Assembler

System software

Programming Languages

Low Level Languages Machine language Assembly language.

High Level Languages

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Translating Languages

English: Display the sum of A times B plus C.

C: printf (A * B + C);

Assembly Language:

Mover areg ,Amul areg,Badd areg,C print areg

Intel Machine Language:

A1 00000000

F7 25 00000004

03 05 00000008

E8 00500000

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Low Level Languages

low level means closeness to the way in which the machine has been built.

Low level languages are machine oriented and require extensive knowledge of computer hardware and its configuration.

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Machine Language first generation language only language that is directly understood by the

computer. It does not needs any translator program. also call it machine code and it is written as strings

of 1's (one) and 0’s (zero). When a sequence of codes is fed to the computer,

it recognizes the codes and converts it in to electrical signals needed to run it.

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Advantage Programs written in machine language

run very fast because no translation program is required for the CPU.

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Disadvantages

1. It is very difficult to program in machine language. The programmer has to know details of hardware to write program.

2. The programmer has to remember a lot of codes to write a program which results in program errors.

3. It is difficult to debug the program.

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Assembly Language Symbolic Representation of a numeric machine

language Why?

Easy to read and understand. Easy to remember ADD, M0V rather than their

hexadecimal numbers. Performance: A good assembly language program can

beat any optimizing compiler. Access to Machine: A high level language does not

have access to underlying machine. One line of assembly corresponds to one

instruction

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ASSEMBLY LANGUAGE PROGRAMMING

Assembly language is a human-readable notation for the machine language that a specific computer architecture uses

Assembly language programming is writing machine instructions in mnemonic form

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Three Basic features

Mnemonic operation codes-eliminates the need to memorize numeric operation codes.Ex: ADD,SUB,MOVEM,MOVER

Symbolic operands-symbolic names can be used as operands

Data declarations-data can be declared in a variety of notations

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Statement Assembly language statements are

written one per line An assembly code program thus consists

of a sequence of assembly language statements, where each statement contains a mnemonic.

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Statement format [Label ] <Opcode> <Operand spec> [,

<operand spec>, …]

[ ] indicates optional symbol. Ex : Read N

Add BREG, One

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Label A label is an identifier. It is Symbolic name for

memory addresses. Labels are used extensively in programs to

reduce reliance upon programmers remembering where data or code is located

A label is a piece of text that gets assigned the memory address of where it is located in the file

It starts with first column A label, when declared, begins with a valid

character (A..Z).

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Opcode Opcode stands for operation code or a

machine code instruction Two Kinds:

OpCode Command to assembler (assembler

directive) Opcode

Symbolic representation of Opcode. Eg : stop,add,sub mover,movem etc

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Operand The operand field consists of additional

information or data that the opcode requires. Operand specification is of the form Register

operand, Memory operand. A statement can have two operands and the

first operand is always a Register. < operand > specification has the syntax <symbolic name > [ + < displacement > ] [ < index register>]

A A + 5 A(5) A+4(5)

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Assemble Language statements

Assembly Language Statement

Imperative Declarative Assembler Statement Statement Directive

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     Imperative Statement

Indicates an action to be performed during the execution of assembled program

Each statement translates into one machine instruction

Eg; MOVER CREG ONE

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Declarative Statement

Indicates some declaration syntax

[Label] DS <constant>

[Label] DC ‘<value>’

Eg:

A DS 2

ONE DC ‘1’

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Assembler Directives

We need to give the assembler information about the program such as:

- where to locate code- allocating memory- setting up constants

We accomplish the above using “Directives”. The assembler directives are specific to the ‘assembler’.

The assembler recognizes keywords and knows what to do

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ASSEMBLER DIRECTIVES

Assembler directives are instructions to the assembler, and are not translated into machine instructions.

The use of directives gives the programmer some control over the operation of the assembler, increasing flexibility in the way programs are written.

Ex: START < constant> START 100 (Giving direction to the assembler to

start execution from the memory 100)

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Assembler Directives

ORIGIN -This tells the assembler where to insert the program code. Any instructions that follow this directive will be inserted sequentially into memory.

Origin < Address space>

EQU –This will associate label with address specification.it is used mainly for readability in code.

<Symbol> EQU <address specification>

END –It indicates the end of the source programEND [< operand spec>] operand specification

Indicates address of instruction where execution should begin.

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Assembler Directives

LTORG– Permits a programmer to specify where literals should be placed.

At every LTORG statement assembler allocates memory to literals of a literal pool

A literal pool contains all literals used in the program since start or since last LTORG statement.

