COMMAND.COM is the BASIC-DOS Command Shell. Unlike other operating systems, which have a Command Interpreter for running operating system commands and a BASIC Interpreter for running BASIC language programs, BASIC-DOS provides a unified command shell.
Each session runs its own copy of COMMAND.COM, as specified by a SHELL= line in CONFIG.SYS. Copies share the same code, so each additional session needs only a small amount of additional memory.
COMMAND [command[:command]…]
Any commands following COMMAND are run when it starts (eg, SHELL=COMMAND.COM AUTOEXEC.BAT). There is no automatic AUTOEXEC.BAT; each session’s startup commands must be given explicitly.
Running COMMAND.COM from the prompt starts another copy of the shell; type EXIT to return to the previous copy.
BAS and BAT files, and commands typed at the prompt, are compiled into 8086 machine code, which is why BASIC-DOS programs run several times faster than the same programs in Microsoft BASIC. A program is compiled as it runs: its lines are compiled in order, as the program reaches them (a jump to a later line compiles the lines before it, too), so even a large program starts immediately. As in Microsoft BASIC, a statement that BASIC-DOS doesn’t support causes a syntax error when the program reaches it, which ends the program.
When a BAS program ends, its compiled code is kept along with the program, if the whole program was compiled, so RUN can start it again without compiling it again.
COMMAND.COM has a resident portion and a transient portion (about 23K). Before running a COM or EXE program, COMMAND.COM frees any variable blocks it doesn’t need and discards its transient portion, leaving more memory for the program. When the program ends, the transient portion is restored: from a copy that COMMAND.COM saves at the top of free memory, if the program didn’t overwrite it, or else by reloading it from A:COMMAND.COM.
The MEM command includes the transient portion in its total of available memory.
Copyright (c) 2020-2026 Jeff Parsons Released under MIT License