Master Segmented Memory Addressing in Real Mode

Topic Overview

Segmented addressing tricks

Different segment:offset pairs can alias the same physical byte. 0000:7C00, 07C0:0000, and 7000:0C00 all map to 0x7C00. For example, a far jump to 0x0000:0x7C00 after loading a sector uses a different segment pair than the BIOS default 0000:7C00 but lands on identical physical memory.

Near vs far addressing

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• Near pointers: Offset only, stay in current segment
• Far pointers: 16-bit offset plus 16-bit segment (4 bytes in memory)
• Paragraph alignment: Segments on 16-byte boundaries simplify math
• Overlapping segments: Can access >64 KB by sliding segment base

Watch wraparound: 0xFFFF:0x0010 becomes 0x00000 without A20.

Why aliasing matters

Boot loaders often normalize CS to 0x0000 with IP=0x7C00 after a manual reload even though the BIOS entered at 07C0:0000. Understanding aliasing prevents off-by-16 bugs when mixing assembly modules assembled with different ORG assumptions. Label addresses in listings are offsets from ORG; always add the load address when reasoning about physical placement.

Why for this exercise

You will convert between segment:offset pairs and physical addresses in both directions. This exercise requires demonstrating overlapping segments and near/far pointer differences with concrete numeric examples.

Write a C program that compares two segment:offset pairs and reports aliasing, canonical form, and 64 KB reachability.

Input: four hex numbers s1 o1 s2 o2.

• Compute physical(s, o) = ((s << 4) + o) & 0xFFFFF for each pair.
• If the physical addresses are equal, print "SAME xxxxx" (5-digit hex) followed by "CANONICAL ssss:oooo", where the canonical form is the representation with the smallest possible segment value (segment = phys/16, offset = phys%16).
• Otherwise print "DIFF xxxxx xxxxx" with both physical addresses.
• Finally, test 64 KB reachability of segment s1: the highest byte it can address is ((s1 << 4) + 0xFFFF) & 0xFFFFF. Print "REACHABLE xxxxx" if that fits in 20 bits, or "WRAPS xxxxx" (the wrapped value) if it does not. If any of the four numbers exceeds 16 bits, print only INVALID.