Open main menu

Brief Introduction to Alpha Systems and Processors

  Brief Introduction to Alpha Systems and Processors
  Neal Crook, Digital Equipment (Editor: David Mosberger
  <mailto:[email protected]>)
  V0.11, 6 June 1997

  This document is a brief overview of existing Alpha CPUs, chipsets and
  systems. It has something of a hardware bias, reflecting my own area
  of expertese. Although I am an employee of Digital Equipment Corpora­
  tion, this is not an official statement by Digital and any opinions
  expressed are mine and not Digital's.

  1.  What is Alpha

  "Alpha" is the name given to Digital's 64-bit RISC architecture. The
  Alpha project in Digital began in mid-1989, with the goal of providing
  a high-performance migration path for VAX customers. This was not the
  first RISC architecture to be produced by Digital, but it was the
  first to reach the market. When Digital announced Alpha, in March
  1992, it made the decision to enter the merchant semicondutor market
  by selling Alpha microprocessors.

  Alpha is also sometimes referred to as Alpha AXP, for obscure and
  arcane reasons that aren't worth persuing. Suffice it to say that they
  are one and the same.

  2.  What is Digital Semiconductor

  Digital Semiconductor <>
  (DS) is the business unit within Digital Equipment Corporation
  (Digital - we don't like the name DEC) that sells semiconductors on
  the merchant market. Digital's products include CPUs, support
  chipsets, PCI-PCI bridges and PCI peripheral chips for comms and

  3.  Alpha CPUs

  There are currently 2 generations of CPU core that implement the Alpha

  ·  EV4

  ·  EV5

  Opinions differ as to what "EV" stands for (Editor's note: the true
  answer is of course "Electro Vlassic" ``[1]''), but the number
  represents the first generation of Digital's CMOS technology that the
  core was implemented in. So, the EV4 was originally implemented in
  CMOS4. As time goes by, a CPU tends to get a mid-life performance kick
  by being optically shrunk into the next generation of CMOS process.
  EV45, then, is the EV4 core implemented in CMOS5 process. There is a
  big difference between shrinking a design into a particular technology
  and implementing it from scratch in that technology (but I don't want
  to go into that now). There are a few other wildcards in here: there
  is also a CMOS4S (optical shrink in CMOS4) and a CMOS5L.

  True technophiles will be interested to know that CMOS4 is a 0.75
  micron process, CMOS5 is a 0.5 micron process and CMOS6 is a 0.35
  micron process.

  To map these CPU cores to chips we get:

        EV4 (originally), EV4S (now)



        LCA4S (EV4 core, with EV4 FPU)

        LCA45 (EV4 core, but with EV45 FPU)



        EV6 <>

  The EV4 core is a dual-issue (it can issue 2 instructions per CPU
  clock) superpipelined core with integer unit, floating point unit and
  branch prediction. It is fully bypassed and has 64-bit internal data
  paths and tightly coupled 8Kbyte caches, one each for Instruction and
  Data. The caches are write-through (they never get dirty).

  The EV45 core has a couple of tweaks to the EV4 core: it has a
  slightly improved floating point unit, and 16KB caches, one each for
  Instruction and Data (it also has cache parity).  (Editor's note: Neal
  Crook indicated in a separate mail that the changes to the floating
  point unit (FPU) improve the performance of the divider.  The EV4 FPU
  divider takes 34 cycles for a single-precision divide and 63 cycles
  for a double-precision divide (non data-dependent).  In constrast, the
  EV45 divider takes typically 19 cycles (34 cycles max) for single-
  precision and typically 29 cycles (63 cycles max) for a double-
  precision division (data-dependent).)

  The EV5 core is a quad-issue core, also superpipelined, fully bypassed
  etc etc. It has tightly-coupled 8Kbyte caches, one each for I and D.
  These caches are write-through. It also has a tightly-coupled 96Kbyte
  on-chip second-level cache (the Scache) which is 3-way set associative
  and write-back (it can be dirty). The EV4->EV5 performance increase is
  better than just the increase achieved by clock speed improvements. As
  well as the bigger caches and quad issue, there are microarchitectural
  improvements to reduce producer/consumer latencies in some paths.

