1616/OS

1616/OS started out as a "monitor program" and just grew.  Yet even the final version 4.7a met with the operating system's initial design constraints:
 

1. Able to run with no disks.

The operating system, editor, shell and assembler ran from EPROM with only cassette tape storage. Late versions had zmodem in ROM, with the ability to bootstrap from attached Unix hosts.
2. No hardware memory management.

Executables were relocated when loaded.  There was no fork().  fork() was possible - Colin did it when he ported Minix, but I didn't find a need.  No VM, of course.
3. Easy to program in assembly language.

• Preemptive multitasking

A simple round-robin scheduler with process groups and per-task prioritisation.
• Multiuser

User ID management in the operating system, with all the usual authentication tools in user-mode applications. Since any process can put itself into supervisor mode, the 68000 always runs in supervisor mode. To prevent non-root users from busting root, executable files have to be owned by root.
• Memory management

A simple but surprisingly efficient bitmap-based memory manager.
• Threads

A process could spawn any number of 1616/OS threads.  These were handled as subprocesses of the spawning thread.  Calling exit() when subthreads were still running was a bad thing to do, because the thread kept on happily running in released memory!
• Around 250 system calls to support assembler programmers as well as C programmers.
• Support for hardware interrupt routines within applications.
• Signals

These were fairly standard.
• Runtime installable character and block device drivers

These were used in MGR, in the virtual console application and in the memory-resident drivers for the memory/SCSI card.
• Character device driver subsystem supporting tty signals.
• The 1616 hierarchical filesystem.
• Standard I/O functions - printf(), fprintf(), getchar(), etc.
• Cassette tape I/O
• Drivers for the Z80 disk controller card
• The drivers on the Z80 side of the disk controller card I wrote in BDS C.  The source is forever lost.
• Serial drivers with hardware and XON/XOFF flow control.
• Video driver which emulated a Hayes something-or-other terminal. Flashing cursor in software!
• Graphics drivers with windowing, clipping, line draw, circle draw, pixel draw, offscreen drawing, etc. Supported multiple frame buffers.
• Non-Y2K compliant time and date routines (so sue me).
• A wonderful line editor (I still use it, and it's better than yours).
• Pipes
• Per-process single stepping via the 68000's trace mode bit.
• Interesting signal semantics: if you send a signal to a process group leader it can optionally be sent to all subprocesses.
• Environment strings
• An asynchronous interprocess communication mechanism which essentially sends a blob of bytes to another task and wakes it up if it's sleeping on a receive.
• Memory-resident drivers which tuck up above the system stack at boot time.
• A full-screen non-modal editor.  The Unix version hangs around here.
• A shell with I/O redirection, pipelines, wildcard expansion, backgrounding of commands, etc.
• Shell backquote substitution was provided via a memory-resident driver extension. (Who remembers why the 'clsub' command printed out "%d backquote commands from %d EXECs.  Why bother?"?)
• An easter egg!  Check rom/ssrom/datetime.c
• Nifty programmable function keys which can learn-as-you-type.
• It's now safe to reveal: a backdoor.  The inbuilt command ^P^A^AE^X^T^O^E gives you root.  I never used it.  Probably couldn't remember it.
• Seventy eight built in commands.  Here they are:
mdb mdw mdl mrdb mrdw mrdl mwb mww mwl mwa mfb mfw mfl mfa mmove mcmp msearch base expr go srec help move mwaz cio terma termb serial fkey setdate date copy syscall delete cat rename dir msave mload tsave tload dirs itload cd echo mkdir edit ssio quit tarchive pause tverify ascii time xpath option volumes touch filemode type ssasm assign ps kill wait set term vmode getz putz geta getb puta
putb printa printb boota bootb
You work 'em out.  rom/ssrom/inbuilt.c.
• Simple scripting.  Fancy scripts are done with cscript.  cscript later became CS+, which gave way to CS++, which is here.
• Joystick routines, audio I/O (stereo via a software switched capacitor thingy), audio channel selection.
• Centronics driver
• Lots of other stuff.  What the heck.  Just download the thing.

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Andrew Morton, 8 March 1999