17 Nov
2012
17 Nov
'12
7:39 a.m.
On 11/16/2012 11:57 PM, ben wrote:
Actually they didn't but in practical terms you cant use/buy them much any
more so your limited to building now with devices that are based on 8/16 bit
interfaces.
As to racks, no the most of the interfaces are less than a fraction of a
rack
like about 4-7u.
Yes, true. save for the 8088/86 was still a 16bit program counter with
an embedded
MMU at no time could it address more than 64K of instruction space or
data space.
Even that was doable with then z80. The PDP11 had the MMU that was
worth emulating.
On 11/16/2012 8:10 PM, allison wrote:
No, the system I was building ran 2167s a 55ns parts not a problem.
The time it takes for the 2901a to do things like propagate a carry and
other internal operations that involve anythingthat ripples through is
the limit.
Then you have to allow for bus timing.
4.77mhz is about as fast as 2901s will run
without pipelining.
I did an 8/16 bitter with 2901Cs and was profoundly disappointed.
How ever in hindsight, memory speed and bus width ( 8 bits ) was the
limiting factor in speed back then. Add length like 20 or 36bits makes for problems
like devices like disks
as most available will be 8 or 16bits. Then again with large disks
half word splits will fit. Makes it troublesome to do 12 or other
odd by
current standards lengths for words.
Disks don't care what bits they get. With my limited knowlage, interface
chips where made for the upcoming 8 bit micro's, other machines still
relied on racks full of equipment.
As to PC killer, that was never needed, the pc
was killing itself with a
cpu that
was 8080 with a bag on the side. I always felt that the 8086/88 was a
market successful while failed design. Maybe why I used PDP11s and Z80s
till 1991 when 32bits made the scene at least interesting. The PC
was an
appliance, not the latest computing tech.
The computer market, did need a machine
with more the 64K and only
Motorola
and Intel had a product that could sell in personal computer. All the
other chips
came out just too late.
Since when is the latest, allways the best. I still
favor analog audio
media,
and film for photos, and REAL IBM keyboards.
The keyboard is ok. But most of my analog media (1/4in tape and vinyl)
are degrading.
Any how since I am doing this computer as hobbie,I can
play with the
design
as "what if" and get a feel why computers ( crappy as they be in some
ways)
are the way they are from that era in time.
Ben.
Yes you can. I only give the benefit of been there and did that as in I
did build fairly
complex transistor logic, TTL based systems and even a few things with
2901s. The
frustration then was more bits were expensive and everything was lower
than I'd
have liked. However some of the architectures were inspired and I
still see the
PDP-8 as an amazing machine for what it could do with so little actual
hardware.
Allison