - and the style(9) transgressions.</para>

      standards - and the style(9) transgressions.</para>

      the history of the FreeBSD project, the commit logs hide the

      the history of the FreeBSD project, the commit logs hide the

      true scale of his impact until you pay attention to

      true scale of his impact until you pay attention to

      "Submitted by", "Reviewed by" and "Pointed out by".</para>

      "Submitted by", "Reviewed by" and "Pointed out by".</para>

      <para>Unfortunately, as of the time this was written &os; did

      <para>Unfortunately, as of the time this was written &os; did

	not support changing user passwords with &man.passwd.1;.

	not support changing user passwords with &man.passwd.1;.

	solution themselves.  I provide some examples here.  Note that

	solution themselves.  I provide some examples here.  Note that

	if you write your own password change script, there are some

	if you write your own password change script, there are some

	security issues you should be made aware of; see <xref

	security issues you should be made aware of; see <xref

	    different API(es).  The M:N library uses the

	    different API(es).  The M:N library uses the

	    <literal>kse_*</literal> family of syscalls while the 1:1

	    <literal>kse_*</literal> family of syscalls while the 1:1

	    library uses the <literal>thr_*</literal> family of

	    library uses the <literal>thr_*</literal> family of

	    thread ID shared between kernel and userspace.  Of course,

	    thread ID shared between kernel and userspace.  Of course,

	    both threading libraries implement the pthread thread ID

	    both threading libraries implement the pthread thread ID

	    API.  Every kernel thread (as described by <literal>struct

	    API.  Every kernel thread (as described by <literal>struct

      <sect3 xml:id="pid-mangling">

      <sect3 xml:id="pid-mangling">

	<title>PID mangling</title>

	<title>PID mangling</title>

	  process ID and thread ID is between &os; and &linux; we have

	  process ID and thread ID is between &os; and &linux; we have

	  to translate the view somehow.  We do it by PID mangling.

	  to translate the view somehow.  We do it by PID mangling.

