System Programming Set 1

8/3/2019 System Programming Set 1

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System Programming MC0073

Set -1

1. Explain the following.a. Lexical Analysisb.

Syntax Analysis

Ans.

a. Lexical AnalysisThe lexical analyzer is the interface between the

source program and the compiler. The lexical analyzerreads the source program one character at a time,

carving the source program into a sequence of atomic

units called tokens. Each token represents a sequence

of characters that can be treated as a single logical

entity. Identifiers, keywords, constants, operators, and

punctuation symbols such as commas and parenthesesare typical tokens. There are two kinds of token:

specific strings such as IF or a semicolon, and classes

of strings such as identifiers, constants, or labels.


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The lexical analyzer and the following phase, the

syntax analyzer are often grouped together into the

same pass. In that pass, the lexical analyzer operates

either under the control of the parser or a co-routine

with the parser. The parser asks the lexical analyzer

returns to the parser a code for the token that it

found. In the case that the token is an identifier or

another token with a value, the value is also passed to

the parser. The usual method of providing this

information is for the lexical analyzer to call abookkeeping routine which installs the actual value in

the symbol table if it is not already there. The lexical

analyzer then passes the two components of the token

to the parser. The first is a code for the token type

(identifier), and the second is the value, a pointer to

the place in the symbol table reserved for the specificvalue found.

b. Syntax AnalysisThe parser has two functions. It checks that

the tokens appearing in its input, which is the

output of the lexical analyzer, occur in patterns

that are permitted by the specification for the

source language. It also imposes on the tokens a


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tree-like structure that is used by the

subsequent phases of the compiler.

The second aspect of syntax analysis is to makeexplicit the hierarchical structure of the

incoming token stream by identifying which

parts of the token stream should be grouped

together.

2. What is RISC and how it is different from theCISC ?

Ans. The Reduced Instruction Set Computer, or RISC,

is a microprocessor CPU design philosophy that favors

a simpler set of instructions that all take about the

same amount of time to execute. The most commonRISC microprocessors are AVR, PIC, ARM, DEC Alpha,

PA-RISC, SPARC, MIPS, and IBMs PowerPC.

RISC characteristics

- Small number of machine instructions : less than 150


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- Small number of addressing modes : less than 4

- Small number of instruction formats : less than 4

- Instructions of the same length : 32 bits (or 64 bits)

- Single cycle execution

- Load / Store architecture

- Large number of GRPs (General Purpose Registers):

more than 32

- Hardwired control

- Support for HLL (High Level Language).

RISC VS CISC

CISC RISC

Emphasis on

hardware Emphasis on software

Includes multi-clock

complex

instructions

Single-clock,

reduced instruction

only

Memory-to-

memory: Register to register:


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"LOAD" and

"STORE"

incorporated ininstructions

"LOAD" and "STORE"

are independent

instructions

Small code sizes,

high cycles per

second

Low cycles per second,

large code sizes

Transistors used

for storing

complex

instructions

Spends more

transistors

on memory registers

3. Explain the following with respect to the

design specifications of an Assembler:

A) Data Structures

B) pass1 & pass2 Assembler flow chart

Ans. A) Data Structures


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The second step in our design procedure is to establish

the databases that we have to work with.

Pass 1 Data Structures1. Input source program

2. A Location Counter (LC), used to keep track of each

instructions location.

3. A table, the Machine-operation Table (MOT), that

indicates the symbolic mnemonic, for each instructionand its length (two, four, or six bytes)

4. A table, the Pseudo-Operation Table (POT) that

indicates the symbolic mnemonic and action to be taken

for each pseudo-op in pass 1.

5. A table, the Symbol Table (ST) that is used to storeeach label and its corresponding value.

6. A table, the literal table (LT) that is used to store

each literal encountered and its corresponding

assignment location.

7. A copy of the input to be used by pass 2.Pass 2 Data Structures

1. Copy of source program input to pass1.


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2. Location Counter (LC)

3. A table, the Machine-operation Table (MOT), that

indicates for each instruction, symbolic mnemonic,length (two, four, or six bytes), binary machine opcode

and format of instruction.

