From mboxrd@z Thu Jan  1 00:00:00 1970
From: cowan@mercury.ccil.org (John Cowan)
Date: Wed, 23 Dec 2015 10:58:52 -0500
Subject: [TUHS] Does anybody recall how the TU10 bootstrap code actually
 operates?
In-Reply-To: <567AB7B1.2050907@gmail.com>
References: <20151223065957.5ED5718C095@mercury.lcs.mit.edu>
 <567AB7B1.2050907@gmail.com>
Message-ID: <20151223155852.GA6677@mercury.ccil.org>

Will Senn scripsit:

> Actually, after reflecting on your comments and walking through it
> again, this is really elegant code. The guys who thought this up
> were amazing.  

As usual, they were standing on the shoulders of giants.  The OS/8
bootstrap (on the PDP-8) for the RK8E disk controller was even more
elegant, only two instructions long and requiring no further operator
actions but "clear" and "start".  The first instruction simply read
the current block into the current memory address, which because of
the "clear" read block 0 into memory location 0; the second was also
a branch-to-self.

But the real cleverness was that the bootstrap was placed at locations
30 and 31.  As the disk block was read in by DMA, location 30 was
overwritten with the "skip if disk is ready" instruction and location
31 with "branch to previous location".  So first the CPU was idling in
a one-instruction infinite loop, then in a two-instruction loop as long
as the block was still loading, and finally would continue executing at
location 32 when the disk bootstrap was fully loaded.  The code there
loaded the full device driver for the RK8E into reserved memory at
07600-07777 and jumped to the code within it that loaded the Keyboard
Monitor (the shell) at location 0 and jumped into it.

-- 
John Cowan          http://www.ccil.org/~cowan        cowan at ccil.org
A rose by any other name may smell as sweet, but if you called it
an onion you'd get cooks very confused.          --RMS