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Tuesday 26 February 2013
Monday 25 February 2013
Linux File System
FILE SYSTEM
File system divided
into two categories:
§ User data - stores actual data contained in
files
§ Metadata - stores file system structural
information such as superblock, inodes, directories
Understanding UNIX / Linux file system
Superblock
Let us take an
example of 20 GB hard disk. The entire disk space subdivided into multiple file
system blocks. And blocks used for what?
Unix / Linux filesystem blocks
The blocks used for two
different purpose:
1.
Most blocks stores user data aka files (user data).
2.
Some blocks in every file system store the file system's
metadata. So what the hell is a metadata?
In simple words Metadata
describes the structure of the file system. Most common metadata structure are
superblock, inode and directories. Following paragraphs describes each of them.
Superblock
Each file system is different
and they have type like ext2, ext3 etc. Further each file system has size like
5 GB, 10 GB and status such as mount status. In short each file system has a
superblock, which contains information about file system such as:
§ File
system type
§ Size
§ Status
§ Information
about other metadata structures
Inodes
The inode (index node) is a
fundamental concept in the Linux and UNIX filesystem.
Each
and every file under Linux (and UNIX) has following attributes:
=>
File type (executable, block special etc)
=> Permissions (read, write etc)
=> Owner
=> Group
=> File Size
=> File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview)
=> File deletion time
=> Permissions (read, write etc)
=> Owner
=> Group
=> File Size
=> File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview)
=> File deletion time
All the above information stored in an inode. In short the
inode identifies the file and its attributes (as above) . Each inode is
identified by a unique inode number within the file system. Inode is also know
as index number.
How do I see file inode number?
You can
use ls -i command to see inode number of file
Sample Output
$ ls -i /etc/passwdSample Output
32820 /etc/passwd
You can
also use stat command to find out inode number and its attribute:
$ stat /etc/passwdOutput:File: `/etc/passwd'
Size: 1988 Blocks: 8 IO Block: 4096 regular file
Device: 341h/833d Inode: 32820 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2005-11-10 01:26:01.000000000 +0530
Modify: 2005-10-27 13:26:56.000000000 +0530
Change: 2005-10-27 13:26:56.000000000 +0530
Tuesday 19 February 2013
Boot Loader
BOOT LOADER
A boot loader, also called a boot manager, is a small program that
places the operating system (OS) of a computer into memory. When a computer is powered-up or restarted, the basic input/output
system (BIOS) performs some initial tests, and then transfers control to the master boot record (MBR) where the boot loader resides.
How does
GRUB work?
When a computer boots, the BIOS transfers
control to the first boot device, which can be a hard disk, a floppy disk, a
CD-ROM, or any other BIOS-recognized device. We'll concentrate on hard disks,
for the sake of simplicity.
The first sector on a hard is called the
Master Boot Record (MBR). This sector is only 512 bytes long and contains a
small piece of code (446 bytes) called the primary boot loader and the
partition table (64 bytes) describing the primary and extended partitions.
By default, MBR code looks for the partition
marked as active and once such a partition is found, it loads its boot sector
into memory and passes control to it.
GRUB
replaces the default MBR with its own code.
Furthermore, GRUB works in stages.
Stage 1 is
located in the MBR and mainly points to Stage 2, since the MBR is too small to
contain all of the needed data.
Stage 2
points to its configuration file, which contains all of the complex user
interface and options we are normally familiar with when talking about GRUB.
Stage 2 can be located anywhere on the disk. If Stage 2 cannot find its
configuration table, GRUB will cease the boot sequence and present the user
with a command line for manual configuration.
Stage 1.5
also exists and might be used if the boot information is small enough to fit in
the area immediately after MBR.
The Stage architecture allows GRUB to be large
(~20-30K) and therefore fairly complex and highly configurable, compared to
most bootloaders, which are sparse and simple to fit within the limitations of
the Partition Table.
For Linux, the two most common boot loaders are known as LILO (LInux LOader) and LOADLIN (LOAD LINux). An
alternative boot loader, called GRUB (GRand Unified Bootloader), is used with Red HatLinux. LILO is the most popular boot loader among computer users that
employ Linux as the main, or only, operating system. The primary advantage of
LILO is the fact that it allows for fast boot-up. LOADLIN is preferred by some
users whose computers have multiple operating systems, and who spend relatively
little time in Linux. LOADLIN is sometimes used as a backup boot loader for
Linux in case LILO fails. GRUB is preferred by many users of Red Hat Linux,
because it is the default boot loader for that distribution.
GRUB (GRand Unified Bootloader) is a boot loader package developed to support multiple operating
systems and allow the user to select among them during boot-up.
GRUB is a bootloader (can also be
spelled boot loader) capable of loading a variety of free and proprietary
operating systems. GRUB will work well with Linux, DOS, Windows, or BSD. GRUB
stands for GRand Unified Bootloader.
GRUB can be run from or be installed to any device (floppy disk, hard
disk, CD-ROM, USB drive, network drive) and can load operating systems from just
as many locations, including network drives. It can also decompress operating
system images before booting them.
What about LILO?
You may have heard about another Linux bootloader called LILO (stands
for LInux LOader). While a sensible option for many Linux users, I believe that
GRUB is a better choice, for several reasons:
1. LILO supports only up to 16 different boot
selections; GRUB supports an unlimited number of boot entries.
2. LILO cannot boot from network; GRUB can.
3. LILO must be written again every time you change
the configuration file; GRUB does not.
4. LILO does not have an interactive command
interface.
