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<HR>
<H2><A NAME="s3">3. Fundamentals</A></H2>
<H2><A NAME="ss3.1">3.1 What is the kernel?</A>
</H2>
<P>The kernel is the "core" of any computer system: it is the "software"
which allows users to share computer resources.
<P>
<P>The kernel can be thought as the main software of the OS (Operating
System), which may also include graphics management.
<P>
<P>For example, under Linux (like other Unix-like OSs), the XWindow
environment doesn't belong to the Linux Kernel, because it manages
only graphical operations (it uses user mode I/O to access video
card devices).
<P>
<P>By contrast, Windows environments (Win9x, WinME, WinNT, Win2K,
WinXP, and so on) are a mix between a graphical environment and kernel.
<P>
<H2><A NAME="ss3.2">3.2 What is the difference between User Mode and Kernel Mode?</A>
</H2>
<H3>Overview</H3>
<P>Many years ago, when computers were as big as a room, users ran
their applications with much difficulty and, sometimes, their applications
crashed the computer.
<P>
<H3>Operative modes</H3>
<P>To avoid having applications that constantly crashed, newer OSs
were designed with 2 different operative modes:
<P>
<P>
<OL>
<LI>Kernel Mode: the machine operates with critical data structure,
direct hardware (IN/OUT or memory mapped), direct memory, IRQ, DMA,
and so on.</LI>
<LI>User Mode: users can run applications.
</LI>
</OL>
<P>
<PRE>
| Applications /|\
| ______________ |
| | User Mode | |
| ______________ |
| | |
Implementation | _______ _______ | Abstraction
Detail | | Kernel Mode | |
| _______________ |
| | |
| | |
| | |
\|/ Hardware |
</PRE>
<P>Kernel Mode "prevents" User Mode applications from damaging the
system or its features.
<P>
<P>Modern microprocessors implement in hardware at least 2 different
states. For example under Intel, 4 states determine the PL (Privilege
Level). It is possible to use 0,1,2,3 states, with 0 used in Kernel
Mode.
<P>
<P>Unix OS requires only 2 privilege levels, and we will use such
a paradigm as point of reference.
<P>
<H2><A NAME="ss3.3">3.3 Switching from User Mode to Kernel Mode</A>
</H2>
<H3>When do we switch?</H3>
<P>Once we understand that there are 2 different modes, we have
to know when we switch from one to the other.
<P>
<P>Typically, there are 2 points of switching:
<P>
<P>
<OL>
<LI>When calling a System Call: after calling a System Call, the
task voluntary calls pieces of code living in Kernel Mode</LI>
<LI>When an IRQ (or exception) comes: after the IRQ an IRQ handler
(or exception handler) is called, then control returns back to the
task that was interrupted like nothing was happened.
</LI>
</OL>
<H3>System Calls</H3>
<P>System calls are like special functions that manage OS routines
which live in Kernel Mode.
<P>
<P>A system call can be called when we:
<P>
<P>
<UL>
<LI>access an I/O device or a file (like read or write)</LI>
<LI>need to access privileged information (like pid, changing scheduling
policy or other information)</LI>
<LI>need to change execution context (like forking or executing some
other application) </LI>
<LI>need to execute a particular command (like ''chdir'', ''kill",
''brk'', or ''signal'')
</LI>
</UL>
<P>
<PRE>
| |
-------&gt;| System Call i | (Accessing Devices)
| | | | [sys_read()] |
| ... | | | |
| system_call(i) |-------- | |
| [read()] | | |
| ... | | |
| system_call(j) |-------- | |
| [get_pid()] | | | |
| ... | -------&gt;| System Call j | (Accessing kernel data structures)
| | | [sys_getpid()]|
| |
USER MODE KERNEL MODE
Unix System Calls Working
</PRE>
<P>System calls are almost the only interface used by User Mode
to talk with low level resources (hardware). The only exception to
this statement is when a process uses ''ioperm'' system call. In
this case a device can be accessed directly by User Mode process
(IRQs cannot be used).
<P>
<P>NOTE: Not every ''C'' function is a system call, only some of
them.
