Assembly Language Fundamentals: Directives, Operands, Expressions

Posted on Jun 22, 2025 in Computers

Assembly Language Directives

Combination Directive

The Combination directive defines how to combine segments with the same name. Possible values include:

  • PUBLIC: All segments are concatenated into one.
  • STACK: Similar to PUBLIC, but with SS (Stack Segment) on registration addresses.
  • COMMON: Overlapping segments created by placing the start of all in the same direction.
  • MEMORY: Indicated by LINK segments, dealing with public as MASM, at a specific address.
  • AT address: Specifies a segment at a particular memory address.

Class Directive

The Class directive indicates the type of segment, marked with any name.

END Directive

The END directive indicates the end of a module. Upon reaching it, the assembler will ignore any statements that follow.

ASSUME Directive

The ASSUME directive allows you to specify the default values for segment registers.

Labels (Tags)

Labels are declared as name: where name is a string.

Data Declaration

Data is declared by type using the rule [name] directive value, where directive can be:

  • DB: Defines Byte (1 byte)
  • DW: Defines Word (2 bytes)
  • DD: Defines Double Word (4 bytes)
  • DQ: Defines Quad Word (8 bytes)
  • DT: Defines Ten Bytes (10 bytes)

You can also use the following directives to declare symbols:

  • LABEL: Creates labels for instructions or data.
  • EQU: Creates symbols of equality (constant values).
  • =: Assigns an absolute value to a symbol.

Structure Declaration

The STRUC directive is used for declaring data structures.

Assembly Instructions

Common categories of assembly instructions include:

  • Linear
  • Motion (Data Transfer)
  • Stack Operations
  • Arithmetic
  • Configuration
  • Compare
  • Jumps (Unconditional, Conditional)
  • Types of Cycles (Loops)
  • Logical Operators
  • Displacement (Shift/Rotate)
  • Linear/Circular
  • Process Control
  • Flags
  • String Operations
  • Load/Store

Operands in Assembly

Operands represent values, registers, or memory locations to be accessed in some way.

Expressions in Assembly

Expressions combine operands and arithmetic or logical operators to calculate a value or an access address.

Types of Operands

The allowed operands are listed below:

  • Constants

    Numbers, strings, or expressions that represent a fixed value.

  • Direct Addressing

    Specifies the memory address to access in the form segment:offset.

  • Relocatable Operands

    Uses a symbol associated with a memory address and can also be used for calls.

  • Location Counter ($)

    Used to indicate the current location in the segment during assembly. Represented by the symbol $ and also known as the sentinel.

  • Registers

    Refers to general-purpose registers, pointers, index registers, or segment registers.

  • Based Addressing

    Represents a memory address based on one of the base registers (BP or BX).

  • Indexed Addressing

    Represents a memory address based on one of the index registers (SI or DI).

  • Base-Indexed Addressing

    Represents a memory address based on the combination of base and index registers.

  • Structure Members

    The syntax is variable.field. variable is the name of the declared structure, and field is the name of the field within the structure.

Assembly Operators & Expressions

Assembly language includes the following types of operators:

  • Arithmetic
  • Shift
  • Relational
  • Bitwise
  • Index
  • Pointer
  • Field Name
  • Special Purpose

OFFSET Operator

OFFSET expression: Returns the offset of the operand.

SHORT Operator

SHORT label: Used for jumps of less than 128 bytes.

LENGTH Operator

LENGTH variable: Returns the number of elements.

SIZE Operator

SIZE variable: Returns the size in bytes occupied by a variable.

SEG Operator

SEG expression: Returns the segment value for an expression.

Example: mov ax, SEG greeting