What is memory segmentation in 8086 architecture?

Memory segmentation in  8086 architecture is a method used to manage memory efficiently. This technique allows the processor to access a larger address space than it could with a single linear address space. In this guide, we will explore what memory segmentation is, how it works in the 8086 microprocessor, and why it is essential. We’ll also discuss the memory organization of 8086 and its benefits.

What is Memory Segmentation?

Memory segmentation is a technique used by the 8086 microprocessor to divide its memory into smaller, manageable segments. Each segment is a block of memory that can be addressed independently. This approach helps in organizing memory logically, making it easier to manage and access. The primary advantage of memory segmentation is that it allows the processor to use a larger address space efficiently.

Why Use Memory Segmentation?

The 8086 microprocessor has a 20-bit address bus, which allows it to address up to 2^20 (1,048,576) memory locations. However, the processor itself can only handle 16-bit addresses directly, which limits it to 2^16 (65,536) memory locations at a time. Memory segmentation solves this problem by dividing the memory into segments, each of which can be accessed using a 16-bit address. This way, the 8086 can access the full 1,048,576 memory locations.

How Does Memory Segmentation Work?

Segment Registers

The 8086 uses four segment registers to handle memory segmentation:

Code Segment (CS):

• Stores the starting address of the code used by the CPU.
• It is used to access instructions that the CPU needs to execute.

Data Segment (DS):

• Holds the starting address of data used by the program.
• It is used to access variables and constants.

Stack Segment (SS):

• Contains the starting address of the stack.
• The stack is used for storing temporary data like function parameters, return addresses, and local variables.

Extra Segment (ES):

• Provides additional data storage.
• It is mainly used for string operations.

Offset Address

Each segment can address up to 64 KB of memory, and the actual address within the segment is specified by an offset address. The combination of a segment address and an offset address gives the physical address. This is calculated using the formula:

Physical Address = (Segment Address x 16) + Offset Address

Example

Let’s say the Code Segment (CS) register contains the value 0x1234 and the Instruction Pointer (IP) contains the value 0x5678. The physical address of the instruction to be executed is calculated as:

Physical Address = (0x1234 x 16) + 0x5678 = 0x12340 + 0x5678 = 0x179B8

Memory Organization of 8086

Memory Map

The 8086 microprocessor divides its 1 MB memory space into segments of 64 KB each. These segments can overlap, providing flexibility in memory usage. The memory map is organized as follows:

Code Segment:

• Contains executable instructions.
• The starting address is stored in the CS register.

Data Segment:

• Holds global and static variables.
• The starting address is stored in the DS register.

Stack Segment:

• Used for stack operations.
• The starting address is stored in the SS register.

Extra Segment:

• Used for additional data storage.
• The starting address is stored in the ES register.

Logical and Physical Addressing

In the 8086 architecture, memory addresses are divided into logical and physical addresses. The logical address is a combination of a segment address and an offset address. The physical address is the actual address in memory, calculated from the logical address.

Advantages of Memory Segmentation

Modularity:

• Programs can be divided into modules.
• Each module can be placed in a separate segment.

Protection:

• Segments can be used to protect data and code.
• For example, the code segment can be made read-only.

Flexibility:

• Segments can overlap, allowing for dynamic memory allocation.
• This is useful for complex programs.

Efficient Memory Usage:

• Only the required segments need to be loaded into memory.
• This reduces memory wastage.

Steps to Implement Memory Segmentation

Step 1: Define Segments

The first step in implementing memory segmentation is defining the segments. This involves specifying the starting address and size of each segment.

Step 2: Initialize Segment Registers

Next, initialize the segment registers (CS, DS, SS, ES) with the starting addresses of the segments. This tells the CPU where each segment begins.

Step 3: Access Memory Using Offsets

To access memory within a segment, use an offset address. The offset specifies the exact location within the segment.

Step 4: Calculate Physical Address

Finally, calculate the physical address using the formula:

Physical Address = (Segment Address x 16) + Offset Address

Practical Example

Let’s consider a practical example to illustrate memory segmentation in the 8086 architecture.

Problem Statement

Suppose we have a program that needs to store instructions, data, and stack information separately. We want to use memory segmentation to achieve this.

Solution

Define Segments:

• Code Segment (CS): Starting address = 0x1000
• Data Segment (DS): Starting address = 0x2000
• Stack Segment (SS): Starting address = 0x3000

Initialize Segment Registers:

• CS = 0x1000
• DS = 0x2000
• SS = 0x3000

Access Memory Using Offsets:

• To access an instruction at offset 0x0040 in the code segment:
• Physical Address = (0x1000 x 16) + 0x0040 = 0x10000 + 0x0040 = 0x10040
  • To access data at offset 0x0020 in the data segment:
  • Physical Address = (0x2000 x 16) + 0x0020 = 0x20000 + 0x0020 = 0x20020
  • To access stack information at offset 0x0010 in the stack segment:
  • Physical Address = (0x3000 x 16) + 0x0010 = 0x30000 + 0x0010 = 0x30010

Final Program

The final program will look something like this:

MOV AX, 0x1000 ; Load segment address of code segment

MOV CS, AX ; Initialize Code Segment Register

MOV AX, 0x2000 ; Load segment address of data segment

MOV DS, AX ; Initialize Data Segment Register

MOV AX, 0x3000 ; Load segment address of stack segment

MOV SS, AX ; Initialize Stack Segment Register

; Access instruction at offset 0x0040

MOV IP, 0x0040 ; Load offset address into Instruction Pointer

; Access data at offset 0x0020

MOV SI, 0x0020 ; Load offset address into Source Index

MOV AX, [SI] ; Load data from memory into AX register

; Access stack information at offset 0x0010

MOV SP, 0x0010 ; Load offset address into Stack Pointer

PUSH AX ; Push AX register onto stack

Also Read : Understanding Quadratic Equations

Memory segmentation in the 8086 architecture is a powerful technique that allows efficient and flexible memory management. By dividing memory into segments, the 8086 can access a larger address space than it could with a single linear address space. This approach offers several advantages, such as modularity, protection, and efficient memory usage.

Understanding memory segmentation and the memory organization of 8086 is essential for anyone working with this microprocessor. It not only helps in writing efficient programs but also in optimizing memory usage.

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