<P>
<P>Below is a list of System Calls under Linux Kernel 2.4.17, from
[ arch/i386/kernel/entry.S ]
<P>
<P>
<PRE>
.long SYMBOL_NAME(sys_ni_syscall) /* 0 - old &quot;setup()&quot; system call*/
.long SYMBOL_NAME(sys_exit)
.long SYMBOL_NAME(sys_fork)
.long SYMBOL_NAME(sys_read)
.long SYMBOL_NAME(sys_write)
.long SYMBOL_NAME(sys_open) /* 5 */
.long SYMBOL_NAME(sys_close)
.long SYMBOL_NAME(sys_waitpid)
.long SYMBOL_NAME(sys_creat)
.long SYMBOL_NAME(sys_link)
.long SYMBOL_NAME(sys_unlink) /* 10 */
.long SYMBOL_NAME(sys_execve)
.long SYMBOL_NAME(sys_chdir)
.long SYMBOL_NAME(sys_time)
.long SYMBOL_NAME(sys_mknod)
.long SYMBOL_NAME(sys_chmod) /* 15 */
.long SYMBOL_NAME(sys_lchown16)
.long SYMBOL_NAME(sys_ni_syscall) /* old break syscall holder */
.long SYMBOL_NAME(sys_stat)
.long SYMBOL_NAME(sys_lseek)
.long SYMBOL_NAME(sys_getpid) /* 20 */
.long SYMBOL_NAME(sys_mount)
.long SYMBOL_NAME(sys_oldumount)
.long SYMBOL_NAME(sys_setuid16)
.long SYMBOL_NAME(sys_getuid16)
.long SYMBOL_NAME(sys_stime) /* 25 */
.long SYMBOL_NAME(sys_ptrace)
.long SYMBOL_NAME(sys_alarm)
.long SYMBOL_NAME(sys_fstat)
.long SYMBOL_NAME(sys_pause)
.long SYMBOL_NAME(sys_utime) /* 30 */
.long SYMBOL_NAME(sys_ni_syscall) /* old stty syscall holder */
.long SYMBOL_NAME(sys_ni_syscall) /* old gtty syscall holder */
.long SYMBOL_NAME(sys_access)
.long SYMBOL_NAME(sys_nice)
.long SYMBOL_NAME(sys_ni_syscall) /* 35 */ /* old ftime syscall holder */
.long SYMBOL_NAME(sys_sync)
.long SYMBOL_NAME(sys_kill)
.long SYMBOL_NAME(sys_rename)
.long SYMBOL_NAME(sys_mkdir)
.long SYMBOL_NAME(sys_rmdir) /* 40 */
.long SYMBOL_NAME(sys_dup)
.long SYMBOL_NAME(sys_pipe)
.long SYMBOL_NAME(sys_times)
.long SYMBOL_NAME(sys_ni_syscall) /* old prof syscall holder */
.long SYMBOL_NAME(sys_brk) /* 45 */
.long SYMBOL_NAME(sys_setgid16)
.long SYMBOL_NAME(sys_getgid16)
.long SYMBOL_NAME(sys_signal)
.long SYMBOL_NAME(sys_geteuid16)
.long SYMBOL_NAME(sys_getegid16) /* 50 */
.long SYMBOL_NAME(sys_acct)
.long SYMBOL_NAME(sys_umount) /* recycled never used phys() */
.long SYMBOL_NAME(sys_ni_syscall) /* old lock syscall holder */
.long SYMBOL_NAME(sys_ioctl)
.long SYMBOL_NAME(sys_fcntl) /* 55 */
.long SYMBOL_NAME(sys_ni_syscall) /* old mpx syscall holder */
.long SYMBOL_NAME(sys_setpgid)
.long SYMBOL_NAME(sys_ni_syscall) /* old ulimit syscall holder */
.long SYMBOL_NAME(sys_olduname)
.long SYMBOL_NAME(sys_umask) /* 60 */
.long SYMBOL_NAME(sys_chroot)
.long SYMBOL_NAME(sys_ustat)
.long SYMBOL_NAME(sys_dup2)
.long SYMBOL_NAME(sys_getppid)
.long SYMBOL_NAME(sys_getpgrp) /* 65 */
.long SYMBOL_NAME(sys_setsid)
.long SYMBOL_NAME(sys_sigaction)
.long SYMBOL_NAME(sys_sgetmask)
.long SYMBOL_NAME(sys_ssetmask)
.long SYMBOL_NAME(sys_setreuid16) /* 70 */
.long SYMBOL_NAME(sys_setregid16)
.long SYMBOL_NAME(sys_sigsuspend)
