Explain the data structures used in the pass1 algorithm of an assembler with a suitable example

The Pass1 algorithm of an assembler uses several data structures to convert assembly language into machine language. Here are the main data structures used:

1. Symbol Table: This is used to store all the labels (symbols) and their addresses. Each entry in the symbol table has two fields: the symbol name and its address. For example, if we have a label 'LOOP' at address '2046', the symbol table will have an entry like this: {LOOP, 2046}.

2. Literal Table: This is used to store all the literals and their addresses. A literal is a value that is used directly in the program. For example, if we have a literal '#10' at address '3000', the literal table will have an entry like this: {#10, 3000}.

3. Opcode Table (OPTAB): This is used to store all the operation codes (opcodes) and their machine equivalent. For example, the opcode 'ADD' might have a machine equivalent of '18'. So, the opcode table will have an entry like this: {ADD, 18}.

4. Base Table (BTAB): This is used to store all the base registers and their current contents. For example, if we have a base register 'B' with a current content of '4096', the base table will have an entry like this: {B, 4096}.

5. Intermediate File: This is used to store the intermediate representation of the source program. This file is used in Pass2 of the assembler to generate the final object code.

6. Location Counter (LC): This is used to keep track of the current instruction address. The LC is incremented after each instruction is processed.

Let's take an example:

Consider the following assembly language program:

START 1000
LDA ALPHA
STA BETA
ALPHA RESW 1
BETA RESW 1
END

In Pass1, the assembler will first initialize the LC to the starting address '1000'. Then it will process each instruction one by one. For the 'LDA ALPHA' instruction, it will add 'ALPHA' to the symbol table with a yet unknown address. For the 'STA BETA' instruction, it will add 'BETA' to the symbol table with a yet unknown address. When it encounters 'ALPHA RESW 1', it will update the address of 'ALPHA' in the symbol table to the current LC value. Similarly, for 'BETA RESW 1', it will update the address of 'BETA' in the symbol table to the current LC value. The intermediate file will contain the intermediate representation of these instructions.