Real Mode Memory Map

In the context of x86 real mode, the memory map is defined by the original IBM PC architecture and includes several well-defined regions.

The real mode memory map of x86 system describes the organization and allocation of memory within the first 1 MB of addressable space. Understanding this memory map is crucial for low-level programming tasks. Real mode is the mode the CPU operates in immediately after a system reset, allowing only 20-bit memory addressing and access to a maximum of 1 MB of memory.

Here's an overview of the memory map in real mode:

Real Mode Memory Map

🤷‍♂️ Explanation:

1 Interrupt Vector Table (IVT):

• Addresses: 0x00000 to 0x003FF
• Size: 1 KB (256 entries, 4 bytes each)
• Description: This table holds pointers to interrupt service routines (ISRs). Each entry is 4 bytes, consisting of a 2-byte segment and a 2-byte offset.
• Usage: The CPU consults the IVT when an interrupt occurs to determine the ISR's location.

2 BIOS Data Area (BDA)

• Addresses: 0x00400 to 0x004FF
• Size: 256 bytes
• Description: This area contains system data used by the BIOS such as hardware configuration and status information.
  • Equipment list (e.g., number of drives).
  • COM and LPT port addresses.
  • Keyboard status.

3 Low Conventional Memory:

• Addresses: 0x00500 to 0x07DFF
• Size: ~31 KB
• Description: This region is available for general-purpose use by the operating system and applications.
• Purpose: General-purpose memory for real-mode programs, used for stacks, variables, and data.

4 Bootloader Area:

• Address Range:0x7c00 - 0x7DFF
• Size: 512 bytes
• Purpose: The BIOS loads the bootloader from the first sector of the bootable storage device into this region.
• Usage: This is the starting point for bootloader execution.

5 Usage Conventional Memory

• Address Range:0x7E00 - 0x9FBFF
• Size:~607 KB
• Purpose: This memory is available for real-mode programs and applications.
• Notes: It is the largest usage memory segment in real mode.

6 Extended BIOS Data Area (EBDA):

• Address Range:0x9FC00 - 0x9FFFF
• Size: 1 KB
• Purpose: Reserved for BIOS-specific use, such as storing ACPI tables and other system-critical data.

7 Video Memory (VGA):

• Addresses:0xA0000 to 0xBFFFF
• Size: 128 KB
• Description: This region is used for video memory, with different segment allocated for various video modes:
  • 0xA0000 to 0xAFFFF: VGA graphics modes.
  • 0xB0000 to 0xB7FFF: Monochrome text mode.
  • 0xB8000 to 0xBFFFF: Color text mode.

8 Video BIOS:

• Addresses:0xC0000 to 0xC7FFF
• Size: 32 KB
• Purpose: Contains firmware specific to video cards, providing low-level hardware control.
• Usage: Accessed during system boot or by applications requiring video services.

9 Optional ROMs and Expansion Cards

• Address Range:0xC8000 - 0xDFFFF
• Size: 96 KB
• Purpose: Reserved for add-on hardware ROMs, such as SCSI controllers or network adapters.

10 System BIOS Extensions

• Address Range:0xE0000 - 0xEFFFF
• Size: 64 KB
• Purpose: Reserved for BIOS extensions, such as advanced power management or system configuration.