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2020 by C. Masloch. Usage of the works is permitted provided that this instrument is retained with the works, so that any entity that uses the works is notified of this instrument. DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY.

This document has been compiled on 2021-10-19.


2021-10-19 Oct Tue
SHUFHOOK first release added
2021-10-13 Oct Wed
RxANSI listed
lClock listed
SEEKEXT listed
TSR example listed
TSR current releases added
2021-10-12 Oct Tue
KEEPHOOK first release added
2021-08-15 Aug Sun
lDebug release 3
2021-05-17 May Mon
Moved from to domain
2021-05-05 May Wed
lDebug release 2
2021-04-07 Apr Wed
The lDebug manual had a News chapter added
2021-02-18 Feb Thu
What is "pushbx"?
2021-02-15 Feb Mon
lDebug release 1


lDebug debugger

lDebug is a 86-DOS debugger based on the MS-DOS Debug clone FreeDOS Debug. It features DPMI client support for 32-bit and 16-bit segments, a 686-level assembler and disassembler, an expression evaluator, an InDOS and a bootloaded mode, script file reading, serial port I/O, permanent breakpoints, conditional tracing, buffered tracing, and auto-repetition of some commands. There is also a symbolic debugging branch being developed.

The manual (text, PDF) is available online. The repo is hosted at and releases are available at

TracList - Trace debugging output in listing files

A companion application to lDebug, connecting to the debugger's serial I/O. It traces execution through NASM format listing files. The manual (text, PDF) is available online. The repo is hosted at

RxDOS 7.2x kernel

RxDOS is an alternative DOS kernel. It is not yet ready for general use. Unlike the FreeDOS kernel, its source is entirely in assembly language. (As of RxDOS 7.20N the source has been ported from MASM to NASM.) Unlike DR-DOS, MS-DOS, and PC-DOS it is (copylefted) free software. It aims to support extended features such as FAT32, LFNs, and LBA. It already supports UMBs (for DOSDATA, DOSCODE) and the HMA (for DOSCODE). Support for additional extensions is planned, such as FAT+, cluster sizes exceeding 32 KiB, sector sizes up to 8 KiB, and moving many DOS data structures into the UMA and HMA. The repo is hosted at

lDOS boot loaders

Advanced boot loading stages used by kernels or other bootloaded programs. The repos are hosted at and

86 Mode NASM macro collection

A macro collection featuring parts such as defaultable defines for conditional assembly, stack frame calculation, anonymous @@ labels, safer sectioning directives, and assisted flag bit accesses. The repo is hosted at

NASM 2.05 based x86 Instruction Reference

An assembly language reference covering all general instructions up to a 686-level instruction set. Includes encoding schemes and some general English-language descriptions. It is available online here (text, PDF). The repo is hosted at

ACEGALS - Assembly Comments Explained: Guide for Advanced Learning and Style

A reference guide for our assembly comments and formatting styles. It is available online here (text, PDF). The repo is hosted at

Boot image FAT FS formatting script

A NASM script that allows formatting an image file with FAT12, FAT16, or FAT32. The filesystem can be preloaded with files. Both diskette or harddisk images can be created. Optionally an MBR and a dosemu image header can be wrapped around the image. For FAT32 a backup boot sector copy and an FSINFO sector can be initialised. Sector sizes between 32 Bytes and 8 KiB are supported. Likewise cluster sizes between 1 to 256 sectors. (Minimum cluster size is 32 Bytes, maximum is 2 MiB.) The repo is hosted at

Terminate and Stay Resident (TSR) programs

The current releases are created automatically from the repos and are updated daily. Old files of the current releases are found at

KEEPHOOK utility

A TSR which aids the uninstalling of other TSRs. The manual (text, PDF) is available online. The repo is hosted at and a current release is available at

SHUFHOOK utility

Not a TSR itself, this tool allows to re-order interrupt handlers. The manual (text, PDF) is available online. The repo is hosted at and a current release is available at

RxANSI - ANSI and LESS sequence parser

RxANSI is a TSR to take over interrupt 29h and parse ANSI and LESS sequences. It only affects output - it does not do any input. The repo is hosted at and some releases are available at and a current release is available at

lClock - Clock display TSR

lClock is a TSR that continuously displays a clock in the upper right-side corner of the screen by writing to the text mode video buffer. It can be customised using build time options. The repo is hosted at and a current release is available at

SEEKEXT - Seek service extension

This TSR extends DOS with an interrupt 21h handler. It hooks the services 21.42 (short seek) and 21.7142 (long seek). The short seek hook is to provide sensible results from positive or negative 32-bit seek offsets for file handles that implement long seek (21.7142). The long seek hook is to redirect calls to the special int 2F.1142 redirector long seek interface, which is used by recent dosemu2 to provide 64-bit seeking on its redirected file systems. The repo is hosted at and a current release is available at

TSR example

This project is an example of a TSR. It was originally based on RxANSI, and was in turn the base for lClock, SEEKEXT, and KEEPHOOK. It implements an AMIS multiplexer and features optimal installation and the advanced deinstallation method. The repo is hosted at (No releases are going to be provided because this is useful only for developers.)


ecm's files
ecm's github
ecm's hgweb
ecm's stackoverflow
ecm's NASM bug reports
BTTR Software
DOS ain't dead

What is "pushbx"?

The username ‘pushbx’ originates in Eric Auer's callver (public domain). A line there reads the instruction ‘push bx’ but has a comment that also reads ‘push bx’. The reason? There's a line further down which has the instruction ‘pop ax’ with the comment ‘but pop ax’. This hints at the fact that, to a beginner, it is unusual to push then pop with two different registers as the source and destination. I've been using pushbx as a username ever since at the latest 2012 March.

Source format of this page

This page has been created using the libre Halibut documentation preparation system.

Source Control Revision ID

hg 1bcfdbbf29df, from commit on at 2021-10-19 18:23:27 +0200

If this is in ecm's repository, you can find it at