Undocumented DOS

What Hacker News says

1. vunderbaSep 2026
   Nice job~

   For the past 6 months I've also been working on a period-accurate QBASIC 1.1 simulator (and corresponding virtual hardware layer) that runs entirely in the browser.

   I've got a stack of books on my desk from when I was a kid in the 90s that have literally been my religious texts (Undocumented DOS, The Programmer's PC Sourcebook, etc), so I know how hard it is to get this right.

   I've actually posted about it a couple times here in HN.

   Anyway small bit of feedback:

   Technically if you don't clear the screen (using CLS, or set a SCREEN mode other than zero), the cursor retains its position even between program executions.

   So basically if you have a program like this:

     PRINT "WHY IS A RAVEN LIKE A WRITING DESK"
     END
   

   And then you run it again - the output screen should display:

     WHY IS A RAVEN LIKE A WRITING DESK
     WHY IS A RAVEN LIKE A WRITING DESK
   
   

   There's a thousand other subtleties like this that make the proverbial "last mile" sometimes feel insurmountable. Fun though!

   * Your definition of "fun" may vary.

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2. vunderbaAug 2026
   Thanks! I went so far with it that I was honestly questioning my own sanity.

   The HAL has a virtualized CPU, and I spent more time than I’d care to admit precisely timing instructions on various period-accurate hardware (386SX, 486DX). I even tuned it to the point where programs that take advantage of it actually run faster on the 486 because of the built‑in math coprocessor.

   > Writing directly to VRAM? WAIT for vsync?

   All screen modes are supported including the ability to draw to hidden pages. It’s got a fully emulated VGA hardware layer and even a synthetic VGA beam clock, so you can do tricks like `WAIT &H3DA, 8, 8` for vertical retrace to minimize flicker.

   > Call absolute for mouse usage?

   Yup! Int 33h is fully supported, we basically use a virtual TSR driver which is pointed to by the interrupt vector table. I actually tested it with an old pixel art editor I made in the 90s!

   PEEK/POKE and much of the BIOS are fully simulated, so you can use more advanced QuickBasic 4.5 features like calling interrupts (including silly stuff like rebooting the computer via int 19h).

   There's a ton more (including a Hayes compatible modem plugged into COM1) as well.

   I basically dug out a stack of my old PC books from the late ’80s and early ’90s, and they’ve been my bibles for the past several months (Undocumented DOS, The Programmer’s PC Sourcebook, etc.).

   It’s about 99% done at this point. I’m hoping to do a Show HN in the next couple of weeks.

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3. burnt-resistorAug 2025
   Seems a bit obsessed with open source when abandonware like Borland C++ 3.1 and Pascal 7.0 are amazing.

   Also, missing the very important, closer to primary sources, physical dead tree resources that are needed as reference to program things.

   - Black Book of Graphics Programming (Special Edition) (now FOSS)

   - Programmer's Guide To The EGA, VGA, And Super VGA Cards (3rd Edition)

   - PC-Intern (where I learned how Central Point, Norton, and later FreeBSD made "GUI" with sub-character graphical pointers in text mode through custom fonts)

   - Undocumented PC

   - Undocumented DOS

   - PC Interrupts (and) Uninterrupted Interrupts (Ralf Brown)

   - Microsoft MS-DOS Programmer's Reference

   - The Programmer's PC Sourcebook: Reference Tables for IBM PCs and Compatibles, PS/2 Systems, EISA-based Systems, MS-DOS Operating System Through Version 5

   - (various hardware books by MindShare)

   - Also useful would be real BIOS dumps and (dis)assembly, and MS-DOS source

   - Emulators are no substitute for the real thing because the problem is that no emulator (commercial or otherwise) is faithful to the quirks, capabilities, and limitations of real hardware (in system, protected mode debuggers/profilers sure are nice though compared to triggering lockup, spontaneous reboot, or a beeping deadlock). If anyone remembers Bochs, its floppy behavior definitely doesn't act or look anything like a real FDC. (I submitted some patches for it many moons ago in college.)

   (Yep, I own a "braindead" 286, 386DX, 486DX-100, Am5, and P5, P2, P Pro, and P4.)

   Because if something can't work on real hardware and original OSes, then it's probably make believe. Prefer to make honest, real things wherever feasible.

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4. ben30Apr 2024
   taking me back to creating fliphide, a utility I wrote in the early '90s. It utilized assembly and INT21, guided by "Undocumented DOS"—a book uncovering the OS's undocumented features. This resource was pivotal for crafting an executable (whether .exe or .com didn't matter) that manipulated file attributes, showcasing the power of accessing deeper OS layers not widely known or used.
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5. stevekempMay 2023
   As documented in the excellent book "Undocumented DOS", whose red cover I can still picture. (Noted in the article, but it was a good book that I still recall fondly.)
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6. ksajNov 2022
   There was a pair of books that were instrumental to me: Undocumented PC and Undocumented DOS. Those books taught me so many tricks!

   I still remember the DWIM (Do what I mean) interrupt they used as the demonstration of how each interrupt was documented.

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