.long SYMBOL_NAME(sys_sigpending)
.long SYMBOL_NAME(sys_sethostname)
.long SYMBOL_NAME(sys_setrlimit) /* 75 */
.long SYMBOL_NAME(sys_old_getrlimit)
.long SYMBOL_NAME(sys_getrusage)
.long SYMBOL_NAME(sys_gettimeofday)
.long SYMBOL_NAME(sys_settimeofday)
.long SYMBOL_NAME(sys_getgroups16) /* 80 */
.long SYMBOL_NAME(sys_setgroups16)
.long SYMBOL_NAME(old_select)
.long SYMBOL_NAME(sys_symlink)
.long SYMBOL_NAME(sys_lstat)
.long SYMBOL_NAME(sys_readlink) /* 85 */
.long SYMBOL_NAME(sys_uselib)
.long SYMBOL_NAME(sys_swapon)
.long SYMBOL_NAME(sys_reboot)
.long SYMBOL_NAME(old_readdir)
.long SYMBOL_NAME(old_mmap) /* 90 */
.long SYMBOL_NAME(sys_munmap)
.long SYMBOL_NAME(sys_truncate)
.long SYMBOL_NAME(sys_ftruncate)
.long SYMBOL_NAME(sys_fchmod)
.long SYMBOL_NAME(sys_fchown16) /* 95 */
.long SYMBOL_NAME(sys_getpriority)
.long SYMBOL_NAME(sys_setpriority)
.long SYMBOL_NAME(sys_ni_syscall) /* old profil syscall holder */
.long SYMBOL_NAME(sys_statfs)
.long SYMBOL_NAME(sys_fstatfs) /* 100 */
.long SYMBOL_NAME(sys_ioperm)
.long SYMBOL_NAME(sys_socketcall)
.long SYMBOL_NAME(sys_syslog)
.long SYMBOL_NAME(sys_setitimer)
.long SYMBOL_NAME(sys_getitimer) /* 105 */
.long SYMBOL_NAME(sys_newstat)
.long SYMBOL_NAME(sys_newlstat)
.long SYMBOL_NAME(sys_newfstat)
.long SYMBOL_NAME(sys_uname)
.long SYMBOL_NAME(sys_iopl) /* 110 */
.long SYMBOL_NAME(sys_vhangup)
.long SYMBOL_NAME(sys_ni_syscall) /* old &quot;idle&quot; system call */
.long SYMBOL_NAME(sys_vm86old)
.long SYMBOL_NAME(sys_wait4)
.long SYMBOL_NAME(sys_swapoff) /* 115 */
.long SYMBOL_NAME(sys_sysinfo)
.long SYMBOL_NAME(sys_ipc)
.long SYMBOL_NAME(sys_fsync)
.long SYMBOL_NAME(sys_sigreturn)
.long SYMBOL_NAME(sys_clone) /* 120 */
.long SYMBOL_NAME(sys_setdomainname)
.long SYMBOL_NAME(sys_newuname)
.long SYMBOL_NAME(sys_modify_ldt)
.long SYMBOL_NAME(sys_adjtimex)
.long SYMBOL_NAME(sys_mprotect) /* 125 */
.long SYMBOL_NAME(sys_sigprocmask)
.long SYMBOL_NAME(sys_create_module)
.long SYMBOL_NAME(sys_init_module)
.long SYMBOL_NAME(sys_delete_module)
.long SYMBOL_NAME(sys_get_kernel_syms) /* 130 */
.long SYMBOL_NAME(sys_quotactl)
.long SYMBOL_NAME(sys_getpgid)
.long SYMBOL_NAME(sys_fchdir)
.long SYMBOL_NAME(sys_bdflush)
.long SYMBOL_NAME(sys_sysfs) /* 135 */
.long SYMBOL_NAME(sys_personality)
.long SYMBOL_NAME(sys_ni_syscall) /* for afs_syscall */
.long SYMBOL_NAME(sys_setfsuid16)
.long SYMBOL_NAME(sys_setfsgid16)
.long SYMBOL_NAME(sys_llseek) /* 140 */
.long SYMBOL_NAME(sys_getdents)
.long SYMBOL_NAME(sys_select)
.long SYMBOL_NAME(sys_flock)
.long SYMBOL_NAME(sys_msync)
.long SYMBOL_NAME(sys_readv) /* 145 */
.long SYMBOL_NAME(sys_writev)
.long SYMBOL_NAME(sys_getsid)
.long SYMBOL_NAME(sys_fdatasync)
.long SYMBOL_NAME(sys_sysctl)
.long SYMBOL_NAME(sys_mlock) /* 150 */
.long SYMBOL_NAME(sys_munlock)
.long SYMBOL_NAME(sys_mlockall)
