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New PowerPC Standard Supports Macs
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/Although still in its early stages, the new Common Hardware
Reference Platform standard promises to let PowerPC systems freely
use various operating systems/
/*Tom Thompson*/
Apple, IBM, and Motorola recently disclosed technical details that were
absent last fall when they announced a new standard for interoperability
among PowerPC computers (including future Power Macs) and PowerPC
operating systems. The CHRP (Common Hardware Reference Platform
)
standard, although still evolving, brings the PowerPC alliance closer to
the goal it stated in 1991 to create a new standard for RISC-based
computers that will let hardware vendors build a computer system for a
specific audience by picking and choosing components. The disclosure at
BYTE's PowerPC forum last December of details pertaining to cache, ASICs
(application specific ICs), controllers, and other components portends
an architecture that will let any CHRP-compliant computer cold-boot any
CHRP-compliant operating system.
Part of the original PowerPC announcement included an ABI (Application
Binary Interface) that would abstract low-level hardware so that Mac,
DOS, and Unix applications would run on the same PowerPC system. While
the ABI has been demonstrated several times over the years, none of the
shipping PowerPC systems have used it. Instead, each uses its own
PowerPC operating system: IBM's PowerPC systems and Motorola's new
PowerStack computers run AIX, and Apple's Power Macs only run System 7.
In the first quarter of this year, a PowerPC version of Windows NT 3.5
will become available for the IBM and Motorola systems.
The rift this creates in the PowerPC market is serious. Users must
choose a system from a vendor based in part on what operating system the
computer can run. This is the very problem that the alliance was trying
to avoid.
The problem has its roots in the hardware itself. The memory
organization that NT uses is different from that of AIX and the Mac OS
(the Endian issue). This problem is further exacerbated by the hardware
dependencies built into each operating system. For example, the Mac OS
makes heavy use of custom ASICs for handling keyboard, sound generation,
and video I/O.
IBM had circulated a draft PReP (PowerPC Reference Platform) document as
early as November 1993, and version 1.0 of the standard was formally
released in June 1994. PReP describes a system hardware standard to
which all PowerPC systems should conform to provide a minimum set of
capabilities (e.g., 16-bit CD-quality sound and 8-bit video) and host a
variety of operating systems. Motorola's systems are PReP 1.0-compliant,
as is the IBM RS/6000 Model 40P. Unfortunately, certain hardware
features were never specified, and support for the Mac OS was lacking.
One notable facet of the November 1994 CHRP announcement was that
vendors will be able to license the Mac OS and thus jump-start the Mac
clone market for Apple. Says Gary Griffis, IBM's director of business
development for Power Personal Systems. ``CHRP is the next step, where
the Mac architecture is merged into the standard. The end result is a
platform that combines the best of PCs and the Mac.''
Complete CHRP specifications are slated for release this spring.
Prototype CHRP systems should appear in the second half of this year and
ship sometime in 1996.
The conceptual block diagram ``Common Hardware Reference Platform''
shows that the core of a CHRP system consists of a PowerPC CPU, DRAM,
and a bootstrap ROM, the same as in PReP 1.0. What's new is that the
standard now suggests the use of a level 2 cache and a ROM SIMM socket.
This ROM socket is for use by manufacturers who will build Mac clones
(much of the Mac OS is housed in ROMs).
Jim Gable, Apple's Power Mac product line manager, says the fees Apple
will collect from Mac ROM licensees will be comparable to DOS and
Windows prices. (Radius and Power Computing have licensed the Mac OS and
say they will build Mac OS-compatible systems.) If Apple keeps this
promise, it augers well for the growth of a Mac-clone market.
Low-level support of the CHRP architecture will be accomplished
initially using ASICs jointly designed by the alliance. Motorola will
help develop the memory and PCI (Peripheral Component Interconnect)
bridge chip, which will be a derivative of the company's Eagle chip. The
new CHRP chip will support the PReP 1.0 and Mac OS memory maps.
Apple and IBM will develop other ASICs that offer I/O support, and
these, too, will leverage off existing chip designs. IBM will use its
Coral chip (an ISA bus interface and IDE device controller), and Apple
will use several ASICs as starting points in these chip designs.
Detailed CHRP specifications will be provided to a wide range of
industry chip-set suppliers, according to Charlie Ashton, Motorola's
PowerPC product marketing manager. ``We will ensure that the core logic
components for CHRP are available from a variety of sources, including
suppliers of standard x86 chip sets and super I/O controllers,'' he says.
