VM/370 Security (and perform

[Andrew Klossner]

Not that many Unix inhabitants care, but here's two more cents worth
about VM/370 before the end-of-Xmas news rush ...

	"(Virtual Machine).  Under VM you can run any operating system
	you want, as long as you have the 370 code for it.  CMS is one
	option.  CP (Control Program) is also part of the package ...
	IBM developed VM, CP and enhanced CMS.  VM is a system control
	program.  It manages the resources of an IBM 370 type computer
	in such a way that multiple users have a functional simulation
	of a computing system (a virtual machine) at their disposal.
	Note that VM can run many different OSs concurrently.  CP
	executes in a real machine controlling the resources of that
	machine.  CP is used to create concurrent virtual machines.
	CMS is a single user OS that can execute in one or more virtual
	machines.  CMS is considered to be a timesharing OS ... Hope
	that clarifies the discussion."

Some corrections.  VM/370 is the name of a system software package
which consists primarily of two components: CP and CMS.  CP is the
kernel running on the real machine (unless it's running under itself
...).  It is the part of VM/370 that presents several virtual machine
environments.  It is responsible for fielding traps and interrupts,
doing resource allocation (like scheduling), and other "kernel" jobs.
There is some user interface in CP, but not much ... it's mostly of the
"simulate a front panel" variety, like the command to boot an operating
system in your virtual machine.

CMS is a single-user IBM 370 operating system.  When it runs
stand-alone on a 370, it supports a single user.  Several instances of
it can be running under CP, each in a distinct virtual machine.  Since
CMS doesn't have to support more than one user, it needn't be concerned
with issues of resource fairness or protection.

Thus, there is no distinct piece of the package called "VM"; the name
refers to the entire system.  Often users of such a system will
use the terms "VM" and "CP" interchangeably.  And the beauty of CMS is
that it is *not* a timesharing OS.

Incidentally, modern versions of CMS will not run stand-alone on a real
computer.  For reasons of efficiency, CP allows programs running in
virtual machines to use an additional means of requesting services,
implemented with the "diagnose" instruction (which on a real machine
causes model-specific results for hardware maintenance, and is useless
to production software).  For example, one variant of "diagnose" means
"read a record from the tape drive, block this virtual machine until
complete."  This is a good deal more efficient than going through the
standard hardware sequence of events: start I/O; wait for an interrupt;
handle the interrupt; test the I/O state; etc.  CMS does all its
standard I/O through diagnose, and so can no longer execute on a bare
machine.

  -- Andrew Klossner   (decvax!tektronix!orca!andrew)       [UUCP]
                       (orca!andrew.tektronix at csnet-relay)  [ARPA]