.long SYMBOL_NAME(sys_munlockall)
.long SYMBOL_NAME(sys_sched_setparam)
.long SYMBOL_NAME(sys_sched_getparam) /* 155 */
.long SYMBOL_NAME(sys_sched_setscheduler)
.long SYMBOL_NAME(sys_sched_getscheduler)
.long SYMBOL_NAME(sys_sched_yield)
.long SYMBOL_NAME(sys_sched_get_priority_max)
.long SYMBOL_NAME(sys_sched_get_priority_min) /* 160 */
.long SYMBOL_NAME(sys_sched_rr_get_interval)
.long SYMBOL_NAME(sys_nanosleep)
.long SYMBOL_NAME(sys_mremap)
.long SYMBOL_NAME(sys_setresuid16)
.long SYMBOL_NAME(sys_getresuid16) /* 165 */
.long SYMBOL_NAME(sys_vm86)
.long SYMBOL_NAME(sys_query_module)
.long SYMBOL_NAME(sys_poll)
.long SYMBOL_NAME(sys_nfsservctl)
.long SYMBOL_NAME(sys_setresgid16) /* 170 */
.long SYMBOL_NAME(sys_getresgid16)
.long SYMBOL_NAME(sys_prctl)
.long SYMBOL_NAME(sys_rt_sigreturn)
.long SYMBOL_NAME(sys_rt_sigaction)
.long SYMBOL_NAME(sys_rt_sigprocmask) /* 175 */
.long SYMBOL_NAME(sys_rt_sigpending)
.long SYMBOL_NAME(sys_rt_sigtimedwait)
.long SYMBOL_NAME(sys_rt_sigqueueinfo)
.long SYMBOL_NAME(sys_rt_sigsuspend)
.long SYMBOL_NAME(sys_pread) /* 180 */
.long SYMBOL_NAME(sys_pwrite)
.long SYMBOL_NAME(sys_chown16)
.long SYMBOL_NAME(sys_getcwd)
.long SYMBOL_NAME(sys_capget)
.long SYMBOL_NAME(sys_capset) /* 185 */
.long SYMBOL_NAME(sys_sigaltstack)
.long SYMBOL_NAME(sys_sendfile)
.long SYMBOL_NAME(sys_ni_syscall) /* streams1 */
.long SYMBOL_NAME(sys_ni_syscall) /* streams2 */
.long SYMBOL_NAME(sys_vfork) /* 190 */
.long SYMBOL_NAME(sys_getrlimit)
.long SYMBOL_NAME(sys_mmap2)
.long SYMBOL_NAME(sys_truncate64)
.long SYMBOL_NAME(sys_ftruncate64)
.long SYMBOL_NAME(sys_stat64) /* 195 */
.long SYMBOL_NAME(sys_lstat64)
.long SYMBOL_NAME(sys_fstat64)
.long SYMBOL_NAME(sys_lchown)
.long SYMBOL_NAME(sys_getuid)
.long SYMBOL_NAME(sys_getgid) /* 200 */
.long SYMBOL_NAME(sys_geteuid)
.long SYMBOL_NAME(sys_getegid)
.long SYMBOL_NAME(sys_setreuid)
.long SYMBOL_NAME(sys_setregid)
.long SYMBOL_NAME(sys_getgroups) /* 205 */
.long SYMBOL_NAME(sys_setgroups)
.long SYMBOL_NAME(sys_fchown)
.long SYMBOL_NAME(sys_setresuid)
.long SYMBOL_NAME(sys_getresuid)
.long SYMBOL_NAME(sys_setresgid) /* 210 */
.long SYMBOL_NAME(sys_getresgid)
.long SYMBOL_NAME(sys_chown)
.long SYMBOL_NAME(sys_setuid)
.long SYMBOL_NAME(sys_setgid)
.long SYMBOL_NAME(sys_setfsuid) /* 215 */
.long SYMBOL_NAME(sys_setfsgid)
.long SYMBOL_NAME(sys_pivot_root)
.long SYMBOL_NAME(sys_mincore)
.long SYMBOL_NAME(sys_madvise)
.long SYMBOL_NAME(sys_getdents64) /* 220 */
.long SYMBOL_NAME(sys_fcntl64)
.long SYMBOL_NAME(sys_ni_syscall) /* reserved for TUX */
.long SYMBOL_NAME(sys_ni_syscall) /* Reserved for Security */
.long SYMBOL_NAME(sys_gettid)
.long SYMBOL_NAME(sys_readahead) /* 225 */
</PRE>
<H3>IRQ Event</H3>
<P>When an IRQ comes, the task that is running is interrupted in
order to service the IRQ Handler.
