MiniUnix/usr/doc/new/mx

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MX - Mini-Unix for PDP-11/10	October 2, 1975
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	H. Lycklama
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A version of the UNIX operating system has been written for
the PDP-11/10 computer.
The system supports all of the standard "system calls"
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of the PDP-11/40, PDP-11/45 and PDP-11/70 versions of UNIX
with the exception of: trace, pipe, prof, getgid and setgid.
The entire system resides in 12K words of memory
and is written in the C-language.
Since the PDP-11/10 computer does not support the extended
instruction set (e.g. mul, div, ash, ashc, etc.)
an emulation package is included in the system to
emulate these instructions.
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User programs are compiled and relocated to start
at address 060000 and may occupy up to 16K words of memory.
Up to 13 processes may exist at any one time,
although only one process may be in core and running.
This should be sufficient to handle up to
4 simultaneous users.
Scheduling is done on a simple round-robin basis
with each process in the "run-state"
receiving a 2 second time-slice.
A process is simply swapped out at the end of its
time-slice and the next runnable
process swapped in.
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The operating system supports all UNIX user programs
which run on the PDP-11/40, PDP-11/45 and PDP-11/70
computers, with the exception of programs which
use the above mentioned system calls.
A few user programs were modified to run on the PDP-11/10
version of UNIX.
These include "ps" to deal with the different process
table structure and "ld" to relocate all user programs
up to address 060000.
All user programs were recompiled in order to relocate
them to start at address 060000.
Filters may be simulated at the command level
by replacing the sh syntax "|" by ">sh.out;<sh.out".
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Since the PDP-11/10 CPU has no segmentation unit,
no protection is provided for the user program.
Thus new user programs must be debugged carefully.
The PDP-11/10 processor is slower than the PDP-11/40
processor and does not have the full instruction set of the
PDP-11/40 processor, thus requiring the
emulation of the missing instructions.
A typical C compilation requires about twice
the total time of that required
on the equivalent PDP-11/40 configuration.
However response to the editor commands
is not significantly longer than
on a more powerful CPU.
Features which are not provided in the MX version
of UNIX include "read-ahead" and physical I/O.
This is due to the limited system address space available.
Only 6 system buffers are provided.
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The current configuration on which the operating system
runs includes:
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PDP-11/10 CPU
28K words memory
2 RK05 disk  drives.
KL11 interface to Decwriter
DL11-E asynchronous line interface for dial-up lines
60 cycle clock.
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The cost of the above configuration to the Bell
System is of the order of $26,000 including an 18% discount.
A minimum configuration with only one RK05 disk and no DL11-E
interface can be obtained for approximately $21,000.
More recent price reductions on PDP-11/10
configurations may result in a total system cost of
less than $20,000.
This provides a very inexpensive tool for software
development in a UNIX time-sharing environment.
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MH-1352-HL-JER	H. Lycklama
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Copy to:
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Dept. 8234
G.L. Baldwin
M.D. McIlroy
D.J. Lando
G.L. Link
G.W.R. Luderer
J.F. Maranzano
H.S. McDonald
E.N. Pinson
D.M. Ritchie
B.A. Tague
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