Re: Reading disk from BIOS

From: Nic Clews (sendspamhere_at_[127.0.0.1)
Date: 01/23/04 

Date: Fri, 23 Jan 2004 14:12:19 +0000

Krish wrote:

> When you power on a Alpha System, running OpenVMS, how does the BIOS
> read the sectors from the disk. Or rather, how does the BIOS access
> the disk without the disk driver being loaded. Becuase, to load the
> driver we need to read the disk, right ? I remember a funda called
> Int13, but I guess this is higly limited to PC bios, am I right. If
> that is the case, what happens in a Alpha system ?

Very briefly, because I suspect you may also get a huge amount of
detail...

This applies to VAX as well as Alpha. The console BIOS can communicate
with all the devices it may be expected to boot from, and going to LBN
zero, this gives it the location of the initial bootstrap program, again
in LBN format, and the console then starts to load the code from the
location given in the bootblock into memory, then starts to execute it.

The "boot block" is an integral part of INDEXF.SYS, if LBN 0 is bad then
this device can not be used as a boot device. The program WRITEBOOT.EXE
in SYS$SYSTEm is used to update the value with the APB.EXE or VMB.EXE
LBN as appropriate. The bootblock is 512 bytes, the VAX LBN is at the
beginning of the bootblock, the Alpha is at the end of the bootblock, so
the pair can co-exist on a single volume.

You can see this if you dump the first block of INDEXF.SYS, and if you
DUMP/HEAD the APB or VMB, you'll see the hex value of the (decimal)
retrieval pointer LBN for the respective file. Note that Alpha value for
the LBN is + 1. For VAX this is the second longword of the INDEXF.SYS.
For Alpha it's the sixth from last longword (remember the DUMP hex
display reads right to left).

The "trickery" that does this is in the console program itself, the
thing that allows you to interact, displaying the >>>, using console
variables, etc. so the console is much more than just a BIOS.

This is changing with Itanium, it boots much more like a PC does, and
the VMS boot process is being moved to "just after" the "standard"
platform boot, so in effect (and I expect I'll be corrected if this is
wrong) the VMS console which will be responsible for the remainder of
the VMS booting process, is first loaded from the system disk to be
booted. This avoids the IPF having to "know" anything about how VMS
wants to boot.

The secondary bootstrap, the VMB, APB, IPB, is what starts to load up
the remainder of the operating system, which is the point that the
operating system begins to load the drivers and everything else.

Your "power on" sequence has missed out the POST, Power On Self Test,
and particularly with an Alpha the point where the console is started is
displayed as a message, and gives you the >>>.

In my younger years, I used to "toggle in a RIM loader". This was a
sequence of octal numbers which represented a program which effectively
told the hardware where the punched paper tape reader was, to load the
contents from the reader into successive memory locations, and then to
start execution. The principles are the same. (RIM - dunno what it
stands for, if anything). 

-- 
Regards, Nic Clews a.k.a. Mr. CP Charges, CSC Computer Sciences
nclews at csc dot com

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