MiniUnix/usr/doc/new/memo5

Find at most related files.
including files from this version of Unix. 
.NH
Summary
.PP
Since MX uses no segmentation unit, no protection
is provided for the user program.
Thus new user programs must be debugged carefully.
In practice, the use of the C language limits the user's program's
use of the program counter and stack pointer
thus limiting damage and usually causing a bus error
before anything drastic happens.
The lack of a segmentation unit does have its advantages.
It means the user can directly access all I/O registers on the UNIBUS
and does not have to write special device drivers
interfaced with the file system to control the special
peripherals.
Thus in cases where a real-time program is to be run,
one may disable the system clock to inhibit
unwanted clock interrupts and also swapping
of processes.
One may also catch clock interrupts during the running
of user programs if the clock is to be used for
user program timing control.
The clock should then be restored to system control
upon exit from the user program.
.PP
There is also another set of user programs available
under MX which may be used to ease transition from
DOS, the DEC operating system, to UNIX for those
installations now using the DEC DOS operating system
on a PDP-11 CPU.
This package consists of a macro-assembler and a
linker-loader for assembling programs
written under DOS for the DEC macro-assembler.
The result is a UNIX 'a.out' file.
.PP
The normal configuration for MX includes a PDP-11/10
CPU with 28K words of memory and two RK05 disk
cartridges for secondary.
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.
The cost of a minimum configuration:
.DS
PDP-11/10 CPU
28K words memory
2 RK05 disk drives
KL11 interface to control console
DL11E interface to dial-up line
60 cycle clock
.DE
is of the order of $20,000 at today's prices (December 1976).
This provides an inexpensive tool for software development
in a UNIX time-sharing environment for those
configurations which have insufficient hardware
to support a full Version 6 UNIX system.