  The EV56 core is fundamentally the same microarchitecture as the EV5,
  but it adds some new instructions for 8 and 16-bit loads and stores
  (see Section ``Bytes and all that stuff''). These are primarily
  intended for use by device drivers. The EV56 core is implemented in
  CMOS6, which is a 2.0V process.

  The 21064 was anounced in March 1992. It uses the EV4 core, with a
  128-bit bus interface. The bus interface supports the 'easy'
  connection of an external second-level cache, with a block size of
  256-bits (2 data beats on the bus). The Bcache timing is completely
  software configurable. The 21064 can also be configured to use a
  64-bit external bus, (but I'm not sure if any shipping system uses
  this mode). The 21064 does not impose any policy on the Bcache, but it
  is usually configured as a write-back cache. The 21064 does contain
  hooks to allow external hardware to maintain cache coherence with the
  Bcache and internal caches, but this is hairy.

  The 21066 uses the EV4 core and integrates a memory controller and PCI
  host bridge. To save pins, the memory controller has a 64-bit data bus
  (but the internal caches have a block size of 256 bits, just like the
  21064, therefore a block fill takes 4 beats on the bus). The memory
  controller supports an external Bcache and external DRAMs. The timing
  of the Bcache and DRAMs is completely software configurable, and can
  be controlled to the resolution of the CPU clock period. Having a
  4-beat process to fill a cache block isn't as bad as it sounds because
  the DRAM access is done in page mode. Unfortunately, the memory
  controller doesn't support any of the new esoteric DRAMs (SDRAM, EDO
  or BEDO) or synchronous cache RAMs. The PCI bus interface is fully
  rev2.0 compliant and runs at upto 33MHz.

  The 21164 has a 128-bit data bus and supports split reads, with upto 2
  reads outstanding at any time (this allows 100% data bus utilisation
  under best-case dream-on conditions, i.e., you can theoretically
  transfer 128-bits of data on every bus clock). The 21164 supports easy
  connection of an external 3-rd level cache (Bcache) and has all the
  hooks to allow external systems to maintain full cache coherence with
  all caches. Therefore, symmetric multiprocessor designs are 'easy'.

  The 21164A was announced in October, 1995. It uses the EV56 core. It
  is nominally pin-compatible with the 21164, but requires split power
  rails; all of the power pins that were +3.3V power on the 21164 have
  now been split into two groups; one group provided 2.0V power to the
  CPU core, the other group supplies 3.3V to the I/O cells. Unlike older
  implementations, the 21164 pins are not 5V-tolerant. The end result of
  this change is that 21164 systems are, in general, not upgradeable to
  the 21164A (though note that it would be relatively straightforward to
  design a 21164A system that could also accommodate a 21164). The
  21164A also has a couple of new pins to support the new 8 and 16-bit
  loads and stores. It also improves the 21164 support for using
  synchronus SRAMs to implement the external Bcache.

  4.  21064 performance vs 21066 performance

  The 21064 and the 21066 have the same (EV4) CPU core. If the same
  program is run on a 21064 and a 21066, at the same CPU speed, then the
  difference in performance comes only as a result of system
  Bcache/memory bandwidth. Any code thread that has a high hit-rate on
  the internal caches will perform the same. There are 2 big performance

  1. Code that is write-intensive. Even though the 21064 and the 21066
     have write buffers to swallow some of the delays, code that is
     write-intensive will be throttled by write bandwidth at the system
     bus. This arises because the on-chip caches are write-through.

  2. Code that wants to treat floats as integers. The Alpha architecture
     does not allow register-register transfers from integer registers
     to floating point registers. Such a conversion has to be done via
     memory (And therefore, because the on-chip caches are write-
     through, via the Bcache).  (Editor's note: it seems that both the
     EV4 and EV45 can perform the conversion through the primary data
     cache (Dcache), provided that the memory is cached already.  In
     such a case, the store in the conversion sequence will update the
     Dcache and the subsequent load is, under certain circumstances,
     able to read the updated d-cache value, thus avoiding a costly
     roundtrip to the Bcache.  In particular, it seems best to execute
     the stq/ldt or stt/ldq instructions back-to-back, which is somewhat

  If you make the same comparison between a 21064A and a 21066A, there
  is an additional factor due to the different Icache and Dcache sizes
  between the two chips.