	  This means that we fake what a PID (=TGID) and TID (=PID) is

	  This means that we fake what a PID (=TGID) and TID (=PID) is

	  <literal>linux_emuldata_shared</literal>.  The

	  <literal>linux_emuldata_shared</literal>.  The

	  <literal>emul_lock</literal> is a nonsleepable blocking

	  <literal>emul_lock</literal> is a nonsleepable blocking

	  mutex while <literal>emul_shared_lock</literal> is a

	  mutex while <literal>emul_shared_lock</literal> is a

	  the per-subsystem locking we can coalesce some locks and

	  the per-subsystem locking we can coalesce some locks and

	  that is why the em find offers the non-locking

	  that is why the em find offers the non-locking

	  access.</para>

	  access.</para>

	<para>Threaded programs should be written with as little

	<para>Threaded programs should be written with as little

	  contention on locks as possible.  Otherwise, instead of

	  contention on locks as possible.  Otherwise, instead of

	  of this, the most well written threaded programs show little

	  of this, the most well written threaded programs show little

	  locks contention.</para>

	  locks contention.</para>

      </sect3>

      </sect3>

	  for the new word can be sent as soon as the Stop Bit for the

	  for the new word can be sent as soon as the Stop Bit for the

	  previous word has been sent.</para>

	  previous word has been sent.</para>

	  synchronizing</quote>, if there is no data to transmit, the

	  synchronizing</quote>, if there is no data to transmit, the

	  transmission line can be idle.</para>

	  transmission line can be idle.</para>

      </sect2>

      </sect2>

	<title>Bits, Baud and Symbols</title>

	<title>Bits, Baud and Symbols</title>

	<para>Baud is a measurement of transmission speed in

	<para>Baud is a measurement of transmission speed in

	  communication technology, this term is frequently misused

	  communication technology, this term is frequently misused

	  when describing the data rates in newer devices.</para>

	  when describing the data rates in newer devices.</para>

	  DCE speed because of the use of compression by the

	  DCE speed because of the use of compression by the

	  modems.</para>

	  modems.</para>

	  varied during the trip between the two machines plus the

	  varied during the trip between the two machines plus the

	  differing bits-per-seconds speeds that are used present on

	  differing bits-per-seconds speeds that are used present on

	  the DTE-DCE and DCE-DCE links, the usage of the term Baud to

	  the DTE-DCE and DCE-DCE links, the usage of the term Baud to

		  technology with various functional flaws

		  technology with various functional flaws

		  corrected. The INS8250A was used initially in PC

		  corrected. The INS8250A was used initially in PC

		  clone computers by vendors who used

		  clone computers by vendors who used

		  corrections in the chip, this part could not be used

		  corrections in the chip, this part could not be used

		  with a BIOS compatible with the INS8250 or

		  with a BIOS compatible with the INS8250 or

		  INS8250B.</para>

		  INS8250B.</para>

	<para>In internal modems, the modem designer will frequently

	<para>In internal modems, the modem designer will frequently

	  emulate the 8250A/16450 with the modem microprocessor, and

	  emulate the 8250A/16450 with the modem microprocessor, and

	  the emulated UART will frequently have a hidden buffer

	  the emulated UART will frequently have a hidden buffer

	  the buffer, these emulations can be as reliable as a 16550A

	  the buffer, these emulations can be as reliable as a 16550A

	  in their ability to handle high speed data.  However, most

	  in their ability to handle high speed data.  However, most

	  operating systems will still report that the UART is only a

	  operating systems will still report that the UART is only a

	<para>When the NS16550 was developed, the National

	<para>When the NS16550 was developed, the National

	  Semiconductor obtained several patents on the design and

	  Semiconductor obtained several patents on the design and

	  they also limited licensing, making it harder for other

	  they also limited licensing, making it harder for other

	  the patents, reverse-engineered designs and emulations had

	  the patents, reverse-engineered designs and emulations had

	  to avoid infringing the claims covered by the patents.

	  to avoid infringing the claims covered by the patents.

	  Subsequently, these copies almost never perform exactly the

	  Subsequently, these copies almost never perform exactly the

	  TI, StarTech, and CMD as well as megacells and emulations

	  TI, StarTech, and CMD as well as megacells and emulations

	  embedded in internal modems were tested with COMTEST.  A

	  embedded in internal modems were tested with COMTEST.  A

	  difference count for some of these components is listed

	  difference count for some of these components is listed

	  not reflect the current performance of the given product

	  not reflect the current performance of the given product

	  from a vendor.</para>

	  from a vendor.</para>

	    produce intelligent serial communication boards.  This type of

	    produce intelligent serial communication boards.  This type of

	    design usually provides a microprocessor that interfaces with

	    design usually provides a microprocessor that interfaces with

	    several UARTs, processes and buffers the data, and then alerts the

	    several UARTs, processes and buffers the data, and then alerts the

	    directly accessed by the PC processor in this type of

	    directly accessed by the PC processor in this type of

	    communication system, it is not necessary for the vendor to use

	    communication system, it is not necessary for the vendor to use

	    UARTs that are compatible with the 8250, 16450, or the 16550 UART.

	    UARTs that are compatible with the 8250, 16450, or the 16550 UART.

    <para>All embedded &os; systems that use flash memory as system

    <para>All embedded &os; systems that use flash memory as system

      disk will be interested in memory disks and memory filesystems.

      disk will be interested in memory disks and memory filesystems.

      flash memory, the disk and the filesystems on the disk will most

      flash memory, the disk and the filesystems on the disk will most

      likely be mounted read-only.  In this environment, filesystems

      likely be mounted read-only.  In this environment, filesystems

      such as <filename>/tmp</filename> and <filename>/var</filename>

      such as <filename>/tmp</filename> and <filename>/var</filename>

  <sect1>

  <sect1>

    <title>Building a File System from Scratch</title>

    <title>Building a File System from Scratch</title>

      as normal IDE hard drives, you could theoretically install &os;

      as normal IDE hard drives, you could theoretically install &os;

