From: leigh@croatia.eng.hou.compaq.com (Kevin Leigh)
Subject: COMPAQ PROPOSED SCALABLE I/O ARCHITECTURE
Summary: Cost effective, higher performance, processor-independent I/O scheme
Keywords: I/O, Point-to-Point, High Performance, Low Cost, Bus
Organization: Compaq Computer Corp.
Date: Thu, 10 Dec 1992 02:56:09 GMT

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*                                                          *
*               COMPAQ COMPUTER CORPORATION                *
*			HOUSTON, TX			   *
*                                                          *
*          STRATEGIC TECHNOLOGY DEVELOPMENT GROUP          *
*                                                          *
*   We're tired of beating our heads against the wall      *
*   trying to expand the performance of I/O buses. Let's   *
*   face it, wide-buses are just not a long term cost      *
*   effective solution for high performance computer       *
*   solutions.  So here is an alternative...               *
*                                                          *
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*                                                          *
*  Compaq presented a proposal at the PCMCIA sub-committee *
*  (CardBus) meeting in Deerfield Beach, Florida, on       *
*  12/7/92 for possible adoption as the CardBus standard.  *
*                                                          *
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Our proposed I/O solution is
	- hierarchical 
	- point-to-point
	- channel I/O architecture
	- scaleable
	- high performance
	- processor & endian independent
	- low cost

In short, NOT yet-another shared-wide-bus!!!
 

                        CPU
                         |
                         |---memory
                       MIOC
                       /  \
                     IOC  dev3
                     / \
                 dev1  dev2

                      Figure-1

The basic I/O subsystem consists of a "main" I/O concentrator
(MIOC) which interfaces the I/O subsystem to any CPU/Memory
architecture [figure-1].  One or more point-to-point 
channels propagate in a hierarchical manner from the MIOC to 
the devices.  One such device is an I/O Concentrator (IOC) 
which allows the bandwidth of a single point-to-point 
channel to be shared amongst several devices.  Other devices 
might include network interfaces, graphics, SCSI, etc.  You 
may optionally allow a few very low bandwidth devices to 
share the same channel under a controlled environment.

A channel is an interconnect between two ports, one residing
in a (M)IOC and the other in a device.  A port closer to 
CPU-memory is referred to as an upstream, and a port further
from CPU-memory is referred to as a downstream.  The physical
channel consists of 12 signals:
    -  a 50 MHz synchronizing clock
    -  two handshake signals
    -  eight data, and 
    -  a parity

The small number of signals on a channel reduces the pin
requirements for interface ICs and the physical size of
add-in boards and connectors, reducing the cost of 
implementation.  Because all the data transfers are in byte-
wide quantities the sequence of data is in increasing address 
order, i.e., there is no little- or big-endianess in the 
data.  Also note that the channel pin-out is NOT a 
derivative of any particular CPU chip's memory interface 
signals, allowing the solution to be non CPU architecture
specific.

The proposed solution minimizes "out-of-band" signals.  All 
operations within the I/O subsystem are carried out via 
packets.  Even the synchronizing channel clock is propagated
from the MIOC to the devices through IOCs.  In other words,
the channel clock is redistributed on every IOC's downstream
port.  The signal timings of a channel is independent from
all other channels.  This virtually eliminates signal skew
problems, especially for the clock, common in today's shared
bus systems.

The standard data transfer rate on a channel is 50 
MBytes/sec (MBPS) and the upper-bound will be limited by the 
physical environment of the channel.  For example, a current 
high performance implemention with GTL (Gunning Transistor
Logic) drivers and receivers could yield a transfer rate of
200 MBPS.

Two ports on a channel "agree" on a transfer rate during the 
initialization.  Each port multiplies the 50 MHz channel 
clock to generate internal clocks to operate at the agreed 
transfer rate.  Each IOC port can operate at a different
transfer rate.  A device designed to support high transfer
rates also works at lower transfer rates.  That means a
device can be used at any I/O hierarchy level.  The I/O "tree"
should be carefully configured to optimize the bandwidth
distributions according to the system and devices requirements.


Operations occur using packet-based commands that flow 
through the channels.  Each packet generally contains four 
fields
- command (Read, Write, Error, Extensions/Interrupts, etc.)
- size (the amount of data to be transferred)
- address
- data

Only the command field is mandatory.  All ports are 
responsible for some encoding and decoding of packets.  Note 
that only one packet at a time can be transferred on a 
channel.  This means that there will be times that
- a devices cannot send or receive when an IOC downstream port
  does not have space or data, respectively, and
- both ports want to send at the same time.
The channel protocol supports handshaking for data transfers
as well as for conflict resolutions.