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Assembler Are programs which generate machine

code instructions from a source code program written in assembly language

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Design specification

How to design an assembler? What information is required? A Language processor has to analyze the

source program and synthesize the target program

There two main functions are

Analyze the source program-Analysis phase

Synthesize the target program-Synthesis phase

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Analysis-Synthesis Model

There are two main parts :1. Analysis of the source code

Breaks the source program into its component parts ,create a symbol table and create an intermediate representation of the source program

2. Synthesis of the target program Constructs the desired target program from

the intermediate representation

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Analysis Phase Its primary function is to build the symbol table and

perform LC Processing. LC processing means determine the memory address

of all program elements. LC is initialized to the constant specified in start

statement and Does the following :

1. It isolates label,opcode and operand fields

2. If label is present,enter label and address in symbol table

3. Check validity of Mnemonic code from mnemonic table

4. Perform LC processing

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Synthesis Phase

Mainly concerned with constructing target language statements which have same meaning as a source statement.

It achieves these by doing the following:

1. Obtain opcode from Mnemonic table

2. Obtain address of operand from symbol table.

3. Synthesize the machine instruction

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Design options in an Assembler

Main issue is whether to have a single pass assembler or a two pass assembler

Pass means one complete scan of the source program

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Assembly Process Two pass process: Pass One: Collects definitions of symbols,

statement labels etc. Pass Two: Re-reads the statements,

replaces symbolic names with values, and translates to target language.

Why Two Passes? Can Handle forward referencing.

Need to find values of names that have not yet been defined.

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Design of a Two Pass Assembler

Pass I :1. Separate label,opcode and operand

fields2. Build the symbol table3. Perform LC processing4. Construct IR

Pass II1. Synthesize the target program

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Data Structures in Pass I

OPTAB – Contains Fields for Opcode, Class,Mnemonic info.

SYMTAB – Contains Fields for symbol, address ,length.

LITTAB – Contains the fields literals,address. POOLTAB – Contains literal number of

starting literal of each literal pool.Used for awareness of different literal pools.

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Intermediate Code forms

Mnemonic fields : of the form

(Statement Class,code)

EX: (AD,02) (DC,01) (IS,09) Operands:of the form

(Operand Class,code)

Ex: (c,200) (S,02) (L,01)

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Pass I pass 1: loop until the end of the program

1. read in a line of assembly code2. If there is a label,save symbol and address

values in symbol table.3. If LTORG then,allocate memory and enter

literals in LITTAB and update LC.4. If assembler directives START or ORIGIN then

LC= Address specified.5. If Equ stmt ,enter in to symbol table with

address specified and generate code(symbol,address)

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Pass I

6. IF Imperative stmt then code= machine opcode , if operand is literal then,enter into literal table else save address values assigned to labels in symbol tables and increment LC and generate IC (IS,code) (S,address).

7. If DC,code=machine opcode, allocate memory and update LC and generate IC ( DL,code)

For processing END statement,

Repeat step 3 and generate IC(AD,02)

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Pass II Uses the information collected in pass I to

synthesize the target program. Pass II:loop until the end of the program

1. read in a line of code2. If an LTORG stmt,assemble the literals in Buffer and size =size of memory required for literals. 3. If start or origin,LC=address specified ,size=0

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Pass II

4. If Declarative stmt,assemble constant in Buffer and size= size required by DC/DS stmt.

5. if an Imperative stmt,get operand address and assemble in buffer,size=size of instruction.

6. If Size !=0 (i)move contents of the buffer to code_area + LC.(ii) LC=LC+size.

Processing end stmt,repeat steps 2 and 6.

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Single Pass Assembler

The problem of Forward references is handled by using a process called back patching.

Operand field of an instruction having forward reference is left blank initially and the address of the instruction is inserted into TII along with the operand

Syntax (< Instruction address> ,<symbol>) Ex ( 105,a).

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Single passAssembler

By the time END stmt is processed, symbol table contains address of all symbols.

TII contains all instructions containing forward references.

All entries in TII can be processed to complete the concerned instruction.

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features -assembler allows the programmer to use mnemonics

when writing source code programs. variables are represented by symbolic names,

not as memory locations symbolic code is easier to read and follow error checking is provided changes can be quickly and easily

incorporated with a re-assembly

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Disadvantages: it is machine dependent. A program written for one computer might not run in

other computers with different hardware configuration. the programmer requires knowledge of the

processor architecture and instruction set many instructions are required to achieve small tasks source programs tend to be large and difficult to follow