      from the network using the kern and mfsroot floppies or from a

      from the network using the kern and mfsroot floppies or from a

      CD.</para>

      CD.</para>

    <para>However, even a small installation of &os; using normal

    <para>However, even a small installation of &os; using normal

      installation procedures can produce a system in size of greater

      installation procedures can produce a system in size of greater

      flash memory devices (128 megabytes is considered fairly large -

      flash memory devices (128 megabytes is considered fairly large -

      mechanisms is not possible—there is simply not enough disk

      mechanisms is not possible—there is simply not enough disk

      space for even the smallest of conventional

      space for even the smallest of conventional

      installations.</para>

      installations.</para>

	successfully run <command>make</command>

	successfully run <command>make</command>

	<buildtarget>install</buildtarget>, we must create a packages

	<buildtarget>install</buildtarget>, we must create a packages

	directory on a non-memory filesystem that will keep track of

	directory on a non-memory filesystem that will keep track of

	your filesystems as read-write for the installation of a

	your filesystems as read-write for the installation of a

	package anyway, it is sensible to assume that an area on the

	package anyway, it is sensible to assume that an area on the

	flash media can also be used for package information to be

	flash media can also be used for package information to be

	    cannot be combined with the next A-B sequence.</para>

	    cannot be combined with the next A-B sequence.</para>

	  <para>Why do we interleave our swap space instead of just tack swap

	  <para>Why do we interleave our swap space instead of just tack swap

	    lot easier to allocate linear swaths of an address space and have

	    lot easier to allocate linear swaths of an address space and have

	    the result automatically be interleaved across multiple disks than

	    the result automatically be interleaved across multiple disks than

	    it is to try to put that sophistication elsewhere.</para>

	    it is to try to put that sophistication elsewhere.</para>

      jmp main-0x7c00+0x600	# Jump to relocated code</programlisting>

      jmp main-0x7c00+0x600	# Jump to relocated code</programlisting>

    </figure>

    </figure>

      <acronym>BIOS</acronym> to address <literal>0x7C00</literal>, it

      <acronym>BIOS</acronym> to address <literal>0x7C00</literal>, it

      copies itself to address <literal>0x600</literal> and then

      copies itself to address <literal>0x600</literal> and then

      transfers control there (recall that it was linked to execute at

      transfers control there (recall that it was linked to execute at

      bytes of <filename>boot</filename> and, because

      bytes of <filename>boot</filename> and, because

      <filename>boot</filename> is written to the first sector of the

      <filename>boot</filename> is written to the first sector of the

      &os; slice, <filename>boot1</filename> fits exactly in this

      &os; slice, <filename>boot1</filename> fits exactly in this

      16 sectors of the &os; slice, it effectively reads the entire

      16 sectors of the &os; slice, it effectively reads the entire

      <filename>boot</filename> file

      <filename>boot</filename> file

      <footnote>

      <footnote>

      flags in the EFLAGS register.  Note that the

      flags in the EFLAGS register.  Note that the

      <literal>popfl</literal> instruction pops out a doubleword (4

      <literal>popfl</literal> instruction pops out a doubleword (4

      bytes) from the stack and places it in the EFLAGS register.

      bytes) from the stack and places it in the EFLAGS register.

