Real Mode is the simplest and most straightforward operating mode of the x86 microprocessor architecture. It is designed to operate as a high-speed mode that mirrors the capabilities of the original Intel 8086 and 8088 processors. Despite being the most basic mode, it serves as a critical foundation for understanding more complex operating modes like Protected Mode and Long Mode. This article provides a comprehensive exploration of Real Mode, covering its history, characteristics, addressing mechanisms, limitations, and typical use cases.

Historical Context

Real Mode has its roots in the original Intel 8086 processor, introduced in 1978. The 8086 was a 16-bit microprocessor that laid the groundwork for the x86 architecture. When Intel released the 80286 processor in 1982, it included Real Mode to maintain compatibility with software written for the 8086. Real Mode remains a part of all subsequent x86 processors to ensure backward compatibility.

Characteristics of Real Mode

1 Addressing:

• Segmented Memory: Real Mode uses a segmented memory model, which allows the processor to access a 20-bit address space despite having only 16-bit registers. This is achieved through a combination of segment and offset addresses.
• Address Calculation: A physical address is calculated by multiplying the segment register value by 16 (shifting it left by 4 bits) and then adding the offset. For example, if the segment register is 0x1234 and the offset is 0x5678, the physical address would be 0x12340 + 0x5678 = 0x179B8.

2 Memory Limitations:

• 1 MB Address Space: The total addressable memory in Real Mode is 1 MB (2^20 bytes), with addresses ranging from 0x00000 to 0xFFFFF.
• Conventional Memory: The first 640 KB (0x00000 to 0x9FFFF) is known as conventional memory, traditionally available for operating systems and applications.
• Upper Memory Area (UMA): The range from 640 KB to 1 MB (0xA0000 to 0xFFFFF) is reserved for system BIOS, video memory, and other hardware devices.

3 No Memory Protection:

• Full Access: All code runs with full access to the entire memory and hardware resources. There is no hardware-level protection against errant code, making it possible for a malfunctioning program to overwrite system memory.

4 No Multitasking:

• Single Task Operation: Real Mode does not support hardware-based multitasking. Only one program can run at a time, which limits the efficiency and capabilities of the system.

5 Interrupt Handling:

• Interrupt Vector Table (IVT): The IVT is located at the beginning of memory (0x0000 to 0x03FF). It contains pointers to interrupt service routines (ISRs) for handling hardware and software interrupts.

Segmented Memory Model

In real mode, memory addresses are calculated using a combination of a segment value and an offset value. This model provides a way to address more memory than could be addressed with a single 16-bit value (which maxes out at 64 KB).

Segment and Offset

1. Segment: A 16-bit value that is shifted left by 4 bits (equivalent to multiplying by 16). This forms the base address.
2. Offset: A 16-bit value added to the segment's base address to form the final physical address.

Calculating the Physical Address

The formula to calculate the physical address in real mode is:

Physical Address = (Segment * 16) + Offset

Both the segment and the offset are 16-bit values, ranging from 0x0000 to 0xFFFF.

Example Calculation:

If the segment is 0x1234 and the offset 0x5678, the physical address is calculated as:

Physical Address = (0x1234 * 16) + 0x5678

Physical Address = 0x12340 + 0x5678

Physical Address = 0x179B8

Addressing Limitations in Real Mode

The 20-bit address bus in real mode limits the addressable memory space to 1 MB (2^20 bytes). The highest possible address is:

Max Address = (0xFFFF * 16) + 0xFFFF
0xFFFF0 + 0xFFFF
0x10FFFF

However, due to the way address wrap around, the effective maximum address is 0xFFFF (1 MB).

When the calculated physical address exceeds this limit, the address "wraps around" due to the limited number of address lines.

Example of Wrap-Around Calculation:

Consider an example where the segment is 0xFFFF and the offset is 0x0020. Let's calculate the physical address and see how wrap-around occurs:

1. Calculate the Base Address:
   Base Address = Segment * 16
   0xFFFF * 16
   0xFFFF0
2. Add the Offset:
   Physical Address = 0xFFFF0 + 0x0020 = 0x100010
3. Apply the 20-bit Address Bus Constraint:
   Since the address bus is 20 bits wide, only the lower 20 bits of the address are used. This is equivalent to taking the result modulo 2^20 (which is 0x100000).
   Wrapped Physical Address = 0x100010 mod 0x100000 = 0x0010

General Formula for Wrap-Around Address:

To generalize, for any segment and offset, the wrapped physical address can be calculated as follows:

`Wrapped Physical Address = (Segment * 16) + Offset mod 0x100000

Example Scenarios:

Segment: 0xFFFF, Offset: 0x0020

Base Address = 0xFFFF0

Physical Address = 0xFFFF0 + 0x0020 = 0x100010

Wrapped Physical Address = 0x100010 mod 0x100000 = 0x0010

Segmentation

Segmentation in real mode is the mechanism used to divide memory into segments. Each segment is a contiguous block of memory that can be addressed independently. This approach allows for more flexible memory management and easier program organization.