4. A table, the Pseudo-Operation Table (POT), that

indicates the symbolic mnemonic and action to be taken

for each pseudo-op in pass 2.

5. A table, the Symbol Table (ST), prepared by pass1,

containing each label and corresponding value.

6. A Table, the base table (BT), that indicates which

registers are currently specified as base registers by

USING pseudo-ops and what the specified contents of

these registers are.

7. A work space INST that is used to hold each

instruction as its various parts are being assembled

together.

8. A work space, PRINT LINE, used to produce a

printed listing.9. A work space, PUNCH CARD, used prior to actual

outputting for converting the assembled instructions

into the format needed by the loader.


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10. An output deck of assembled instructions in the

format needed by the loader.

Fig. 1.3: Data structures of the assembler

B) pass1 & pass2 Assembler flow chart

The third step in our design procedure is to specify

the format and content of each of the data structures.Pass 2 requires a machine operation table (MOT)

containing the name, length, binary code and format;

pass 1 requires only name and length. Instead of using

two different tables, we construct single (MOT). The

Machine operation table (MOT) and pseudo-operation

table are example of fixed tables. The contents of

these tables are not filled in or altered during the

assembly process.


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The following figure depicts the format of the

machine-op table (MOT)

6 bytes per entry

Mnemonic

Opcode

(4bytes)characters

Binary

Opcode

(1byte)(hexadecimal)

Instruction

length

(2 bits)(binary)

Instruction

format

(3bits)(binary)

Not

used

here

(3bits)

Abbb 5A 10 001

Ahbb 4A 10 001

ALbb 5E 10 001

ALRB 1E 01 000

. . . .

b represents blank

Fig.:

The flowchart for Pass 1:


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The primary function performed by the analysis phase

is the building of the symbol table. For this purpose it

must determine the addresses with which the symbol

names used in a program are associated. It is possible

to determine some address directly, e.g. the address

of the first instruction in the program, however others

must be inferred.

To implement memory allocation a data structure

called location counter (LC) is introduced. The location

counter is always made to contain the address of the

next memory word in the target program. It is


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initialized to the constant. Whenever the analysis

phase sees a label in an assembly statement, it enters

the label and the contents of LC in a new entry of the

symbol table. It then finds the number of memory

words required by the assembly statement and

updates; the LC contents. This ensure: that LC points

to the next memory word in the target program even

when machine instructions have different lengths and

DS/DC statements reserve different amounts of

memory. To update the contents of LC, analysis phaseneeds to know lengths of different instructions. This

information simply depends on the assembly language

hence the mnemonics table can be extended to include

this information in a new field called length. We refer

to the processing involved in maintaining the location

counter asLC processing


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Flow chart for Pass 2

Fig. : Pass2 flowchart


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4. Define the following,

A) Parsing

B) ScanningC) Token

Ans.

A) Parsing

Parsing transforms input text or string into a data

structure, usually a tree, which is suitable for later

processing and which captures the implied hierarchy of

the input. Lexical analysis creates tokens from a

sequence of input characters and it is these tokens

that are processed by a parser to build a data

structure such as parse tree or abstract syntax trees.

Conceptually, the parser accepts a sequence of tokens

and produces a parse tree. In practice this might notoccur.

1. The source program might have errors. Shamefully,

we will do very little error handling.


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2. Real compilers produce (abstract) syntax trees not

parse trees (concrete syntax trees). We dont do this

for the pedagogical reasons given previously.

There are three classes for grammar-based parsers.

1. Universal

2. Top-down

3. Bottom-up

The universal parsers are not used in practice as they

are inefficient; we will not discuss them.

As expected, top-down parsers start from the root of

the tree and proceed downward; whereas, bottom-up

parsers start from the leaves and proceed upward. The

commonly used top-down and bottom parsersare not universal. That is, there are (context-free)

grammars that cannot be used with them.

