Assembly Language Programming and Memory Models

Assembly language programming

A low-level programming language that is closely tied to a computer's hardware architecture, using mnemonics and symbolic names instead of binary machine code.

Low-Level Language

Assembly language is considered a low-level language because it operates directly with the computer's hardware, allowing precise control over hardware components.

Mnemonics for Machine Instructions

Assembly language uses mnemonics (short codes) to represent machine instructions, making programming easier than dealing with raw binary.

Direct Hardware Interaction

Assembly interacts directly with the hardware, often used for tasks requiring speed and efficiency, such as device drivers and embedded systems.

Architecture-Specific

Assembly language is specific to a particular CPU architecture, with each processor family having its own instruction set architecture (ISA).

Assembler

An assembler converts assembly language programs into machine code, translating mnemonics into machine-readable binary instructions.

.MODEL

This directive defines the memory model of the program, with TASM supporting several models like TINY, SMALL, and LARGE.

TINY Model

Code and data in a single segment, used for very small programs (COM files) under 64KB.

SMALL Model

Code and data in separate segments, each fitting within a single 64KB segment, suitable for moderate-size applications.

MEDIUM Model

Multiple code segments and a single data segment, allowing code to span up to 1MB while data is limited to 64KB.

COMPACT Model

Single code segment with multiple data segments, allowing larger data structures while keeping code size manageable.

LARGE Model

Multiple code and data segments, both can span up to 1MB each, supporting large applications or systems software.

HUGE Model

Similar to the large model, but allows individual data structures to exceed 64KB.

.DATA

The ________ segment directive is used to declare variables and constants, holding data accessible by the program.

.CODE

This segment contains the actual instructions (the code) that the CPU will execute.

.STACK

The __________ directive defines a segment for the stack, used for storing return addresses, local variables, and passing parameters.

END

The END directive marks the end of the program and specifies the entry point (optional).

Memory Variables

Data stored in memory that can be accessed and manipulated by the program, depending on size, type, and specific needs.

Byte Variables (DB)

Size: 1 byte (8 bits); used for storing small integer values or characters, with a range of 0 to 255 (unsigned) or -128 to 127 (signed).

Word Variables (DW)

Size: 2 bytes (16 bits); used for storing larger integers or addresses, with a range of 0 to 65535 (unsigned) or -32768 to 32767 (signed).

Double Word Variables (DD)

Size: 4 bytes (32 bits); used for storing 32-bit integers or addresses, with a range of 0 to 4,294,967,295 (unsigned) or -2,147,483,648 to 2,147,483,647 (signed).

Quad Word Variables (DQ)

Size: 8 bytes (64 bits); used for storing very large integer values, especially in 64-bit systems.

Ten Byte Variables (DT)

Size: 10 bytes (80 bits); typically used for storing floating-point numbers or extended precision data.

String Variables

Defined using a sequence of bytes and are often null-terminated (ending with a 0 or $ in DOS assembly).

Array Variables

Defined as a sequence of values (bytes, words, etc.) and accessed using index values or offsets.

Pointer Variables

Store memory addresses, typically represented as words (16-bit) or double words (32-bit), pointing to other memory locations.