The commands are designed to allow optimal use of the
distributed I/O bandwidth and to let devices maintain high
bandwidths even when latencies are long.  For example, the
read operations are split transactions, where each read
packet is later responded by a corresponding read-response
packet.

The proposed solution is designed to allow the CPU to 
directly read and write registers of MIOCs, IOCs, and
devices in a programmed I/O manner.  However, the
architecture particularly lends itself to Command List
Processing Master devices.  These entities run device-
specific command contexts placed in system memory that define
their operation.  This potentially frees the main CPU to
perform other operations.


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*                                                          *
*  So, what do you think?  We are interested in making     *
*  this proposal an open Industry Standard.  As such, you  *
*  may request further information, give us your comments, *
*  or ask questions.                                       *
*                                                          *
*                                                          *
*  Contact:                                                *
*                                                          *
*   David Wooten, Manager,                                 *
*     Strategic Technology Development                     *
*     Compaq Computer Corporation,                         *
*     20555, SH 249,                                       *
*     Houston, TX                                          *
*                                                          *
*   Email: davidw@twisto.compaq.com                        *
*   Phone: (713)378-7231                                   *
*   Fax:   (713)374-2580                                   *
*                                                          *
*                                                          *
*                                                          *
*  CONTINUE READING IF YOU'RE INTERESTED IN WHY WE SPENT   *
*  TIME DEVELOPING THIS CONCEPT!!!                         *
*                                                          *
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The question we asked ourselves was how can one build an I/O 
subsystem that is high performance, easy to interface, easy 
to expand, and inexpensive?

A shared wide-bus is not the solution due to its inherent 
limitations, such as distributed capacitance, one-at-a-time 
transmit, and high pin count for the mother board and add-in 
cards.

In the past, shared bus architectures made sense because of 
the TTL technology and the high-cost-and-low-integration of
silicon.  The PC architecture that started almost a decade
ago with a single, shared, 8-bit memory-I/O bus (known as 
"ISA") has evolved into many variants due to the ever-
escalating performance requirements of x86 based CPU
systems.  This phenomenon is also seen in other system
architectures based on non-x86 CPUs.

One of the first changes in PC evolution was to move the CPU 
and the memory to a faster proprietary local bus when the 
ISA bus became a bottleneck.  The I/O data bus was widened 
to 16-bit and then to 32-bit to provide higher bandwidth 
for some devices.  There were two PC "wide-bus" solutions: 
EISA, a super-set of ISA for backward compatibility, and 
Micro Channel (MCA).

EISA system is more flexible than ISA (and MCA) system
because it can accept either EISA or ISA card.  Also, EISA
cards in EISA systems can provide higher performance than
ISA cards in ISA (or EISA) systems.  EISA has served quite
well for several high-performance devices (e.g., Compaq's
high performance QVision graphics card).  As a tradeoff
for the higher bandwidth, the system and the add-in cards
for the wider I/O buses are more expensive than comparable
ISA solutions.  Because ISA can provide adequate bandwidth
for the majority of devices, several board vendors continue
to build more boards for ISA because of the larger available
market.

More recently, color graphics and video applications have
become popular for PCs.  In addition, high pixel 
resolution and more bits-per-pixel (for color) displays
have become more affordable.  Consequently, the bandwidth
demands for the graphics/video devices have become too
large for even EISA and MCA.  To correct this deficiency,
several system OEMs have moved graphics onto higher
bandwidth proprietary local buses.  As the practice of
moving faster devices onto the local bus became common,
there was a need for a standard.  Currently, there are
two local bus standard proposals, namely, PCI (by Intel)
and VL-Bus (by VESA).

Existing processors do not have a PCI bus and future 
processors will not directly support the VL-Bus.  Besides, 
PCI and VL have limited plug-in (card) support.  
Consequently, PC systems utilizing PCI or VL may have 
multiple levels of buses (CPU bus, local bus, standard I/O 
bus) and IC bridges to interface between different bus 
protocols.  All of this leads to higher system and add-in 
board costs.  Chip and board vendors also need to choose 
between many different interfaces and form-factors to 
support a wide range of system types.