<P>
<P>After the IRQ is handled, control returns backs exactly to point
of interrupt, like nothing happened.
<P>
<P>
<PRE>
Running Task
|-----------| (3)
NORMAL | | | [break execution] IRQ Handler
EXECUTION (1)| | | -------------&gt;|---------|
| \|/ | | | does |
IRQ (2)----&gt;| .. |-----&gt; | some |
| | |&lt;----- | work |
BACK TO | | | | | ..(4). |
NORMAL (6)| \|/ | &lt;-------------|_________|
EXECUTION |___________| [return to code]
(5)
USER MODE KERNEL MODE
User-&gt;Kernel Mode Transition caused by IRQ event
</PRE>
<P>The numbered steps below refer to the sequence of events in the
diagram above:
<P>
<P>
<OL>
<LI>Process is executing</LI>
<LI>IRQ comes while the task is running.</LI>
<LI>Task is interrupted to call an "Interrupt handler".</LI>
<LI>The "Interrupt handler" code is executed.</LI>
<LI>Control returns back to task user mode (as if nothing happened)</LI>
<LI>Process returns back to normal execution
</LI>
</OL>
<P>Special interest has the Timer IRQ, coming every TIMER ms to
manage:
<P>
<P>
<OL>
<LI>Alarms</LI>
<LI>System and task counters (used by schedule to decide when stop
a process or for accounting)</LI>
<LI>Multitasking based on wake up mechanism after TIMESLICE time.
</LI>
</OL>
<H2><A NAME="ss3.4">3.4 Multitasking</A>
</H2>
<H3>Mechanism</H3>
<P>The key point of modern OSs is the "Task". The Task is an application
running in memory sharing all resources (included CPU and Memory)
with other Tasks.
<P>
<P>This "resource sharing" is managed by the "Multitasking Mechanism".
The Multitasking Mechanism switches from one task to another after
a "timeslice" time. Users have the "illusion" that they own all resources.
We can also imagine a single user scenario, where a user can have
the "illusion" of running many tasks at the same time.
<P>
<P>To implement this multitasking, the task uses "the state" variable,
which can be:
<P>
<P>
<OL>
<LI>READY, ready for execution</LI>
<LI>BLOCKED, waiting for a resource
</LI>
</OL>
<P>The task state is managed by its presence in a relative list:
READY list and BLOCKED list.
<P>
<H3>Task Switching</H3>
<P>The movement from one task to another is called ''Task Switching''.
many computers have a hardware instruction which automatically performs
this operation. Task Switching occurs in the following cases:
<P>
<P>
<OL>
<LI>After Timeslice ends: we need to schedule a "Ready for execution"
task and give it access.</LI>
<LI>When a Task has to wait for a device: we need to schedule a new
task and switch to it *
</LI>
</OL>
<P>* We schedule another task to prevent "Busy Form Waiting", which
occurs when we are waiting for a device instead performing other
work.
<P>
<P>Task Switching is managed by the "Schedule" entity.
<P>
<P>
<PRE>
Timer | |
IRQ | | Schedule
| | | ________________________
|-----&gt;| Task 1 |&lt;------------------&gt;|(1)Chooses a Ready Task |
| | | |(2)Task Switching |
| |___________| |________________________|
| | | /|\
| | | |
| | | |
| | | |
| | | |
|-----&gt;| Task 2 |&lt;-------------------------------|
| | | |
| |___________| |
. . . . .
