4 Aug
2008
4 Aug
'08
11:54 a.m.
Tony Duell wrote:
Alas, as is well known, I don't design for a living. I wish I did...
However, I have done so in the past.
I am also not arrogant enough to suggest that my designs were ever
perfect. I am quite sure I've made my fair share of brain-dead decisions.
And IMHO one common expression of 'brain
damage' is when the design
omitted to ccosider soemthing that later became a major issue.
Do you design for a living? If so, then my hat is off to you for thinking of *EVERY POSSIBLE* use of the design you
have made. I design
I think that;'s going a bit far....
I don't consider that the Z80 is brain-dead because it only has a 16 bit
address bus, for all 64K is nowhere near enough memory for a personal
computer now. 16 bit address was a reasonable choice at the time.
I do consider the IBM PC interrupt system to be brain dead because
-- It lead to major problems later on
-- The 'correct way to do it' was well-known by every electronic engineer
worth his NaCL, and had been used in many previous machines.
-- The 'cost' to do it right was 1 or 2 inexpensive ICs.
for a living, and I think you're asking way too
much of a designer. The
IMHO part of being a designer is thinking how the design could be used
(other than its originally-intended use),
best designers plan for the stated requirements and
try to consider the
long term, but they are not prophets.
I think subjecting the designers choices to things that came much later
is ludicrous.
And on the RAM at $0000 issue, I think that is simply Intel and the PC
environment responding to bad choices that people made. Intel 8080
designers no doubt assumed (as the MOS folks did), that startup vectors
would be in ROM. The Development Board chose RAM + switches to offer
some flexibility, which is to be expected. That CP/M and later systems
assumed RAM at $0 can't be blamed on Intel. The fact that they later
Agreed, and nor do I blame it on Intel.
tried to fix the issue by moving the jump table to the
top segment seems
The poiont is that that fixed nothing. The issue, IMHO, was not caused by
_where_ the procesor started execuding on rest.
to me to be reacting to the "status quo",
not creating a bad design.
CPUs need to fetch vectors from a fixed address. Anywhere you put them
This is not a $deity-given requirement. For example, after a reset, the
CPU could 3-state the address bus, assert an extra output pin (or assert
a normally-unused combination of pins, for example Rd and Wr asserted at
the same time), and then read in the location to start executing from on
the address pins. I know of no commericla processor which did it this
way, but it's certainly possible. Whether it's a sensible thing to do is
another matter.
will offend someone. I think Intel putting them at
the bottom sounds
like a fine design, at least in the '70s. 'C' and it's desire to have
address 0 be NULL was not around on the micros, and putting it at the
I wasn't aware that this was a requirement of C, or any other language.
bottom alleviated the need to keep moving it as the
"top" of memory
keeps moving up.
No it doesn't, given that a PDP11 address to a program is always 16 bits.
The 18 or 22 bit phuysicall addresses were created by the MMU.
And what maces the design worse is that the
problem had been considered
(and correctly solved) before. On the PDP11, the top part of the address
space was used for I/O devices. And it was the top part on all PDP11s,
whether they had 16, 18, or 22 bit addressing.
I don't think I would call that a correct design. It forces all
programs to treat all IO as relative addresses, since they will move as the design moves. If there are processing penalties
for relative
addressing, you've sealed the programmer's fate.
It was a good choice for minicomputers, as they specced out multiple
addressing options for various levels of the market, and programmers all
designed with that in mind. How was Intel to know the 8080 would sell
as well as a DEC CPU?
The issue here was witht he 80286 .vs. the 8086. Not the 8080. It was
clear the 8088 had sold very well (IBM and all that :-)), it was likely
the 80286 would sell well as well.
-tony