Re: Architectural implications of locate mode I/O and channels

An Historical Perspective::
EricP wrote:

Michael S wrote:

On Wed, 03 Jul 2024 08:44:01 -0400
EricP <ThatWouldBeTelling@thevillage.com> wrote:
>

An alternative is each device has its own DMA controller
which is just a couple of counters.
>

>
The main cost is not counters, but bus mastering logic.
For PCI it was non-trivial cost even as late as year 2000. For example,
PLX PCI to local bus bridges with bus mastering capability, like PCI
9080 costed non-trivially more than slave-only 9060.

>
PCI is a different matter. I think they shot themselves in the foot.
That is because the PCI design used FAST TTL and was ridiculously
complex and had all sorts of unnecessary optional features like bridges.

I don't think it was as much "shot themselves in the foot" as it was
not looking forward enough. CPUs had just dropped from 5.0V to 3.3V
and few peripherals were going 3.3--yet.
There were no real "popcorn" parts on 3.3V. CMOS was gradually taking
over, but was "essentially" compatible voltage wise with TTL.

>
To my eye the choice of FAST TTL looks wrong headed. They needed FAST
because they wanted to run at 33 MHz which is beyond LS TTL limit.

Was also faster than popcorn CMOS of the day.

With a bus at 33 MHz and 4 bytes it superficially sounds like 133 MB/s.
But 33 MHz was too fast to decode or latch the address and data,
plus it multiplexes address and data, and took 5 cycles to do a
transfer.
So the bus actual data transfer rate was more like 133/5 = 26 MB/s.

Welcome to "back when computers were hard".

>
I looked into PCI bus interface chips when it first came out and there
were just *TWO* manufacturers for them on the planet, and they charged
$50,000 just to answer the phone, and you had to pay them to design a
custom chip for you even though it was supposed to be a standard design.

When PC shipped in the thousands per month this was the way things were.
When PC started to ship hundreds of thousands per months things changed
(early-mid 90s).

This all pushed the price of PCI cards way up so, for example,
an ISA bus modem card cost $50 but the exact same modem on PCI was $250.
No wonder most people stuck with ISA bus cards.

Exacerbating the above.

>
Whereas 74LS TTL was cheap and available from manufacturers everywhere.
I would have used 74LS TTL and done a straight 32-bit bus with no
options,
multiplexed address and data to keep down the connector pin count and
cost.
That could have run at 20 MHz which leaves 50 ns for address and data
decode and latch, and driven 8 bus loads with no bridges.
That gives 10 MT/s = 40 MB/s. Plus cheap and widely available.

It would take "too many" TTL parts to implement a small form factor
initerface, so integration was needed.