Mini-Unix/usr/doc/new/memo2

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Introduction
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The MINI-UNIX Operating System was written to run on all
PDP-11 processors without the memory management unit
available on the PDP-11/40, 11/45 and 11/70 processors
and is therefore restricted to a 28K word address space.
The operating system itself (MX) is basically a modified version
of the Version 6 UNIX Operating System (1)
and as such supports most of the standard UNIX "system calls".
The system resides in 12K words of memory and is written
in the C language.
An instruction emulation package is included in the system
for those machines which do not support the
extended instruction set (e.g. mul, div, ash, etc.).
The system supports up to four users using a simple
round-robin time-slicing algorithm.
It supports all of the UNIX user programs unmodified.
User programs which have been slightly modified
are discussed in a later section.
MINI-UNIX thus provides an inexpensive software development
system in a UNIX time-sharing environment
for those installations with a minimum amount of
hardware which is insufficient to support
the full Version 6 UNIX Operating System.
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Other software tools are also available for
easing the transition from software written under the DEC DOS
Operating System.
This includes a macro assembler and linker.
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Hardware
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The MINI-UNIX system runs on any PDP-11 processor
with 28K words of memory.
The PDP-11 computer is a 16-bit word mini-computer
with a UNIBUS for interfacing DEC peripherals to the CPU.
The typical configuration consists of a PDP-11/10 CPU
with 28K words of memory, a console terminal and an RK05
moving-head disk controller with two removable
disk cartridges for swapping and file system storage.
Each RK05 disk pack has 2.5 Megabytes (8-bit byte)
of storage.
However, the MX system also supports the RF fixed-head disk
(1 Megabyte) and the RP03 and RP04 moving-head
disk controllers with 40M bytes and 80M bytes, respectively.
Other peripherals supported include line-printer, Dectape,
magtape and various asynchronous and synchronous interface units.
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The system is normally configured to be 12K words in size.
This includes an emulation package for interpreting
the 10 extended instructions normally performed
by the EIS hardware available as an option
on some PDP-11 processors and standard
on the PDP-11/45 processor.
A minimum system has room for 6 or 7 system buffers.
As new drivers are added to the system,
the number of system buffers must be decreased
if the system size is maintained at 12K words.
Thus it is recommended that for some applications
it may be appropriate to add the drivers for only a few peripherals
on any one version of the system and thus maintain
a few versions of the system, one for each set of drivers
desired concurrently in the system.
This keeps the system size at 12K words
in order to be able to support all of the
user software of Version 6 UNIX.