Two Pass Assemblers

Design of 2 Pass Assemblers

 

An assembler is a translator, that translates an assembler program into a conventional machine language program. Basically, the assembler goes through the program one line at a time, and generates machine code for that instruction. 

Then the assembler processes to the next instruction. In this way, the entire machine code program is created

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What is the difference between one pass and two pass assembler?

What is a single pass assembler?

It is a assembler that generally generates the object code directly in memory for immediate execution. It parses through your source code only once and your done.

Now let us see how a two pass assembler works.

Simple, while on its way, if the assembler encounters an undefined label, it puts it into a symbol table along with the address where the undefined symbol's value has to be placed, when the symbol is found in future

 

Why do we need a two pass assembler?

As explained, one-pass assembler cannot resolve forward references of data symbols. It requires all data symbols to be defined prior to being used. A two-pass assembler solves this dilemma by devoting one pass to exclusively resolve all (data/label) forward references and then generate object code with no hassles in the next pass.

If a data symbol depends on another and this another depends on yet another, the assembler resolved this recursively. If I try explaining even that in this post, the post will become too big. Read this ppt for more details

Topics covered:
How 2 pass assembler works?
2 pass assembler algorithm.
2 pass assembler Design.
2 pass assembler program


 Agenda

Introduction 
 
Advanced Assembler Directives

1. ORIGIN
2. EQU
3. LT ORG

Pass I of the Assembler

Data Structure used in Pass I

1. OPTAB
2. SYMTAB
3. LITTAB
4. POOLTAB

Algorithm
Intermediate code
Declaration and Assembler Directive Processing


Pass II of the Assembler
 

 Introduction
 

• Convert mnemonic operation codes to their machine language equivalents
• Convert symbolic operands to their equivalent machine addresses
• Build the machine instructions in the proper format
• Convert the data constants to internal machine representations
• Write the object program and the assembly listing

Two Pass Assembler

• Read from input line
  • LABEL, OPCODE, OPERAND

Two Pass Assembler

  

 Design of 2 - Pass Assembler

Pass 1:

1. Separate the Symbol, Mnemonic opcode and operand fields
2. Build the symbol table
3. Perform LC Processing
4. Construct Intermediate Representation

Pass 2

Synthesize the target program

Advanced Assembler Directives

• ORIGIN
• EQU

ORIGIN

Syntax:
            ORIGIN < Address Specification>

 

EQU

Syntax:
        <Symbol> EQU <Address Specification>

E.g.     MAXLEN    EQU    4096

Pass I of Assembler 

• Pass I Uses following Data Structures
  • OPTAB
  • SYMTAB
  • LITTAB
  • POOLTAB 

OPTAB

SYMTAB: Symbol Table

LITTAB: Table of Literals used in program

POOLTAB: A table of information concerning literal pool

Algorithm

The main reason why most assemblers use a 2-pass system is to address the problem of forward references -- references to variables or subroutines that have not yet been encountered when parsing the source code. A strict 1-pass scanner cannot assemble source code which contains forward references. Pass 1 of the assembler scans the source, determining the size and address of all data and instructions; then pass 2 scans the source again, outputting the binary object code.

Some assemblers have been written to use a 1.5 pass scheme, whereby the source is only scanned once, but any forward references are simply assumed to be of the largest size necessary to hold any native machine data type . The unknown quantity is temporarily filled in as zero during pass 1 of the assembler, and the forward reference is added to a 'fix-up list'. After pass 1, the '.5' pass goes through the fix-up list and patches the output machine code with the values of all resolved forward references. This can result in sub-optimal opcode construction, but allows for a very fast assembly phase.

Anykind of questions and suggestions are welcome below in comments