Memory Layout in Real Mode:

In real mode, the CPU can address up to 1MB of memory, even though it operates in a 16-bit mode. This is achieved through the use of segments and offsets. The memory layout looks like this:

 0x00000  +---------------------------------------------------+
          |                                                   |
          |                     1MB Memory                    |
          |                                                   |
 0xFFFFF  +---------------------------------------------------+

Segment Registers:

The x86 CPU uses segment registers to hold the base addresses of segments. There are four main segment registers:

• CS (Code Segment): Points to the segment containing the executable code.
• DS (Data Segment): Points to the segment containing data.
• SS (Stack Segment): Points to the segment containing the stack.
• ES (Extra Segment): Used for additional data segment addressing.

Logical Address: In real mode, a logical address is composed of a segment and an offset, written as segment:offset. This format allows the CPU to access memory beyond the initial 64KB limit.

Physical Address Calculation: The physical address is calculated by shifting the segment value left by 4 bits (or multiplying by 16) and then adding the offset. The formula is:

Physical Address = (Segment * 16) + Offset

How Segmentation Works in Real Mode

To understand segmentation, let's look at an example. Assume the following values for segment and offset:

• Segment: 0x1234
• Offset: 0x5678

The physical address is calculated as follows:

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

This calculation allows the CPU to address up to 1MB of memory (20-bit address space) in real mode, even though each segment is only 64KB.

Segment Registers and Their Use

Code Segment (CS): When the CPU fetches instructions to execute, it uses the CS register to determine the base address of the code segment. The instruction pointer (IP) register holds the offset within this segment.

Data Segment (DS): The DS register typically points to where the program's variables are stored. For example, to access a variable at offset 0x1000 in the data segment, the CPU uses DS:0x1000.

Stack Segment (SS): The SS register is used with the stack pointer (SP) to manage function calls and local variables. The stack grows downward, meaning the SP register is decremented to push values onto the stack and incremented to pop values off.

Extra Segment (ES): The ES register serves as an additional data segment. It is often used in string operations and data transfers, particularly in conjunction with the DS register.

	xor ax, ax         ; Zero out AX register
    mov ds, ax         ; Set DS to 0
    mov es, ax         ; Set ES to 0
    mov ss, ax         ; Set SS to 0
    mov sp, 0x7c00     ; Initialize stack pointer

Finding Segment:Offset Pairs

There exists different Segment:Offset pairs for any physical address, means any physical address is achievable through different Segment:Offset.

For example the physical address 0x123450 is achievable through:

1 Using Segment 0x0000 and Offset 0x123450:

Segment = 0x0000

Offset = 0x123450

Physical Address = 0x0000 * 16 + 0x123450 = 0x123450

Pair (0x0000:0x123450)

2 Using Segment 0x12340 and Offset 0x0050

Segment = 0x12340

Offset = 0x0050

Pair (0x12340:0050)

3 Using Segment 0x12300 and Offset 0x0450

Segment = 0x12300

Offset = 0x0450

Pair (0x12300:0x0450)

4 Using Segment 0x12000 and Offset 0x3450

Segment = 0x12000

Offset = 0x3450

Pair (0x12000:0x3450)

5 Using Segment 0x12345 and Offset 0x0000

Segment = 0x12345

Offset = 0x0000

Pair (0x12345:Ox000)

There can be more possible segment:offset pairs that map to the same physical address:

To find all possible pairs, you need to consider the general formula:

Physical Address = (Segment * 16) + Offset

For a given physical, Offset can vary from 0 to 0xFFFF (64KB), and Segment will adjust accordingly. Here's step-by-step process to generate more segment:offset pairs for the physical address.

General Approach-

For a given physical address P, calculate the segment and offset as follows:

1. Choose a segment value.
2. Compute the offset using the formula:
   Offset = P - (Segment * 16)
3. Ensure the offset is within the valid range (0 to 0xFFFF).

Example:

Physical Address 0x123450.

Let's generate more pairs.

1. Segment 0x1234
   Offset = 0x123450 - (0x1234 * 16)
   0x123450 - 0x12340 = 0x0110 (It's in the range of 0x0000 to 0xFFFF)
   Pair: (0x1234: 0x0110)
2. Segment 0x1233
   Offset = 0x123450 - (0x1233 * 16)
   0x12350 - 0x12330 = 0x0120 (It's in the range of 0x0000 to 0xFFFF)
   Pair: (0x1233: 0x0120)
3. Segment 0x1232
   Offset = Ox123450 - (0x1232 * 16)
   0x123450 - 0x12320 = 0x0130 (It's in the range of 0x0000 to 0xFFFF)
   Pair: (0x1232: 0x0130)

There a lot more pairs which can be generated.