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heading	Welcome to BASIC-DOS
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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, DEL, DIR, EXIT, HELP, KEYS, MEM, RESTART, TIME, TYPE, and VER
• Running COM, EXE, BAT, and BAS files, and loading programs into other sessions
• BAT and BAS files can run other BAT and BAS files and then continue (no CALL command required); a nested BAS file gets its own variables
• Pipes (|) and output redirection (> creates or truncates the output file, >> appends to it), including redirection at the end of a pipeline (eg, DIR | CASE > TEST)
• COPY creates (or truncates) the output file, and refuses to copy a file onto itself
• DEL/ERASE
• Input redirection (<)
• 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)
• Type suffixes (% for integers, # or ! for doubles, and $ for strings), where the type is part of a variable's identity (eg, after DEFINT A-Z and then DEFDBL A-Z, A is a new variable, separate from A%)
• String variables, concatenation, and comparisons
• String functions: ASC, CHR$, DATE$, FRE, HEX$, INKEY$, INSTR, LCASE$, LEFT$, LEN, MID$, OCT$, RIGHT$, SPACE$, STR$, STRING$, TIME$, UCASE$, and VAL
• String pool management: temporary strings are released as soon as they're consumed, strings are compacted (and empty string blocks freed) when space runs out, and runtime string errors (eg, "String too long") abort the program cleanly
• CLS, COLOR, DEF FN (including string functions and parameters), DEFDBL, DEFINT, DEFSNG, DEFSTR, ECHO, GOTO, IF/THEN/ELSE, LET, PRINT, REM, and RETURN
• Assignments without LET in BAS and BAT files (LET is still required on the command line)
• LOAD, LIST, NEW, and RUN
• Functions: ERRORLEVEL, MAXINT, and RND%
• Code blocks grow as needed (up to 64K) for large programs
• Control flow: GOSUB, FOR/NEXT, WHILE/WEND, and ON ... GOTO/GOSUB
• INPUT, LINE INPUT, READ, DATA, and RESTORE
• Arrays (DIM)
• The MID$ statement (ie, MID$(A$,N[,M]) = B$)
• &H and &O prefixes in VAL
• Comma print zones in PRINT (commas currently print a tab)
• File I/O statements (eg, OPEN, CLOSE, PRINT #, INPUT #)
• Error handling (eg, ON ERROR, ERR, ERL), and runtime error messages for numeric errors

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)
• Numeric variables default to doubles (unless DEFINT is used), and DEFSNG is treated as DEFDBL
• MBF-based MSLIB option removed
• FPU$ device driver, which detects an 8087 at boot and provides a table of floating-point functions (IOCTL_GETFPU)
• 8087 functions: arithmetic (including ^), comparisons, conversions between longs and doubles, ABS, INT, FIX, and SQR
• Software emulation of all FPU$ functions for systems without an 8087
• String-to-double (FPU_ATOD) and double-to-string (FPU_DTOA) conversions, and %f support in sprintf
• Expression generator support for mixed integer/floating-point operations (promotion and demotion rules), using FPU$ calls
• Floating-point constants and PRINT output in BASIC programs
• BASIC math functions: ABS, ATN, COS, EXP, FIX, INT, LOG, SIN, SQR, and TAN
• FPUTESTS, run with and without an 8087 by tools/tests/quick.sh
• Saving and restoring 8087 state on session switches (FPU$ functions currently disable interrupts instead), and better error reporting for FPU exceptions
• Utility functions DOS_UTL_ATOF64, DOS_UTL_I32F64, and DOS_UTL_OPF64 (still stubs; possibly superseded by FPU$)

Build, Tests, and Documentation

• Builds with MASM 4.0 using mk.sh (PC.js), which also updates the BASIC-DOS demo disks after a successful build
• DOSTESTS: CALL 5, memory allocation, file create/write/read-back, and file rename/delete tests
• STRFUNCS and STRPOOL: BASIC string function tests, and string pool stress and leak tests
• Unattended test runs using tools/tests/quick.sh (boots BASIC-DOS with and without an 8087, runs FPUTESTS, DOSTESTS, STRFUNCS, and STRPOOL, and reports whether the tests passed)
• tools/tests/chkdsk.sh runs MS-DOS 3.20 CHKDSK on a diskette image saved by a test session (see QUIT /S in pc.js)
• More tests (eg, BASIC language and CMD command tests)
• HELP for functions (HELP currently covers only keywords), and an update of HELP.TXT (eg, it still says floating-point isn't supported)
• Complete the BASIC-DOS manual

Roadmap

These are the next steps, roughly in priority order:

1. Add the REN command and input redirection
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 file I/O statements, and the remaining string features (the MID$ statement, LINE INPUT, and comma print zones)
6. Runtime error handling (ON ERROR, ERR, ERL)
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.