The LL and LR parsers are important in practice. Hand

written parsers are often LL. Specifically, the

predictive parsers we looked at in chapter two are for

LL grammars. The LR grammars form a larger class.Parsers for this class are usually constructed with the

aid of automatic tools.


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B) ScanningCompiler is a program which converts the source

program into machine level language. It is a translator.Compiler performs analysis for sentence generations

and interpretations. One phase output will go to the

next phase as input. Conceptually, there are three

phases of analysis with the output of one phase the

input of the next. Each of these phases changes the

representation of the program being compiled. Thephases are called lexical analysis or scanning, which

transforms the program from a string of characters to

a string of tokens.

C)

TokenSyntax Analysis or Parsing, transforms the program

into some kind of syntax tree; and Semantic Analysis,

decorates the tree with semantic information.

The character stream input is grouped into meaningful

units called lexemes, which are then mappedinto tokens, the latter constituting the output of the

lexical analyzer.

For example, any one of the following C statements


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x3 = y + 3;

x3 = y + 3 ;

x3 = y+ 3 ;

but not

x 3 = y + 3;

would be grouped into the lexemes x3, =, y, +, 3, and ;.

A token is a

pair. The hierarchical

decomposition above sentence is given figure

Fig.

A token is a pair.


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5. Describe the process of Bootstrapping in the

context of Linkers

Ans. The discussions of loading up to this point have all

presumed that theres already an operating system or

at least a program loader resident in the computer to

load the program of interest. The chain of programs

being loaded by other programs has to start

somewhere, so the obvious question is how is the first

program loaded into the computer?

In modern computers, the first program the computer

runs after a hardware reset invariably is stored in a

ROM known as bootstrap ROM. as in "pulling ones self

up by the bootstraps."

When the CPU is powered on orreset, it sets its registers to a known state. On x86

systems, for example, the reset sequence jumps to the

address 16 bytes below the top of the systems

address space. The bootstrap ROM occupies the top

64K of the address space and ROM code then starts up

the computer. On IBM-compatible x86 systems, theboot ROM code reads the first block of the floppy disk

into memory, or if that fails the first block of the

first hard disk, into memory location zero and jumps to

location zero. The program in block zero in turn loads a


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slightly larger operating system boot program from a

known place on the disk into memory, and jumps to that

program which in turn loads in the operating system

and starts it. (There can be even more steps, e.g., a

boot manager that decides from which disk partition to

read the operating system boot program, but the

sequence of increasingly capable loaders remains.)

Why not just load the operating system directly?

Because you cant fit an operating system loader into512 bytes. The first level loader typically is only able

to load a single-segment program from a file with a

fixed name in the top-level directory of the boot disk.

The operating system loader contains more

sophisticated code that can read and interpret a

configuration file, uncompress a compressed operatingsystem executable, address large amounts of memory

(on an x86 the loader usually runs in real mode which

means that its tricky to address more than 1MB of

memory.) The full operating system can turn on the

virtual memory system, loads the drivers it needs, and

then proceed to run user-level programs.

Many Unix systems use a similar bootstrap process to

get user-mode programs running. The kernel creates a

process, then stuffs a tiny little program, only a few


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dozen bytes long, into that process. The tiny program

executes a system call that runs /etc/init, the user

mode initialization program that in turn runs

configuration files and starts the daemons and login

programs that a running system needs.

None of this matters much to the application level

programmer, but it becomes more interesting if you

want to write programs that run on the bare hardware

of the machine, since then you need to arrange tointercept the bootstrap sequence somewhere and run

your program rather than the usual operating system.

Some systems make this quite easy (just stick the

name of your program in AUTOEXEC.BAT and reboot

Windows 95, for example), others make it nearly

impossible. It also presents opportunities forcustomized systems. For example, a single-application

system could be built over a Unix kernel by naming the

application /etc/init.

6. Describe the procedure for design of a Linker.

Ans. Relocation and linking requirements in

segmented addressing


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The relocation requirements of a program are

influenced by the addressing structure of the

computer system on which it is to execute. Use of the

segmented addressing structure reduces the relocation

requirements of program.

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System Programming Set 1