Understand x86 Real Mode: Segmented Memory and Register Limitations

Topic Overview

x86 real mode fundamentals

Real mode is how every x86 CPU starts: 16-bit registers, segmentation, and no paging. Physical addresses combine a 16-bit segment and offset: (segment << 4) + offset. For example, CS=0x07C0 and IP=0x0000 yield physical 0x7C00, exactly where the BIOS places your boot sector.

Registers and limits

• General purpose: AX, BX, CX, DX (each splittable into high/low bytes)
• Segments: CS, DS, ES, SS, FS, GS select 64 KB windows
• Pointers: IP (code), SP/BP (stack), SI/DI (strings)
• Address space: 20-bit, 1 MB maximum without A20 tricks

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IVT occupies 0x00000; BIOS interrupts remain callable via INT n.

Transition preview

Everything you do here runs before CR0.PE is set. Once you enter protected mode, segment registers index the GDT instead of shifting by four. Practicing physical address arithmetic now prevents subtle bugs when mixing real-mode loaders with 32-bit kernels linked at 0x100000. Keep a cheat sheet of default BIOS values for CS:IP at 0x7C00 entry.

Why for this exercise

You will compute physical addresses and demonstrate segment register usage in real mode. This exercise requires showing the segment:offset formula on paper and in code before writing boot-sector assembly.

Write a C program that simulates real-mode address formation.

Input: two hex numbers, segment and offset (e.g. "07C0 0"). Output:

• SEG=xxxx OFF=xxxx echoing the inputs in uppercase 4-digit hex
• PHYS=xxxxx, the 20-bit physical address ((segment << 4) + offset) & 0xFFFFF
• ALIASES=n, the number of distinct segment:offset pairs that also map to this exact physical byte (segment ranges from phys/16 down, limited to 0x0000..0xFFFF and the 16-byte window below the address) If segment or offset exceeds 16 bits, print only INVALID.