  Now, the 21164 solves both these problems: it achieve much higher
  system bus bandwidths (despite having the same number of signal pins -
  yes, I know it's got about twice as many pins as a 21064, but all
  those extra ones are power and ground! (yes, really!!))  and it has
  write-back caches. The only remaining problem is the answer to the
  question "how much does it cost?"

  5.  A Few Notes On Clocking

  All of the current Alpha CPUs use high-speed clocks, because their
  microarchitectures have been designed as so-called short-tick designs.
  None of the sytem busses have to run at horrendous speeds as a result

  ·  on the 21066(A), 21064(A), 21164 the off-chip cache (Bcache) timing
     is completely programmable, to the resolution of the CPU clock. For
     example, on a 275MHz CPU, the Bcache read access time can be
     controller with a resolution of 3.6ns

  ·  on the 21066(A), the DRAM timing is completely programmable, to the
     resolution of the CPU clock (not the PCI clock, the CPU clock).

  ·  on the 21064(A), 21164(A), the system bus frequency is a sub-
     multiple of the CPU clock frequency. Most of the 21064 motherboards
     use a 33MHz system bus clock.

  ·  Systems that use the 21066 can run the PCI at any frequency
     relative to the CPU. Generally, the PCI runs at 33MHz.

  ·  Systems that use the APECs chipset (see Section ``'') always have
     their CPU system bus equal to their PCI bus frequency. This means
     that both busses tends to run at either 25MHz or 33MHz (since these
     are the frequencies that scale up to match the CPU frequencies). On
     APECs systems, the DRAM controller timings are software
     programmable in terms of the CPU system bus frequency

  Aside: someone suggested that they were getting bad performance on a
  21066 because the 21066 memory controller was only running at 33MHz.
  Actually, it's the superfast 21064A systems that have memory
  controllers that 'only' run at 33MHz.

  6.  The chip-sets

  DS sells two CPU support chipsets. The 2107x chipset (aka APECS) is a
  21064(A) support chiset. The 2117x chipset (aka ALCOR) is a 21164
  support chipset. There will also be 2117xA chipset (aka ALCOR 2) as a
  21164A support chipset.

  Both chipsets provide memory controllers and PCI host bridges for
  their CPU. APECS provides a 32-bit PCI host bridge, ALCOR provides a
  64-bit PCI host bridge which (in accordance with the requirements of
  the PCI spec) can support both 32-bit and 64-bit PCI devices.

  APECS consists of 6, 208-pin chips (4, 32-bit data slices (DECADE), 1
  system controller (COMANCHE), 1 PCI controller (EPIC)). It provides a
  DRAM controller (128-bit memory bus) and a PCI interface. It also does
  all the work to maintain memory coherence when a PCI device DMAs into
  (or out of) memory.

  ALCOR consists of 5 chips (4, 64-bit data slices (Data Switch, DSW) -
  208-pin PQFP and 1 control (Control, I/O Address, CIA) - a 383 pin
  plastic PGA).  It provides a DRAM controller (256-bit memory bus) and
  a PCI interface. It also does all the work required to support an
  external Bcache and to maintain memory coherence when a PCI device
  DMAs into (or out of) memory.

  There is no support chipset for the 21066, since the memory controller
  and PCI host bridge functionality are integrated onto the chip.

  7.  The Systems

  The applications engineering group in DS produces example designs
  using the CPUs and support chipsets. These are typically PC-AT size
  motherboards, with all the functionality that you'd typically find on
  a high-end Pentium motherboard. Originally, these example designs were
  intended to be used as starting points for third-parties to produce
  motherboard designs from. These first-generation designs were called
  Evaluation Boards (EBs). As the amount of engineering required to
  build a motherboard has increased (due to higher-speed clocks and the
  need to meet RF emission and susceptibility regulations) the emphasis
  has shifted towards providing motherboards that are suitable for
  volume manufacture.

  Digital's system groups have produced several generations of machines
  using Alpha processors. Some of these systems use support logic that
  is designed by the systems groups, and some use commodity chipsets
  from DS. In some cases, systems use a combination of both.