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Building BASIC-DOS

Everything needed to build BASIC-DOS is in this repository, including the pc.js utility (in tools/pc) and the MS-DOS 3.20 disk image with MASM 4.0 and associated tools that the build runs on. Just clone the repository and install its Node.js dependencies:

git clone https://github.com/jeffpar/basicdos
cd basicdos
npm install

If you also want successful builds to update the BASIC-DOS demo disks, you'll need the PCjs repository (for its diskimage.js utility), with the PCJS environment variable set to its location:

git clone https://github.com/jeffpar/pcjs
export PCJS="$HOME/pcjs"

Now you're ready to build BASIC-DOS, using the mk.sh script, which runs pc.js with MS-DOS 3.20 as drive C and the BASIC-DOS source code as drive D:

$ ./mk.sh
[Press CTRL-D to enter command mode]

C:\>D:

D:\>MK
Microsoft (R) Program Maintenance Utility  Version 4.02
Copyright (C) Microsoft Corp 1984, 1985, 1986.  All rights reserved.

...

D:\>QUIT

Any files that the build modifies are written back to the software/pcx86/src directory. A successful build runs QUIT automatically, after which mk.sh updates the demo disks (if PCJS is set); if a build fails and you don't want the demo disks updated, use pc.js's "abort" command instead.

The boot.sh script then builds and boots a 360K floppy (the largest floppy supported by an IBM PC XT Model 5160) containing the BASIC-DOS boot sector and system files, along with the files in the configs/console/serial/fpe folder (including CONFIG.SYS and AUTOEXEC.BAT) and the test binaries that tools/tests/prep.sh copies there:

$ ./boot.sh
[Press CTRL-D to enter command mode]
BASIC-DOS 2.00B
Press a key to start...

The bootfpu.sh script does the same thing, but on an ibm5160-fpu machine (which has an 8087 coprocessor), using the configs/console/serial/fpu folder; boot.sh uses a machine without an 8087, where BASIC-DOS must emulate floating-point operations.

Finally, tools/tests/quick.sh boots both configurations unattended, runs the FPUTESTS and DOSTESTS programs, and reports whether the tests passed.

Tool Trivia

In keeping with the era for which BASIC-DOS is designed, it's built with Microsoft Assembler (MASM) 4.0 and associated tools, circa 1985. While we could have opted for even older tools, the MASM 4.0 release strikes a nice balance between vintage operation and modern tooling.

However, as with any old tools, there are idiosyncrasies that can catch you by surprise.

One is how MASM encodes 32-bit and 64-bit floating-point numbers: by default, it uses the Microsoft Binary Format (MBF) instead of the IEEE 754 format used by 80x87 Intel coprocessors. This is probably because Microsoft's original floating-point emulation libraries were written using MBF and they didn't want to spend time and resources creating new emulation libraries that supported the newer IEEE 754 format.

So, by default, an assembly language data directive such as:

DQ  1.0

will generate:

00 00 00 00 00 00 00 81

instead of:

00 00 00 00 00 00 F0 3F

And while neither the MASM 4.0 User Guide or Reference Manual discuss this, it turns out that if you pass /R on the MASM command-line, MASM will use the IEEE 754 format instead. The stated purpose of /R is to generate 80x87 opcodes instead of emulation calls whenever using coprocessor instructions, but another important consideration is encoding all floating-point number in a compatible format (ie, IEEE 754 for coprocessors or MBF for the Microsoft emulation library) -- which /R happens to do as well.

Other idiosyncrasies include some minor code generation quirks. For example, an instruction like:

cmp	ah,UTILTBL_SIZE

should always generate 3 bytes (2 bytes for the opcode and 1 byte for the immediate operand), but if a constant like UTILTBL_SIZE is calculated using 16-bit values, even if the result is 8 bits, MASM 4.0 will reserve 16 bits and then replace the upper 8 bits with a NOP (0x90).

To eliminate the NOP, one work-around is to explicitly truncate the operand with an 8-bit mask, as in:

cmp	ah,UTILTBL_SIZE AND 255

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