      EFLAGS register is effectively cleared (IA-32 requires that bit

      EFLAGS register is effectively cleared (IA-32 requires that bit

      2 of the EFLAGS register always be 1).</para>

      2 of the EFLAGS register always be 1).</para>

      abstraction.  The IA-32 architecture demands the creation and

      abstraction.  The IA-32 architecture demands the creation and

      use of <emphasis>at least</emphasis> one <acronym>TSS</acronym>

      use of <emphasis>at least</emphasis> one <acronym>TSS</acronym>

      if multitasking facilities are used or different privilege

      if multitasking facilities are used or different privilege

      client is executed in privilege level 3, but the

      client is executed in privilege level 3, but the

      <acronym>BTX</acronym> server does in privilege level 0, a

      <acronym>BTX</acronym> server does in privilege level 0, a

      <acronym>TSS</acronym> must be defined:</para>

      <acronym>TSS</acronym> must be defined:</para>

    <para>For this reason, no serious applications rely on block

    <para>For this reason, no serious applications rely on block

      devices, and in fact, almost all applications which access

      devices, and in fact, almost all applications which access

      disks directly take great pains to specify that character

      disks directly take great pains to specify that character

      the implementation of the aliasing of each disk (partition) to

      the implementation of the aliasing of each disk (partition) to

      two devices with different semantics significantly complicated

      two devices with different semantics significantly complicated

      devices as part of the modernization of the disk I/O

      devices as part of the modernization of the disk I/O

      infrastructure.</para>

      infrastructure.</para>

  </sect1>

  </sect1>

        with PnP. This feature is not implemented in any existing

        with PnP. This feature is not implemented in any existing

        driver and is not considered further in this document.</para>

        driver and is not considered further in this document.</para>

        legacy devices they will not be attached twice (once as legacy

        legacy devices they will not be attached twice (once as legacy

        and once as PnP).  But in case of device-dependent identify

        and once as PnP).  But in case of device-dependent identify

        routines it is the responsibility of the driver to make sure

        routines it is the responsibility of the driver to make sure

              Free the memory allocated by

              Free the memory allocated by

              <function>bus_dmamem_alloc()</function>. At present,

              <function>bus_dmamem_alloc()</function>. At present,

              freeing of the memory allocated with ISA restrictions is

              freeing of the memory allocated with ISA restrictions is

              of use is to keep and re-use the allocated areas for as

              of use is to keep and re-use the allocated areas for as

              long as possible. Do not lightly free some area and then

              long as possible. Do not lightly free some area and then

              shortly allocate it again. That does not mean that

              shortly allocate it again. That does not mean that

          Before calling the callback function from

          Before calling the callback function from

          <function>bus_dmamap_load()</function> the segment array is

          <function>bus_dmamap_load()</function> the segment array is

          stored in the stack. And it gets pre-allocated for the

          stored in the stack. And it gets pre-allocated for the

          this the practical limit for the number of segments on i386

          this the practical limit for the number of segments on i386

          architecture is about 250-300 (the kernel stack is 4KB minus

          architecture is about 250-300 (the kernel stack is 4KB minus

          the size of the user structure, size of a segment array

          the size of the user structure, size of a segment array

          array is allocated based on the maximal number this value

          array is allocated based on the maximal number this value

          must not be set higher than really needed. Fortunately, for

          must not be set higher than really needed. Fortunately, for

          most of hardware the maximal supported number of segments is

          most of hardware the maximal supported number of segments is

          int error = 0;</programlisting>

          int error = 0;</programlisting>

        <para>Then allocate and activate all the necessary

        <para>Then allocate and activate all the necessary

          before returning from probe, it has to be allocated

          before returning from probe, it has to be allocated

          again. We expect that the probe routine had properly set all

          again. We expect that the probe routine had properly set all

          the resource ranges, as well as saved them in the structure

          the resource ranges, as well as saved them in the structure

      <indexterm><primary>Linksys</primary></indexterm>

      <indexterm><primary>Linksys</primary></indexterm>

      <indexterm><primary>D-Link</primary></indexterm>

      <indexterm><primary>D-Link</primary></indexterm>

	numeric IDs for device identification.  Using numeric IDs and

	numeric IDs for device identification.  Using numeric IDs and

	a centralized database complicates adding IDs and support for

	a centralized database complicates adding IDs and support for

	cards to the system.  One must carefully check to see who

	cards to the system.  One must carefully check to see who

      then also converting the SCSI commands to the native commands of

      then also converting the SCSI commands to the native commands of

      the hardware).</para>

      the hardware).</para>

      here, from this point on we will consider everything from the

      here, from this point on we will consider everything from the

      SIM standpoint.</para>

      SIM standpoint.</para>

	  the timeout to make sure that the target is not sleeping

	  the timeout to make sure that the target is not sleeping

	  forever.  If the command would not get aborted in some

	  forever.  If the command would not get aborted in some

	  reasonable time like 10 seconds the timeout routine would go

	  reasonable time like 10 seconds the timeout routine would go

	  will be aborted in some reasonable time we can just return

	  will be aborted in some reasonable time we can just return

	  the abort request now as successfully completed, and mark

	  the abort request now as successfully completed, and mark

	  the aborted CCB as aborted (but not mark it as done

	  the aborted CCB as aborted (but not mark it as done

    return;</programlisting>

    return;</programlisting>

	<para>That is all for the ABORT request, although there is one

	<para>That is all for the ABORT request, although there is one

	  ongoing transactions on a LUN we have to mark all the other

	  ongoing transactions on a LUN we have to mark all the other

	  active transactions on this LUN as aborted.  That should be

	  active transactions on this LUN as aborted.  That should be

	  done in the interrupt routine, after the transaction gets

	  done in the interrupt routine, after the transaction gets

      routine (or the other way around, the poll routine may be doing

      routine (or the other way around, the poll routine may be doing

      the real action and the interrupt routine would just call the

      the real action and the interrupt routine would just call the

      poll routine).  Why bother about a separate function then?

      poll routine).  Why bother about a separate function then?

      <function>xxx_poll</function> routine gets the struct cam_sim

      <function>xxx_poll</function> routine gets the struct cam_sim

      pointer as its argument when the PCI interrupt routine by common

      pointer as its argument when the PCI interrupt routine by common

      convention gets pointer to the struct

      convention gets pointer to the struct

    <para>Example: Firmware download Many devices that have been

    <para>Example: Firmware download Many devices that have been

      developed are based on a general purpose processor with an

      developed are based on a general purpose processor with an

      drivers and firmware for USB devices is still very new, many

      drivers and firmware for USB devices is still very new, many

      devices require the downloading of the firmware after they have

      devices require the downloading of the firmware after they have

      been connected.</para>

      been connected.</para>