CHRP emphasizes the use of an ISA bus or a PCI bus, whereas the original
standard only suggested their use. The ISA bus will provide
compatibility with existing expansion card designs. The PCI expansion
bus's throughput makes it suitable for high-performance applications. A
big advantage of PCI is that it's a plug-and-play bus, which could make
the job of adding expansion devices to a CHRP system easy.
Thanks to PCI 2.0 and Open Firmware, a CHRP system can host different
operating systems while using the same expansion devices. The PCI 2.0
specification provides for multiple firmware images in the expansion
device's firmware. And CHRP requires that PCI expansion devices use Open
Firmware for the boot process.
Open Firmware, an evolving IEEE standard (P1275), specifies a
processor-independent mechanism by which a system can interrogate
expansion devices, configure them, and install device drivers. Open
Firmware provides the mechanism by which a CHRP system configures and
operates all its expansion devices as Mac peripherals under the Mac OS
for one session and then configures and operates these devices as PC
peripherals when the computer is restarted to run an NT or OS/2 session.
Apple is using Open Firmware to implement PCI expansion devices in
PCI-bus Power Macs that will be introduced the first half of this year.
Neither IBM nor Motorola have Open Firmware development tools at this
time, but they plan to work with Apple and many third-party vendors in
this area.
Each member of the alliance is responsible for porting a specific
operating system to the new platform: For IBM, it is AIX and OS/2; for
Motorola, NT; and for Apple, the Mac OS. Because the PowerPC processor
can use little-Endian or big-Endian addressing, the memory organizations
that the operating systems use are not an issue. Although CHRP will
minimize some of the hardware dependency problems, all three companies
agree that this area still needs work.
At the very least, all the operating systems will require rewritten
device drivers. For example, modifications to NT's HAL and drivers are
necessary for it to run on the CHRP platform. Apple says current Mac
device drivers will also need modification, but at least the company has
already laid the groundwork with Open Firmware on the PCI Macs.
Once this work is complete, the ability to seamlessly run different
operating systems will be compelling. ``CHRP Macs will run whatever OS
is shipping in 1996,'' Apple's Gable says. ``The hardware and software
will operate as a no-apologies Power Mac.''
Other vendors also find this capability valuable. Greg Galanos,
president of Metrowerks (Montreal, Canada), a supplier of PowerPC
compilers for the Mac, says that CHRP should make life easier for
developers. ``If CHRP is implemented as promised, a developer would need
to buy only one CHRP system,'' Galanos says.
However, at the time of this writing, the CHRP standard is not complete.
Vendors are free to attach devices or subsystems where it makes sense
for their design. For example, a vendor might connect the video
subsystem directly to the PowerPC system bus for performance reasons.
The alliance has to balance flexibility for PowerPC system manufacturers
with the danger that comes when too much leeway defeats the purpose of a
standard (e.g., the original SCSI standard didn't precisely describe
certain low-level details, and many early SCSI devices that complied
with the standard didn't function with other SCSI devices).
The PowerPC alliance will have to resolve hardware details if the CHRP
standard is to avoid a similar fate to that of the first SCSI standard.
One such detail lies in the expansion bus. While the PCI bus lets
expansion devices function with different operating systems, the same
may not be true of a CHRP system that uses an ISA bus. By the time the
standard is final, the ISA bus may be eliminated from the standard.
Another unresolved question is whether the CHRP boot process will
support Plug and Play peripherals in either bus. Even the ROM socket
that's crucial for the Mac OS is optional.
When selecting core logic components, vendors can unintentionally impact
the use of their systems. For example, a PowerPC manufacturer might save
costs by not adding the ROM socket and an ASIC that provides an ADB
(Apple Desktop Bus) interface to its design. However, this decision
disappoints users who expected to use the Mac OS on the CHRP system at a
future date. The result: Users have to choose a CHRP system based on the
operating system they want--which is the problem CHRP is supposed to solve.
However, this is just a snapshot of the situation while the CHRP
standard is in its early stages. As the standard matures, the alliance
is actively soliciting input from other PowerPC OEMs. ``The original
PReP 1.0 standard was too open,'' says IBM's Griffis. ``We learned from
that to narrow CHRP down to specifics.''
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Common Hardware Reference Platform
illustration_link (17 Kbytes)
/The PowerPC common reference platform shown here is a high-level
conceptual diagram. Actual implementation may vary as details are
finalized and as manufacturers make their own decisions. What's new is a
level 2 cache and a ROM SIMM socket, which is for manufacturers who will
build Mac clones. The controller function blocks, particularly the I/O
controller, might be implemented using two or more ASICs. System I/O
support includes Mac and PC standards, such as SCSI, ADB (Apple Desktop
Bus), Ethernet, serial, and parallel port standards. /
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© 1994-1996