MSCP Boot Block Code Size for PDP-11 (Was: Defining Disk Image Dump Standard)

From: Jerome Fine <>
Date: Mon Jun 5 21:45:48 2000

>Sellam Ismail wrote:

> I still maintain that the less than 256 byte Disk ][ boot ROM code is one
> of the most amazing pieces of code ever written for a computing device.

Jerome Fine replies:

I am not sure if we are considering the same thing, but the single block
(512 bytes) of code for the secondary MSCP boot program on a PDP-11
(the primary MSCP boot program being the code in the BOOT ROM which
is much less than 256 bytes - actually one that can be entered by hand using
ODT is only 110 bytes) which is on block zero on the hard disk drive of a
PDP-11 for RT-11 is also a squeeze. And while including the mapping table
for eight RT-11 partitions made life difficult (only a 16 word table was used), I
challenge anyone to manage to add the other 56 available partitions and make
all 64 RT-11 partitions allowable for boot purposes since it would seem that
a 128 word mapping table would be required. I don't think it is possible to fit
the rest of the code, data and buffers into just the remaining 128 words even
when just one controller or host adapter is allowed and still manage to utilize
many hard disk drives each with a maximum total of 8 GBytes - obviously
not all physical partitions could be booted from one 64 entry mapping table
since even just one 8 GByte drive has 256 physical partitions (as mentioned
by Tim Shoppa - how do you even keep track of what is on all 256 partitions,
let alone use them all? - envy!!!). But at least with a 64 entry mapping table
in the boot block, all 64 partitions mapped for data use can also be booted
from inside RT-11 using DUP if they have the necessary system files on
those partitions. The problem is to squeeze an extra 56 entries into the
mapping table so that DUP can boot all 64 possible partitions (D00: => D77:).
>From what I can understand of what the MSCP boot requires, it is not
possible to do the rest using just the other 128 words for code and data
(buffers can occupy the same space as the mapping table).

Sincerely yours,

Jerome Fine
Received on Mon Jun 05 2000 - 21:45:48 BST

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