24 Sep
2001
24 Sep
'01
6:15 a.m.
Warning Heresy follows :) DG alert!
I'm not the expert on microcoding versus writable control stores. I seem to
dimely remember that there was a whole section about microcoding in the 1981
book by Tracy Kidder "Soul of a new machine". Lots of microcode was used to
emulate "old" instructions from previous machines. Also to perform complex
series of instructions.
Mike
mmcfadden(a)cmh.edu
24 Sep
24 Sep
6:56 a.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
Mike McFadden wrote:
I'm not the expert on microcoding versus writable
control stores. I seem to
dimely remember that there was a whole section about microcoding in the 1981
book by Tracy Kidder "Soul of a new machine". Lots of microcode was used to
emulate "old" instructions from previous machines. Also to perform complex
series of instructions.
The Eagle (MV) 32-bit members of the Eclipse family, like the 16-bit members
before them, were indeed microcoded -- with the difference that the MVs had
soft microcode that was loaded off of magnetic media (floppies on the original
MV8K; later from files stored in the native AOS/VS file system).
Faced with the issue of _how_ to read the microcode the MV designers
did something modestly clever. Since the MV is an almost-proper superset
of the Nova, and since the Nova ISA was well understood, when a MV first
powers up it thinks it's a slightly brain damaged Nova 800 (there's no
page zero auto inc/auto dec instructions and most "do nothing" I/O an
skip instructions are not honored), which gave them a reasonably
complex abstraction upon which to open and traverse file systems
looking for the MV microcode...
--
Chris Kennedy
chris(a)mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97
11:42 a.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
On Mon, Sep 24, 2001 at 11:56:30AM -0700, Chris Kennedy wrote:
Faced with the issue of _how_ to read the microcode
the MV designers
did something modestly clever. Since the MV is an almost-proper superset
of the Nova, and since the Nova ISA was well understood, when a MV first
powers up it thinks it's a slightly brain damaged Nova 800 (there's no
page zero auto inc/auto dec instructions and most "do nothing" I/O an
skip instructions are not honored), which gave them a reasonably
complex abstraction upon which to open and traverse file systems
looking for the MV microcode...
But where is the microcode for the brain-damaged Nova 800 instruction set
stored, and can it be overwritten?
-- Derek
4:28 p.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
Derek Peschel wrote:
But where is the microcode for the brain-damaged Nova
800 instruction set
stored, and can it be overwritten?
PROMs, and no, I know of no way to overwrite _that_ code. Instead you
execute a load microstore operation, which moves data from memory to
microstore and resets the machine such that it begins execution from
soft microstore rather than the proms.
I'll have a look-see in the POP for the details.
--
Chris Kennedy
chris(a)mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97
8:34 p.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
On Mon, Sep 24, 2001 at 09:28:19PM -0700, Chris Kennedy wrote:
Derek Peschel wrote:
... And once you've loaded your own microcode, do the PROMs take up any
room in the microaddress space? And does the microcode handle the load
operation, or is there some sort of dedicated circuit?
I assume the load operation isn't supposed to happen while an ordinary
program is running (though that _would_ be an opportunity for spectacularly
efficient and inscrutable code).
It's scary how similar this is to a popular booting technique (switch on
boot ROM, use it to load RAM, switch off boot ROM) except, of course, that
you're one level lower.
-- Derek
But where is the microcode for the brain-damaged
Nova 800 instruction set
stored, and can it be overwritten?
PROMs, and no, I know of no way to overwrite _that_ code. Instead you
execute a load microstore operation, which moves data from memory to
microstore and resets the machine such that it begins execution from
soft microstore rather than the proms.
25 Sep
25 Sep
2:42 a.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
Derek Peschel wrote:
... And once you've loaded your own microcode, do
the PROMs take up any
room in the microaddress space? And does the microcode handle the load
operation, or is there some sort of dedicated circuit?
I can't find any _architectural_ specification for this. The PROMs clearly
do not form part of the MV microcode set; in effect a bank switch takes
place after the microcode is loaded.
I assume the load operation isn't supposed to
happen while an ordinary
program is running (though that _would_ be an opportunity for spectacularly
efficient and inscrutable code).
There's no functionality in the MV instruction set that allows the
control store to be manipulated -- it's implemented in the dorked-up
Nova 800 instruction set in the tried-and-true fashion of the
16-bit Eclipse instruction set: Recycle meaningless-but-
valid instructions (for example, things like register-to-register
moves with the same source and destination register with no skip
or always skip conditions set) to create space for the new instruction.
It's scary how similar this is to a popular
booting technique (switch on
boot ROM, use it to load RAM, switch off boot ROM) except, of course, that
you're one level lower.
It was modestly brilliant. By creating a hardwired ISA using a model
that was well understood (i.e., unlikely to ever need revision), the
designers allowed themselves a much higher level of abstraction when
building code that had to, among other things, interact with the
I/O subsystem. Since they already had architectural verification
programs in place for the Nova it was trivial to modify them to
verify the correctness of the boot firmware.
--
Chris Kennedy
chris(a)mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97
9:45 a.m.
New subject: DG MVs (was RE: microcoding a PC into a PDP-11 (was: RE: Classic Computers
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ard＠p850ug1.demon.co.uk -
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mmcfadden＠cmh.edu