Introduction to VM - The User Directory
=======================================
OK, so you've now got a little understanding of VM, and some sense of
the way it works. As with all computer stuff, there's a lot more to it
than that, and any questions you may have probably all have the
answer 'it depends'.
To recap:
We have VM installed on our swish new mainframe, and have successfully
IPLed CP. 'IPL' is IBM-speak for 'boot' ('Initial Program Load').
You *do* remember what CP is, don't you?
So what now?
Virtual machines are defined to CP via the 'user directory'. Now, I
found this a bit confusing at first. The virtual machines are known as
'users', and there is no concept of userids or logins in the same way
as on eg. Unix or Windows systems. The reason for this is that
originally, VM was intended to enable time-sharing and multi-user
operation by giving each end-user their own single-user virtual
machine. This of course, is the opposite way to how most current
systems work ie. one operating system image accessed concurrently by
many users.
Strictly speaking, it *is* possible to log on to and remain in CP as a
'user' in the more conventional sense, but as CP only really provides
facilities for managing hardware and virtual machines, you're not
going to get much actual work done there!
Let's take a look at an example user directory entry:
USER LEEB SECRET1 16M 64M ABCDEFG
MACHINE ESA
IPL CMS
CONSOLE 009 3215
SPOOL 00C 2540 READER *
SPOOL 00D 2540 PUNCH A
SPOOL 00E 1403 A
LINK MAINT 190 190 RR
LINK MAINT 19E 19E RR
LINK MAINT 19D 19D RR
MDISK 191 3390 0001 0020 USER01 MR ALL
MDISK 400 3390 0021 0500 USER01 R ALL
MDISK 123 3390 000 END 240RES MW ALL
OK. Doesn't make a great deal of sense at first glance. Let's go
through it line-by-line.
USER LEEB SECRET1 16M 64M ABCDEFG
This defines a user (virtual machine) 'leeb', with password 'secret1'.
The leeb virtual machine starts with 16MB of RAM ('real storage' in
mainframe-speak), and is permitted to extend that up to 64MB if
required. The last field, seemingly a partial recitation of the
alphabet, defines the privilege levels this user holds. Privilege
levels in CP detemine what you can request of CP in terms of virtual
machine operations, and CP management functions. So, for example, 'C'
(system programmer) can change system-wide parameters in CP. 'G'
(general user) is restricted to CP commands associated with their
virtual machine. Naturally, I gave myself full access :-) There are
other levels available, and a big slice of the alphabet is given over
to user-definable levels.
MACHINE ESA
This directive describes the architecture of the virtual
machine. The capabilities of this directive do depend on the version
of VM and the underlying real hardware, but it may be possible to eg.
run z/VM (64-bit) at the top level with a VM/ESA (31-bit) guest second
level, and a VM/SP (24-bit) guest under VM/ESA (with say, an ancient
version of MVS running as a VM/SP guest). The question then arises:
"What kind of madman would do such a thing?". I don't know the answer
to that question, but you can see how it might be useful (albeit not
officially supported) to keep some of those legacy systems running
that you just don't seem to be able to shake off. You may have seen
VMWare et al put to similar use on x86.
IPL CMS
This line will automatically boot the named operating system when you
start up (ie. log in as) your virtual machine. CMS is definitely going
to be in another article. For now, just know that CMS is a single-user
OS commonly used with (and provided as part of) VM. I know you haven't
already forgotten what 'IPL' means.
CONSOLE 009 3215
SPOOL 00C 2540 READER *
SPOOL 00D 2540 PUNCH A
SPOOL 00E 1403 A
These lines define some virtual devices for the virtual
machine. CONSOLE is fairly self-explanatory, the first SPOOL defines a
virtual punch-card reader, the second a virtual card punch, and the
third a virtual printer. I think this stuff will be in a separate
article. For the time being, it's enough to know that VM can provide
virtual devices to your virtual machine, which have no real device
equivalent.
The three-digit hex number after the SPOOL and CONSOLE directives is
the address of the device *as it will appear inside your virtual machine*
I'll skip over LINK just now, and go on to:
MDISK 191 3390 0001 0020 USER01 MR ALL
MDISK 400 3390 0021 0500 USER01 R ALL
MDISK 123 3390 000 END 240RES MW ALL
These lines define virtual hard disks to your virtual machine. VM uses
'minidisk' to denote virtual disk storage devices, hence MDISK. I
think I'll break out disk storage into another article too...
Similar to SPOOL and CONSOLE, the hex number after MDISK defines the
address of the disk inside your virtual machine.
Now lets look at LINK:
LINK MAINT 190 190 RR
LINK MAINT 19E 19E RR
LINK MAINT 19D 19D RR
The LINK directive allows your virtual machine to access other virtual
disks in the VM system. This is very useful for sharing software and
data between virtual machines under the control of CP.
For instance, maybe a development suite (compiler, debugger etc.) is
installed somewhere. Accessing the minidisk(s) from your virtual
machine means you can now make use of that software, and so get
cracking on your Zork port for VM. Or maybe something less
interesting. Anyway. These LINK directives specify that we're going to
link to three minidisks (owned by user MAINT) at addresses 190, 19E,
and 19D (in the MAINT virtual machine), and we're going to access them
at the same addresses in our virtual machine. 'RR' is read-only,
because to be honest, MAINT probably doesn't want us screwing up all
his hard work.
So, you log in ('logon' in the mainframe world) as your virtual
machine user 'leeb', enter the correct password, then CP goes away and:
- allocates you a bit of RAM
- sets up some virtual devices for you
- locates your virtual disks in the system (probably on some SAN or
disk array across the computer room somewhere)
- Tries to access the MAINT disks and add them to the devices
available to your virtual machine
- Attempts to start your OS (CMS) in the virtual environment it has
just prepared for you.
All being well, you see:
FILES: 0001 RDR, 0001 PRT, NO PUN
LOGON AT 15:30:49 JST SATURDAY 01/12/19
VM/ESA V2.4.0 1999-09-24 09:18