. . . . .
. . . . .
| | | |
| | | |
------&gt;| Task N |&lt;--------------------------------
| |
|___________|
Task Switching based on TimeSlice
</PRE>
<P>A typical Timeslice for Linux is about 10 ms.
<P>
<P>
<PRE>
| |
| | Resource _____________________________
| Task 1 |-----------&gt;|(1) Enqueue Resource request |
| | Access |(2) Mark Task as blocked |
| | |(3) Choose a Ready Task |
|___________| |(4) Task Switching |
|_____________________________|
|
|
| | |
| | |
| Task 2 |&lt;-------------------------
| |
| |
|___________|
Task Switching based on Waiting for a Resource
</PRE>
<H2><A NAME="ss3.5">3.5 Microkernel vs Monolithic OS</A>
</H2>
<H3>Overview</H3>
<P>Until now we viewed so called Monolithic OS, but there is also
another kind of OS: ''Microkernel''.
<P>
<P>A Microkernel OS uses Tasks, not only for user mode processes,
but also as a real kernel manager, like Floppy-Task, HDD-Task, Net-Task
and so on. Some examples are Amoeba, and Mach.
<P>
<H3>PROs and CONTROs of Microkernel OS </H3>
<P>PROS:
<P>
<P>
<UL>
<LI>OS is simpler to maintain because each Task manages a single
kind of operation. So if you want to modify networking, you modify
Net-Task (ideally, if it is not needed a structural update).
</LI>
</UL>
<P>CONS:
<P>
<P>
<UL>
<LI>Performances are worse than Monolithic OS, because you have to
add 2*TASK_SWITCH times (the first to enter the specific Task, the
second to go out from it).
</LI>
</UL>
<P>My personal opinion is that, Microkernels are a good didactic
example (like Minix) but they are not ''optimal'', so not really
suitable. Linux uses a few Tasks, called "Kernel Threads" to implement
a little microkernel structure (like kswapd, which is used to retrieve
memory pages from mass storage). In this case there are no problems
with perfomance because swapping is a very slow job.
<P>
<H2><A NAME="ss3.6">3.6 Networking</A>
</H2>
<H3>ISO OSI levels</H3>
<P>Standard ISO-OSI describes a network architecture with the following
levels:
<P>
<P>
<OL>
<LI>Physical level (examples: PPP and Ethernet)</LI>
<LI>Data-link level (examples: PPP and Ethernet)</LI>
<LI>Network level (examples: IP, and X.25)</LI>
<LI>Transport level (examples: TCP, UDP)</LI>
<LI>Session level (SSL)</LI>
<LI>Presentation level (FTP binary-ascii coding)</LI>
<LI>Application level (applications like Netscape)
</LI>
</OL>
<P>The first 2 levels listed above are often implemented in hardware.
Next levels are in software (or firmware for routers).
<P>
<P>Many protocols are used by an OS: one of these is TCP/IP (the
most important living on 3-4 levels).
<P>
<H3>What does the kernel?</H3>
<P>The kernel doesn't know anything (only addresses) about first
2 levels of ISO-OSI.
<P>
<P>In RX it:
<P>
<P>
<OL>
<LI>Manages handshake with low levels devices (like ethernet card
or modem) receiving "frames" from them.</LI>
<LI>Builds TCP/IP "packets" from "frames" (like Ethernet or PPP ones),
</LI>
<LI>Convers ''packets'' in ''sockets'' passing them to the right
application (using port number) or</LI>
<LI>Forwards packets to the right queue
</LI>
</OL>
<P>
<PRE>
frames packets sockets
NIC ---------&gt; Kernel ----------&gt; Application
| packets
--------------&gt; Forward
- RX -
</PRE>
<P>In TX stage it:
<P>
<P>
<OL>
<LI>Converts sockets or </LI>
<LI>Queues datas into TCP/IP ''packets''</LI>
<LI>Splits ''packets" into "frames" (like Ethernet or PPP ones)</LI>
<LI>Sends ''frames'' using HW drivers
</LI>
</OL>
<P>
<PRE>
sockets packets frames
Application ---------&gt; Kernel ----------&gt; NIC
packets /|\
Forward -------------------
- TX -
</PRE>
<H2><A NAME="ss3.7">3.7 Virtual Memory</A>
</H2>
<H3>Segmentation</H3>
<P>Segmentation is the first method to solve memory allocation problems:
it allows you to compile source code without caring where the application
will be placed in memory. As a matter of fact, this feature helps
applications developers to develop in a independent fashion from
the OS e also from the hardware.
