Understanding Assembly Language: Instructions and Assemblers

Understanding Assembly Language

Assembly language is a low-level programming language where instructions are written using mnemonic codes rather than binary machine language. An assembler translates these instructions into executable machine code.

Classification of Assembly Instructions

Assembly language instructions are categorized into three primary types:

• Imperative Statements (IS): Specify operations for the processor to perform. These generate executable machine code. Examples include: STOP, ADD, SUB, MULT, MOVER, MOVEM, COMP, BC, DIV, READ, and PRINT.
• Declarative Statements (DL): Used for memory allocation and data definition. DS (Define Storage) reserves memory locations, while DC (Define Constant) stores a constant value in memory.
• Assembler Directives (AD): Provide instructions to the assembler and do not generate machine code. Key directives include: START (starting address), END (program termination), ORIGIN (changes Location Counter), EQU (assigns values to symbols), and LTORG (allocates storage for literals).

The Location Counter (LC)

The Location Counter (LC) is used by the assembler to assign addresses to instructions and data. It is initialized by the START directive and increments as each instruction is processed. Declarative statements reserve memory, and literals receive addresses when LTORG or END is encountered.

Example Program Analysis

START 200
LOOP MOVER AREG, ='5'
ADD AREG, NUM
MOVEM AREG, RESULT
SUB AREG, ='1'
BC ANY, LOOP
NUM DC 10
RESULT DS 1
END

LC Processing Breakdown:

• START initializes LC to 200.
• MOVER (200), ADD (201), MOVEM (202), SUB (203), BC (204).
• NUM (DC) receives address 205; RESULT (DS) receives address 206.
• At END, literals are assigned: ='5' (207) and ='1' (208).

Symbol and Literal Tables

• Symbol Table: Stores symbol names and their addresses (e.g., LOOP: 200, NUM: 205, RESULT: 206).
• Literal Table: Stores literals and their assigned addresses (e.g., ='5': 207, ='1': 208).

During Pass I of the assembler, LC processing is performed, and the Symbol and Literal Tables are generated to facilitate machine code generation.