Page 67 - DCAP507_SYSTEM_SOFTWARE

Unit 4: Design of Assembler

Notes
Figure 4.3: Pass 2 Overview

4.1.4 Format of the Databases

The third step in our design process is to identify the format and content of each of the databases
– a mission that must be undertaken even before depicting the particular algorithm underlying
the assembler design.
Pass 2 needs a Machine Operation Table containing name, length, binary code, and format; Pass
1 requires only name, format, and length. We could use two separate tables with different
formats and contents or use the same table for both passes; the same is true of the Directive/
Pseudo Operation Table. By simplifying the table formats, we could merge the tables into one
table. For this particular design, we will use disconnected tables.

Once we decide what information fist in each database, it is essential to identify the format of
each entry.

Example: In what format are symbols stored (e.g., left justified, padded with blanks,
coded in ASCII) and what are the coding conventions?

Example: The Machine-Op Table and Pseudo Op Tables are examples of fixed tables.
The contents of these tables are not filled in or tainted during the assembly process.

Notes In real life, the algorithm, database, and formats are all interconnected. Their
specification is in practical designs, circular, in that the designer has in mind some traits of
the format and algorithm he/she plans to use and continues to iterate their design until all
cases function.

LOVELY PROFESSIONAL UNIVERSITY 61