Worse yet, both PCI and VL buses will very likely try to 
satisfy future needs, such as higher bandwidth and 3.3V 
technology, by CHANGING the physical layer.  Examples 
might include: a wider (64-bit) bus once 32-bits runs out of 
steam, and a different driver/receiver technology for higher 
frequencies.  When a 32-bit bus is expanded to 64-bit, a
high-bandwidth (e.g., video) device will not gain 
performance on the 64-bit bus unless it is redesigned for
64-bit transfers.  Protocols might also have to change and 
many of these changes will render today's PCI and VL devices 
incompatible (PCI does have a compatible transition to 64-
bits).  The additional drivers/receivers and new design will
make the new devices to be more expensive.  The trick is to
allow the aggregate system bandwidth to go up without having
to redesign everything.

Neither PCI nor VL supports ease-of-use (hot-insertion and
plug-and-play) features.  Depending on implementation,
PCMCIA does not support hot-swap and may not offer enough
performance for some applications.  A PCMCIA sub-committee
was created to come up with a PCMCIA super-set called
CardBus.  As mentioned earlier, board vendors made more ISA
cards than EISA cards for cost reasons and larger market share.
Similarly, we are guessing that, board vendors will build
more PCMCIA cards than CardBus cards, if CardBus is a
super-set of PCMCIA, i.e., a wider and more expensive 32-bit
bus and backward compatible.

To summarize: the PC industry has tried several times to 
solve increasingly higher performance I/O requirements by 
utilizing a multitude of wide, shared buses.  Most solutions 
did not last long because they were not scaleable and they 
were EXPENSIVE.  As a result most PC users continue to buy 
cheap and reasonable performance solutions.  What the 
computer industry needs is a solution that:
(a) solves today's problems (performance, expansion and
    ease-of-use) in a cost effective manner AND
(b) offers a migration path for those who want it AND
(c) is processor independent AND
(d) will provide longevity.

Our proposed solution can offer ALL of these properties!


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*                                                          *
*   To summarize the major Features:                       *
*                                                          *
************************************************************

Our proposed solution is scaleable in multiple dimensions:
performance, expandability and cost.  Specifically

FOR PERFORMANCE:
(a) the CPU-memory bandwidth is decoupled from the slower 
    device latencies,
(b) point-to-point minimizes the physical constraints and 
    GTL enables higher signal rates than TTL,

FOR EXPANDABILITY:
(c) virtually unlimited number of devices can be connected
    on-board or off-connectors,
(d) expansion can also be made in external boxes via a small
    pin-count robust protocol (e.g., a fast serial link)
(e) hot plug-and-play is supported,

FOR COST:
(f) small packages/connectors/boards and high integration 
    can be achieved because of the small number of pins to 
    interface,
(g) the number of PCB layers can be minimized because of 
    "clean" layout and no long traces (even in the case of 
    busing, the fanouts are fairly short),
(h) development cost can be reduced by utilizing common 
    functional blocks (e.g., state machines, FIFOs),
(i) time-to-market can be reduced by integrating common 
    parts,
(j) common parts for a wide range of systems enable large 
    volume, fewer inventory part types and consistent 
    test/manufacturing techniques,

FOR COMPATIBILITY:
(k) standard I/Os such as ROM and keyboards can be tapped 
    off of an MIOC or IOC,
(l) existing standard buses (e.g., ISA, PCMCIA) can also be 
    implemented off MIOC or IOC,
(m) Existing application software and operating systems will
    be compatible with some driver updates.

FOR LONGEVITY:
(n) the same devices can be reused in future systems because
    they are processor-neutral,
(o) the same I/O subsystem or architecture can be used in a 
    family of products (portables, desktops, workstations, 
    servers),
(p) the same device can be used for different transfer rates 
    without changing the physical interface, and
(q) changing a device interface in the future on a channel 
    does not affect the rest of the devices [we call this 
    "damage control"].

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*                                                          *
*   Thanks for reviewing our thoughts!  Feel free to       *
*   to contact us for more information, we're here to      *
*   help!!!                                                *
*                                                          *
*   Happy Holidays from (Strat. Tech. Dev.)                *
*     David Wooten, Kevin Leigh, Reynold Starnes,          *
*     Thanh Tran, Chris Simonich, Brett Costly,            *
*     David Murray, Craig Miller, Roger Tipley             *
*                                                          *
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Newsgroups: comp.arch