[TUHS] Re: UNIX "Machine Layer" Standards

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Paul Ruizendaal

    > something like a boot rom only became the norm in the late
    > 70's. Before that, one keyed in two dozen words with a tiny program to
    > load the first boot stage.

A little wrong on that date. Even the PDP-11/20 (the first -11) had a boot
ROM:

  https://gunkies.org/wiki/BM792_ROM

which appreared in mid-1971 (about a year after the release of the /20). DEC
sold them pre-programmed, but one could 'program' one onself, if one wanted -
with a soldering iron! (Check out the image! I actually did that to one that
I was given, that had been eviscerated by someone.) From then on (follow the
category link), the rest used PROM chips.

  
    > From: Warner Losh

    > Oftentimes, the interrupt vector was in the lowest core addresses

It's worth remembering that in the early period, that restriction to low
addresses was built into the hardware (in an amusing way :-).

Take the DL11:

  https://gunkies.org/wiki/DL11_asynchronous_serial_line_interface

which was sort of mandatory as the 'console' serial interface on most early
-11's (until the DL11-W appeared; more on its big improvement in a second).
It set the interrupt vector with _jumpers_. (You want to change the interrupt
vector? Dig out your soldering iron! :-) There were only 6 jumpers - one each
for address bits 3 through 8. So the largest vector you could set was 0770.

The DL11-W was a big step forward - it replaced the jumpers with a DIP
switch! :-) Still only six bits, though. :-)

	Noel