<P>
<P>
<PRE>
| Stack |
| | |
| \|/ |
| Free |
| /|\ | Segment &lt;---&gt; Process
| | |
| Heap |
| Data uninitialized |
| Data initialized |
| Code |
|____________________|
Segment
</PRE>
<P>We can say that a segment is the logical entity of an application,
or the image of the application in memory.
<P>
<P>When programming, we don't care where our data is put in memory,
we only care about the offset inside our segment (our application).
<P>
<P>We use to assign a Segment to each Process and vice versa. In
Linux this is not true. Linux uses only 4 segments for either Kernel
and all Processes.
<P>
<H3>Problems of Segmentation</H3>
<P>
<PRE>
____________________
-----&gt;| |-----&gt;
| IN | Segment A | OUT
____________________ | |____________________|
| |____| | |
| Segment B | | Segment B |
| |____ | |
|____________________| | |____________________|
| | Segment C |
| |____________________|
-----&gt;| Segment D |-----&gt;
IN |____________________| OUT
Segmentation problem
</PRE>
<P>In the diagram above, we want to get exit processes A, and D
and enter process B. As we can see there is enough space for B, but
we cannot split it in 2 pieces, so we CANNOT load it (memory out).
<P>
<P>The reason this problem occurs is because pure segments are continuous
areas (because they are logical areas) and cannot be split.
<P>
<H3>Pagination</H3>
<P>
<PRE>
____________________
| Page 1 |
|____________________|
| Page 2 |
|____________________|
| .. | Segment &lt;---&gt; Process
|____________________|
| Page n |
|____________________|
| |
|____________________|
| |
|____________________|
Segment
</PRE>
<P>Pagination splits memory in &quot;n&quot; pieces, each one with
a fixed length.
<P>
<P>A process may be loaded in one or more Pages. When memory is
freed, all pages are freed (see Segmentation Problem, before).
<P>
<P>Pagination is also used for another important purpose, "Swapping".
If a page is not present in physical memory then it generates an
EXCEPTION, that will make the Kernel search for a new page in storage
memory. This mechanism allow OS to load more applications than the
ones allowed by physical memory only.
<P>
<H3>Pagination Problem</H3>
<P>
<PRE>
____________________
Page X | Process Y |
|____________________|
| |
| WASTE |
| SPACE |
|____________________|
Pagination Problem
</PRE>
<P>In the diagram above, we can see what is wrong with the pagination
policy: when a Process Y loads into Page X, ALL memory space of the
Page is allocated, so the remaining space at the end of Page is wasted.
<P>
<H3>Segmentation and Pagination</H3>
<P>How can we solve segmentation and pagination problems? Using
either 2 policies.
<P>
<P>
<PRE>
| .. |
|____________________|
-----&gt;| Page 1 |
| |____________________|
| | .. |
____________________ | |____________________|
| | |----&gt;| Page 2 |
| Segment X | ----| |____________________|
| | | | .. |
|____________________| | |____________________|
| | .. |
| |____________________|
|----&gt;| Page 3 |
|____________________|
| .. |
</PRE>
<P>Process X, identified by Segment X, is split in 3 pieces and
each of one is loaded in a page.
<P>
<P>We do not have:
<P>
<P>
<OL>
<LI>Segmentation problem: we allocate per Pages, so we also free
Pages and we manage free space in an optimized way.</LI>
<LI>Pagination problem: only last page wastes space, but we can decide
to use very small pages, for example 4096 bytes length (losing at
maximum 4096*N_Tasks bytes) and manage hierarchical paging (using
2 or 3 levels of paging)
</LI>
</OL>
<P>
<PRE>
| | | |
| | Offset2 | Value |
| | /|\| |
Offset1 | |----- | | |
/|\ | | | | | |
| | | | \|/| |
| | | ------&gt;| |
\|/ | | | |
Base Paging Address ----&gt;| | | |
| ....... | | ....... |
| | | |
Hierarchical Paging
</PRE>
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