2.1Basic Assemblers Functions

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Basic Assembler Functions

Basic assembler functions 2

Function of an assembler

Assembly language program

Assembler

Database

Machine language and other information for the loader

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Fundamental assembler functions

Translate mnemonic operation codes their machine language equivalents.Assign machine addresses to symbolic labels used by the programmer.

The features and design of an assembler depend heavily on –the source language it translates and–the machine language it produces.

e.g., the instruction format and addressing modes


Assembler directives

The assembler can also process assembler directives.• Assembler directives (or pseudo-instructions)

provide instructions to the assembler itself. They are not translated into machine instructions.

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Assembler directives (cont’d)

The SIC assembler language has the following assembler directives.• START Specify name and staring address for the program• END Indicate the end of the source program and (optionally)

specify the first executable instruction in the program• BYTE Generate character or hexadecimal constant, occupying

as many bytes as needed to represent the constant

• WORD Generate one-word integer constant• RESB Reserve the indicated number of bytes for a data area• RESW Reserve the indicated number of words for a data area


Example of a SIC assembler language program

a forward reference: a reference to a label that is defined later in the program

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Example of a SIC assembler language program (cont’d)

Lines beginning with “.” contain comments only.


Example of a SIC assembler language program (cont’d)

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Translation of source program to object codeRequire to accomplish the following functions• Convert mnemonic operation codes to their machine language equivalents

• e.g. translate STL to 14• process assembler directives

• Convert symbolic operands to their equivalent machine addresses• e.g. translate RETADR to 1033• handle forward references

• two passes• the first pass scans the source program for label definitions and assigns addresses• the second performs most of the actual translation.

• Build the machine instructions in the proper format• Convert the data constants specified in the source program into their internal

machine representation• e.g. translate EOF to 454F46

• Write the object program and the assembly listing• Object program format


Program with object codes

RETADR

STL

a large memory space

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Program with object codes (cont’d)


Program with object codes (cont’d)

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Format of SIC object program

Address of first executable instruction in object program (hex)Col. 2-7ECol. 1

End record:Object code, represented in hexadecimal (2 columns per byte of object code)Col. 10-69Length of object code in this record in bytes (hex.)Col. 8-9Starting address for object code in this record (hex.)Col. 2-7TCol. 1

Text record:Length of object program in bytes (hex.)Col. 14-19Starting address of object program (hex.)Col. 8-13Program nameCol. 2-7HCol. 1

Header record:


Object program

No object code corresponds to addresses 1033-2038.This storage is reserved by the loader for use by the program during execution.

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A simple two-pass assemblerPass 1 (define symbols)• Assign addresses to all statements in the program.• Save the values(addresses) assigned to all labels for use in Pass 2.• Perform some processing of assembler directives.

• Include processing that affects address assignment such as determining the length of data areas defined by BYTE, RESW, etc.

Pass 2 (assemble instructions and generate object program)• Assemble instructions.

• translate operation codes• look up addresses

• Generate data values defined by BYTE, WORD, etc.• Perform processing of assembler directives not done during Pass 1.• Write the object program and the assembly listing.

Assembler Algorithm and Data Structures

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Internal data structures

the Operation Code Table (OPTAB)• OPTAB is used to look up mnemonic operation codes

and translate them to their machine language equivalents.

the Symbol Table (SYMTAB)• SYMTAB is used to store values (addresses) assigned

to labels.a Location Counter (LOCCTR)• This is a variable that is used to help in the

assignment of address.


Internal data structures (cont’d)

SourceProgram

SYMTAB

OPTAB

LOCCTR

Intermediate file

Pass 1 of assembler

ObjectProgram

Pass 2 of assembler

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OPTAB

In most cases, OPTAB is a static table.OPTAB must contain the mnemonic operation code and its machine language equivalentIn more complex assemblers, OPTAB also contains information about instruction format and length.OPTAB is usually organized as a hash table, with mnemonic operation code as the key.


OPTAB (cont’d)class optab_entry {

public:

char mnemonic[6];

char opcode;

};

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SYMTABSYMTAB includes the name and value (address) for each label in the source program together with flags to indicate error conditions. • e.g., a symbol defined in two different places

This table may also contain information, such as type or length, about the data area or instruction labeled.SYMTAB is usually organized as a hash table for efficiency of insertion and retrieval.• the label is the key of SYMTAB.

• non-random key


SYMTAB (cont’d)class symtab_entry {

public:

char symbol[8];

int address;

};

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Intermediate file

Pass 1 usually generates an intermediate file that contains• each source statement together with its assigned address,

error indicators, etc.This file is used as the input to Pass 2.This file retains some results of operations performed during Pass 1• the scanned operand field for symbols and addressing

flags• pointers into OPTAB and SYMTAB for each operation

code and symbol used.


LOCCTR

LOCCTR is a variable.LOCCTR is initialized to the beginning address specified in the START statement.After each source statement is processed, the length of the assembled instruction or data area to be generated is added to LOCCRT.When a label is reached, the current value of LOCCTR gives the address to be associated with that label.

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Algorithm for Pass 1


Algorithm for Pass 1 (cont’d)

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Algorithm for Pass 2


Algorithm for Pass 2 (cont’d)

trace an objectprogram

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Table processingSearching in a table• Key: symbol name• Methods:

• Linear search: compare exhaustively every entry in the table

• Binary search:• ordered table

• sorting• Hash

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2.1Basic Assemblers Functions