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Editorial Team
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Asked: 2026-06-12T17:16:13+00:00

I’ve been programming x86 asm programs (bootloaders) in real mode, and I know how

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I’ve been programming x86 asm programs (bootloaders) in real mode, and I know how to use segments, registers and stuff like that.

I see from debuggers like OllyDbg and similar that DS registers, SS, etc.. are obviously used.. but how do normal “Windows-like” processes use them? I know that segmentation is partially used (just to separate ring0 from ring3) and that their entries are in the GDT, I know that there’s paging involved and that would totally mess the addresses with PDE and PTE, but I can’t quite “link” everything together and understand what data stack extra segments are all about.. does each process have a different DS/SS/ES?

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  1. Editorial Team

    From INTEL 80386 PROGRAMMER’S REFERENCE MANUAL 1986:

    Figure 5-1.  Address Translation Overview

             15           0      31                           0
    LOGICAL ╔═══════════════╗   ╔══════════════════════════════╗
    ADDRESS ║    SELECTOR   ║   ║            OFFSET            ║
            ╚═══════════╤═══╝   ╚═══╤══════════════════════════╝
                        v           v
                     ╔══════════════════════════════╗
                     ║     SEGMENT TRANSLATION      ║
                     ╚══════════════╤═══════════════╝
                                 ╔══╧═╗       PAGING ENABLED
                                 ║PG ?╟────────────────────┐
                                 ╚══╤═╝                    │
                   31        PAGING v DISABLED       0     │
          LINEAR  ╔═══════════╦═══════════╦═══════════╗    │
          ADDRESS ║    DIR    ║   PAGE    ║  OFFSET   ║    │
                  ╚═══════════╩═════╤═════╩═══════════╝    │
                                    v                      │
                     ╔══════════════════════════════╗      │
                     ║       PAGE TRANSLATION       ║      │
                     ╚══════════════╤═══════════════╝      │
                                    │<─────────────────────┘
                      31            v              0
            PHYSICAL ╔══════════════════════════════╗
            ADDRESS  ║                              ║
                     ╚══════════════════════════════╝


Figure 5-2.  Segment Translation

           15              0    31                                   0
  LOGICAL ╔════════════════╗   ╔═════════════════════════════════════╗
  ADDRESS ║    SELECTOR    ║   ║                OFFSET               ║
          ╚═══╤═════════╤══╝   ╚═══════════════════╤═════════════════╝
       ┌──────┘         v                          │
       │ DESCRIPTOR TABLE                          │
       │  ╔════════════╗                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ╠════════════╣                           │
       │  ║  SEGMENT   ║ BASE          ╔═══╗       │
       └─>║ DESCRIPTOR ╟──────────────>║ + ║<──────┘
          ╠════════════╣ ADDRESS       ╚═╤═╝
          ║            ║                 │
          ╚════════════╝                 │
                                         v
              LINEAR  ╔════════════╦═══════════╦══════════════╗
              ADDRESS ║    DIR     ║   PAGE    ║    OFFSET    ║
                      ╚════════════╩═══════════╩══════════════╝

    In Windows, DS=ES=SS in most processes at most times and the values of CS and DS are shared across all processes. Processes may change their segment registers, but its rarely needed, so you’re going to see the same set of CS and DS/ES/SS values most of the time. The kernel uses its own CS and DS.

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