QLB - QuickBASIC Quick Library (.QLB) format

This folder is the result of reverse engineering of the Quick Library format of Microsoft QuickBASIC 4.0 / 4.5: the file format, how QuickBASIC loads a library, how it calls into it, and how to write one. Everything is written down, checked against the real QuickBASIC 4.50 and 4.0 and against Microsoft's own QB.QLB, and released into the public domain (Unlicense, (c) DosWorld 2026). Use it for anything, without conditions.

What is a .QLB?

A Quick Library is the file that QuickBASIC loads with QB /L name.QLB. It contains compiled routines (assembler, C, anything that produces .OBJ) that a BASIC program in the QuickBASIC editor can then CALL. Without it the QuickBASIC interpreter can only run BASIC: no interrupts, no absolute addresses, no hand-written inner loops, no hardware access. With it a program can do CALL INTERRUPT(...), CALL ABSOLUTE(...) (that is the library QB.QLB that ships with QuickBASIC) or call your own routines.

Technically it is a DOS MZ executable that nobody runs, used as a container: code image, data, a symbol block with name tables, and an MZ relocation table. QuickBASIC maps the pieces into memory, relocates them, and then talks to the library through a small table of far pointers (a dispatcher with a life cycle: load, init, find a routine by name, terminate).

QB /L MYLIB.QLB /RUN PROG.BAS

DECLARE FUNCTION SUMI% (a AS INTEGER, b AS INTEGER)
PRINT SUMI%(1000, 234)     ' assembler routine from MYLIB.QLB

How useful is it for DOS software?

• For QuickBASIC 4.x programmers: essential. It is the only way to extend the editor/interpreter. Anything that needs a DOS or BIOS call, a port, a TSR, speed, graphics or a hardware driver goes through a Quick Library. The same assembler routines can also be linked into a compiled .EXE (as .OBJ), so a library is a convenient way to develop them. With the documentation here you can write libraries with NASM and the free RTools linker (rlink qlb) and need neither Microsoft LINK nor BQLB45.LIB.
• For other DOS programs: a small, ready-made plug-in format
  • useful, but niche. A .QLB is a relocatable module with named exports, its own data segment, a list of relocations and life-cycle hooks, loadable without EXEC, at any address, by a small 8086 assembler loader (ALOADER\ALOADER.ASM). You can use it as an overlay, plug-in, driver or script-extension container for your own DOS software, and any QuickBASIC 4.x user can load the same file. Its limits: one 64 KB data segment laid out for QuickBASIC 4.5, no import table (a library can take only what QuickBASIC offers at fixed places), names found through a dispatcher, and RLINK and Microsoft LINK /Q as the only writers. If you do not care about QuickBASIC, a plain RDF or MZ overlay with your own loader is simpler.
• For preservation: the format was undocumented; now it is documented and checked by working code.

The documents

• QLBFMT.MD
  • what it is: The format specification. File layout, MZ header, QB header, the three tables, memory model, relocation, the dispatcher and its selectors, the calling convention, imports, limits, QuickBASIC's refusal checklist, an annotated byte dump, assembler examples.
• QLBUSE.MD
  • what it is: Consumer manual. Part A: using a library from a QuickBASIC program (parameters, return values, errors, pitfalls). Part B: loading a library from your own DOS program: load, relocate, resolve exports and imports, phases with codes, call routines, unload - with the assembler code.
• QLBWRT.MD
  • what it is: Writer guide. Writing routines in NASM (stack frames for every parameter type, return values), the life cycle hook, imports, data, linking with rlink qlb, testing, hang checklist, writing a .QLB without RLINK.
• MSRT.LST
  • what it is: Disassembly of the runtime that Microsoft's LINK /Q embeds (dispatcher, GETPROC, service thunks, Init, Term ...), annotated, with the variables of QuickBASIC's data segment.

The demos (built: .QLB, and .EXE for the loader)

• QLOADER\
  • content: A QuickBASIC program calls a library written in NASM, with parameters of every kind. DEMO.ASM (the library), DEMO.QLB (built), DEMO.BAS (the BASIC program), MAKE.BAT, RUN.BAT, OUTPUT.TXT (output of the run under QuickBASIC 4.50). It needs QB.EXE to run: a .QLB is not an .EXE.
• ALOADER\
  • content: A DOS program in NASM that loads a .QLB and calls it, playing the part of QuickBASIC, with all the phases (06h, 10h, 00h, 0Ah per name, 00h, 02h, 04h), the imports (DS:00D3, the gateway DS:00BE/00C2) and the calls. ALOADER.ASM, ALOADER.EXE (built), DEMO.ASM, DEMO.QLB, MAKE.BAT, RUN.BAT, OUTPUT.TXT (full run), OUTLOAD.TXT (aloader QB.QLB /L: Microsoft's own library passes every check).

Try it:

cd ALOADER
aloader DEMO.QLB              full run, no QuickBASIC needed
aloader C:\QB45\QB.QLB /L     load and check any library

cd ..\QLOADER
qb /l DEMO.QLB /run DEMO.BAS   same routines from BASIC
                              (needs QuickBASIC 4.5 or 4.0)

RLINK and the QLB format

The linker rlink qlb of RTools follows these findings:

• Imports. A module may extern the names of QuickBASIC (B$RUNERR, B$ERR_FC, b_errnum, b_ULSymSeg ...); RLINK builds the thunks and data symbols itself (DEMO.ASM uses it).
• Lookup. Like Microsoft's runtime: CDECL names get the underscore, case is ignored, the temporary name string is freed (QLBFMT.MD section 12), and the CODE table lists the exported names.

Rebuild

NASM 0.98.39 and the linker RLINK of RTools (BIN\NASM.EXE, BIN\RLINK.EXE), under MS-DOS or DOSBox-X:

nasm -s -f rdf DEMO.ASM
rlink qlb /o=DEMO.QLB DEMO.RDF
nasm -s -f rdf ALOADER.ASM
rlink mzs /o=ALOADER.EXE ALOADER.RDF

Tested with

• QuickBASIC 4.50 and 4.0 (QB.EXE) for the file format and the life cycle; the demo programs of QLOADER under 4.50.
• Microsoft's QB.QLB of QuickBASIC 4.5: read and accepted by ALOADER (all checks); BASIC PDS 7.1 files were read only.
• QBasic (QBASIC.EXE of MS-DOS 5/6) has no /L option and does not load libraries.

Known problems and open points

• Only one library can be loaded by QuickBASIC (/L twice prints the usage line); all modules have to be linked into one.
• Not examined: selectors 04h 08h 0Ch 0Eh at run time, STRING function results, far data/BSS areas, QuickBASIC PDS at run time.
• ALOADER runs libraries made by RLINK; libraries made by LINK /Q are loaded and checked only.

License

(c) DosWorld 2026. Public domain: the Unlicense, full text in LICENSE.TXT and in the header of every source file. Microsoft, QuickBASIC and MS-DOS are trademarks of Microsoft Corporation; this project is not affiliated with Microsoft and contains no Microsoft code or files, except MSRT.LST, a commented disassembly of the runtime that LINK /Q embeds, published for interoperability and documentation; check the rights that apply to you. QB.QLB is not included; the loader only reads it.