Assembler Introduction
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and
other programmable devices in which each statement corresponds to a single machine language instruction. An assembly language is thus specific to a certain physical (or
virtual) computer architecture, in contrast to most high-level programming languages, which, ideally, are portable.
Assembly
language allows the use of symbolic representation for machine operation codes(usually
called mnemonics), memory
locations, registers and other parts of an instruction.
A utility program called an assembler is used to translate assembly language
statements into the target computer's machine code.
Many
advanced assemblers offer additional mechanisms to facilitate program
development, control the assembly process, and aid debugging. In
particular, most modern assemblers include a macrofacility (described below), and are
called macro assemblers.
Assembler
Typically a modern assembler creates object code by translating assembly instruction mnemonics into opcodes, and by
resolving symbolic
names for memory locations and other entities.[1] The use of symbolic references is a key feature of assemblers,
saving tedious calculations and manual address updates after program
modifications. Most assemblers also include macro facilities for
performing textual substitution—e.g., to generate common short sequences of
instructions as inline, instead of called subroutines.
Basic
Assembler functions:
The assembler is responsible for translating the assembly language
program into machine code. When the source language is essentially a symbolic
representation for a numerical machine language, the translator is called an
assembler and the source language is
called an assembly language. A pure assembly language is a language in which
each statement produces exactly one machine instruction figure below show the
assembler functions.
1 . Translate mnemonic
opcodes to machine language.
2 . Convert symbolic
operands to their machine addresses.
3 . Build machine
instructions in the proper format.
4 . Convert data constants
into machine representation.
5 . Error checking is
provided.
6 . Changes can be quickly
and easily incorporated with a reassembly.
7 . Variables are
represented by symbolic names, not as memory locations.
Assembly language statements are written one per line. A machine
code program thus consists of sequence of assembly language statements, where
each statement contains a mnemonic. Each line of an assembly language program
is split into four fields, as show below:
LABEL OPC ODE OPERAND COMMENTS
Label: it is an identifier
and optiona l field. Labels are used extensively in programs to reduce reliance upon programmers
remembering where data or code is located. The maximum length of label differs
between assemblers. Some accept upto 32 characters long, other only four
characters. A label when declared is suffixed by a colon and begins with a valid character
(A..Z)
For example
START: LDAA #24 H
Here, the label START is equal to the address of instruction
LDAA #24 H
An advantage of using labels is that inserting or rearranging code
stat elements do not necessitate reworking actual machine instructions .
Opcode: this field contains a mnemonic. Opcode stands for operation
code i.e. a machine code instruction.
The opcode may also require additional information i.e. operands.
Operand: this field consists of additional information or data that the
opcode requires. In certain types of
addressing modes, the operand is used to specify
- Constants or labels
- Immediate data
- Data contained in another
accumulator or register .
- An address
The figure below shows the
machine instruction format
Assembly
Language Statements
It consists of three types of statements:
1 - Imperative
2 - Declaration
3 - Assembler directive
Imperative statement: it indicates an action
to be performed during the execution of the assembled program. Imperative
programs focus on how to solve a problem popular imperative languages such as C
and Pascal are based on side effects on memory. Each imperative statement
translates into one machine instruction.
An imperative statement indicates an action to
be performed during the execution of the assembled program. Each imperative
statement typically translates into one machine instruction.
Declaration statement: it focus on what the problem is and leave solution
mechanism up to the language
implementation. Within the declarative paradigm, most notable are the function
languages such as lisp. Declarative lang uages are typically quite abstract and
hence can be harder to implement efficiently.
The
syntax of declaration statements is as follows:
The DS (short for declare
storage) statement reserves
areas of memory and associates names with them. Consider the following DS
statements:
The first statement reserves a memory area of 1 word and associates the name A with it. The second statement reserves a block of 200 memory words. The name G is associated with the first word of the block. Other words in the block can be accessed through offsets from G, e.g. G+5 is the sixth word of the memory block, etc.
The DC
(short for declare constant) statement constructs memory words
containing constants. The statement
associates the name ONE with a memory word containing the value ‘ 1′. The programmer can declare constants in different forms–decimal, binary, hexadecimal, etc. The assembler converts them to the appropriate internal form.
Assembler directive: assembler directives instruct the assembler to perform
certain actions during the assembly of a
program. They can be used to declare variables, create storage space for
results, to declare constants. The following assembler directives are used in
the program:
1 - START-specify name and
starting address for the program. START <constant>
2 - END-indicate the end of
the source program and specify the first execut able instruction in the
program. END [ <operand spec> ]
3 - BYTE-generate character
or hexadecimal constant, occupying as many bytes as needed to represent the
constant.
4 - WORD-generate one word integer
constant.
Assembler
directives instruct the assembler to perform certain actions during the
assembly of a program. Some assembler directives are described in the
following.
START <constant>
This
directive indicates that the first word of the target program generated by the
assembler should be placed in the memory word with address<constant>.
END [<operand spec>]
This
directive indicates .the end of the source program. The optional <operand spec> indicates the address of the
instruction where the execution of the program should begin. (By default,
execution begins with the first instruction of the assembled program.).
Advantages of assembly
language
1 . Reduced errors.
2 . Faster translation
times.
3 . Changes could be made
easier and faster.
Disadvantages
1 . Many instructions are
required to achieve small tasks.
2 . Source programs tend to
be large and difficult to follow.
3 . Programmer requires
knowledge of the processor architecture and instruction set.
4 . Programs are machine
dependent, requires complete rewrites if the hardware is changed.
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