PC DOS Reimagined

Read the Blog, then check out the Preview, which highlights a few of the original Demos.

Project Status

BASIC-DOS began as a technology demo: a reimagining of what the first IBM PC operating system could have been, with a unified DOS and BASIC command interpreter, preemptive multitasking sessions, and other features that PC DOS wouldn’t offer for years (if ever). The goal now is to turn it into a usable product.

This section tracks what’s been completed ([x]) and what remains ([ ]). Items marked partial work, but with known gaps.

Boot and System Configuration

• Boot sector that loads the BASIC-DOS drivers, kernel, and interpreter
• Boot prompt when a hard disk is detected (press Esc to boot from it)
• CONFIG.SYS support for BOOTKEY, CONSOLE, DEBUG, FILES, MEMSIZE, SESSIONS, SHELL, and SWITCHAR
• Installable device drivers (DEVICE=)
• Critical (“hard”) error handling (eg, “Abort, Retry, Ignore”)

Device Drivers

• CON, with multiple console contexts (one per session)
• COM, AUX, LPT, PRN, NUL, CLOCK$, and PIPE$ devices
• Interrupt-driven, asynchronous I/O for CON and COM
• Floppy disk reads, including multi-track requests and requests that cross 64K boundaries
• Floppy disk writes
• Media check and BPB rebuilds (reads PC DOS 1.x and 2.x diskettes)
• Write-with-verify (DDC_WRITEV), formatting, and hard disk support
• “Popup” and background console contexts

Kernel: Processes, Memory, and Sessions

• COM and EXE program loading, PSPs, EXEC, and exit codes
• Memory allocation (MCBs), including per-program heap requests
• Preemptive multitasking of multiple sessions
• CTRL-C/CTRL-Break handling, and CTRL-ALT-DEL session aborts
• Session STOP/END operations (currently TODOs)
• TSR support (INT 27h and INT 21h function 31h)
• Environment segments for EXEC (EPB_ENVSEG)

Kernel: File System

• FAT12 file reading
• Handle-based open, read, seek, IOCTL, and close
• Find first/next, disk info (free space), and current drive/DTA
• Handle-based file creation (function 3Ch) and writing (function 40h)
• File deletion (function 41h) and renaming (function 56h)
• FAT cluster allocation and freeing
• Write-back of modified FAT and directory buffers (every FAT copy is updated), including when the buffer is reused, on file close, disk reset, and restart
• partial: FCB support (open, close, and reads only; no create, write, delete, or rename)
• Truncating/extending a file with a zero-length write
• Enforcing the read-only attribute on open, and getting/setting file attributes (function 43h) and date/time (function 57h)
• Subdirectories (MKDIR, RMDIR, CHDIR, and paths)
• Absolute disk reads and writes (INT 25h and INT 26h)
• FAT16, hard disks, and a larger buffer cache (there are currently only two sector buffers: one for FAT sectors and one for directory sectors)

Command Interpreter: DOS Commands

• COPY, DATE, DIR, EXIT, HELP, KEYS, MEM, RESTART, TIME, TYPE, and VER
• Running COM, EXE, BAT, and BAS files, and loading programs into other sessions
• Pipes (|) and redirection (>)
• COPY creates (or truncates) the output file, and refuses to copy a file onto itself
• partial: Output redirection works only with existing files (it should use DOS_HDL_CREATE, like COPY)
• DEL/ERASE, REN/RENAME, and SAVE (for BASIC programs)
• Disk utilities (eg, FORMAT, CHKDSK, SYS)

Command Interpreter: BASIC Language

• 32-bit integer variables, constants, and expressions (including AND, OR, XOR, EQV, IMP, NOT, MOD, and shifts)
• String variables, concatenation, and comparisons
• CLS, COLOR, DEF FN, DEFINT, DEFSTR, ECHO, GOTO, IF/THEN/ELSE, LET, PRINT, REM, and RETURN
• LOAD, LIST, NEW, and RUN
• Functions: ERRORLEVEL, MAXINT, and RND%
• Control flow: GOSUB, FOR/NEXT, WHILE/WEND, and ON … GOTO/GOSUB
• INPUT, READ, DATA, and RESTORE
• Arrays (DIM)
• String functions (eg, LEFT$, MID$, RIGHT$, LEN, CHR$, ASC, STR$, VAL)
• File I/O statements (eg, OPEN, CLOSE, PRINT #, INPUT #)
• Error handling (eg, ON ERROR, ERR, ERL)

Floating-Point

BASIC-DOS will support only one floating-point type: IEEE 754 64-bit (double-precision) values. It won’t use any MBF (Microsoft Binary Format) code.

• Parser support for floating-point constants (CLS_FLOAT tokens)
• DEFDBL and DEFSNG are accepted (currently no effect)
• MBF-based MSLIB option removed
• Utility functions DOS_UTL_ATOF64, DOS_UTL_I32F64, and DOS_UTL_OPF64 (currently stubs)
• Arithmetic and comparison operations
• Conversions between integers and floating-point values
• Floating-point input (parsing) and output (PRINT formatting)
• Expression generator support for mixed integer/floating-point operations (promotion and demotion rules)
• Math functions (eg, SQR, SIN, COS, ATN, LOG, EXP)
• Optional 8087 coprocessor support

Build, Tests, and Documentation

• Builds with MASM 4.0 using mk.sh (PC.js) or the in-browser Build Machine
• DOSTESTS: CALL 5, memory allocation, file create/write/read-back, and file rename/delete tests
• Unattended test runs using tools/tests/quick.sh (boots BASIC-DOS, runs DOSTESTS, and reports whether the tests passed)
• More tests (eg, BASIC language and CMD command tests)
• Complete the BASIC-DOS manual

Roadmap

These are the next steps, roughly in priority order:

1. Use DOS_HDL_CREATE for output redirection, and add the DEL and REN commands
2. Handle zero-length writes (truncation), the read-only attribute, and file attribute/date/time functions
3. FCB create, write, delete, and rename functions
4. Essential BASIC statements: GOSUB, FOR/NEXT, INPUT, READ/DATA, and DIM
5. BASIC string functions and file I/O statements
6. IEEE 754 floating-point support, starting with conversions and I/O
7. Critical error handling
8. Subdirectory support

License

BASIC-DOS is an open-source project on GitHub released under the terms of an MIT License.

Copyright (c) 2020-2026 Jeff Parsons Released under MIT License