  Various third-parties build systems using Alpha processors. Some of
  these companies design systems from scratch, and others use DS support
  chipsets, clone/modify DS example designs or simply package systems
  using build and tested boards from DS.

  The EB64: Obsolete design using 21064 with memory controller
  implemented using programmable logic. I/O provided by using
  programmable logic to interface a 486<->ISA bridge chip. On-board
  Ethernet, SuperI/O (2S, 1P, FD), Ethernet and ISA. PC-AT size. Runs
  from standard PC power supply.

  The EB64+: Uses 21064 or 21064A and APECs. Has ISA and PCI expansion
  (3 ISA, 2 PCI, one pair are on a shared slot). Supports 36-bit DRAM
  SIMs. ISA bus generated by Intel SaturnI/O PCI-ISA bridge. On-board
  SCSI (NCR 810 on PCI) Ethernet (Digital 21040), KBD, MOUSE (PS2
  style), SuperI/O (2S, 1P, FD), RTC/NVRAM. Boot ROM is EPROM. PC-AT
  size. Runs from standard PC power supply.

  The EB66: Uses 21066 or 21066A. I/O sub-system is identical to EB64+.
  Baby PC-AT size. Runs from standard PC power supply. The EB66
  schematic was published as a marketing poster advertising the 21066 as
  "the first microprocessor in the world with embedded PCI" (for trivia
  fans: there are actually 2 versions of this poster - I drew the
  circuits and wrote the spiel for the first version, and some Americans
  mauled the spiel for the second version)

  The EB164: Uses 21164 and ALCOR. Has ISA and PCI expansion (3 ISA
  slots, 2 64-bit PCI slots (one is shared with an ISA slot) and 2
  32-bit PCI slots.  Uses plus-in Bcache SIMMs. I/O sub-system provides
  SuperI/O (2S, 1P, FD), KBD, MOUSE (PS2 style), RTC/NVRAM. Boot ROM is
  Flash. PC-AT-sized motherboard.  Requires power supply with 3.3V

  The AlphaPC64 (aka Cabriolet): derived from EB64+ but now baby-AT with
  Flash boot ROM, no on-board SCSI or Ethernet. 3 ISA slots, 4 PCI slots
  (one pair are on a shared slot), uses plug-in Bcache SIMMs.  Requires
  power supply with 3.3V output.

  The AXPpci33 (aka NoName), is based on the EB66. This design is
  produced by Digital's Technical OEM (TOEM) group. It uses the 21066
  processor running at 166MHz or 233MHz. It is a baby-AT size, and runs
  from a standard PC power supply. It has 5 ISA slots and 3 PCI slots
  (one pair are a shared slot). There are 2 versions, with either PS/2
  or large DIN connectors for the keyboard.

  Other 21066-based motherboards: most if not all other 21066-based
  motherboards on the market are also based on EB66 - there's really not
  many system options when designing a 21066 system, because all the
  control is done on-chip.

  Multia (aka the Universal Desktop Box): This is a very compact
  pedestal desktop system based on the 21066. It includes 2 PCMCIA
  sockets, 21030 (TGA) graphics, 21040 Ethernet and NCR 810 SCSI disk
  along with floppy, 2 serial ports and a parallel port. It has limited
  expansion capability (one PCI slot) due to its compact size. (There is
  some restriction on when you can use the PCI slot, can't remember
  what) (Note that 21066A-based and Pentium-based Multia's are also

  DEC PC 150 AXP (aka Jensen): This is a very old Digital system - one
  of the first-generation Alpha systems. It is only mentioned here
  because a number of these systems seem to be available on the second-
  hand market. The Jensen is a floor-standing tower system which used a
  150MHz 21064 (later versions used faster CPUs but I'm not sure what
  speeds). It used programmable logic to interface a 486 EISA I/O bridge
  to the CPU.

  Other 21064(A) systems: There are 3 or 4 motherboard designs around
  (I'm not including Digital systems here) and all the ones I know of
  are derived from the EB64+ design. These include:

  ·  EB64+ (some vendors package the board and sell it unmodified); AT

  ·  Aspen Systems motherboard: EB64+ derivative; baby-AT form-factor.

  ·  Aspen Systems server board: many PCI slots (includes PCI bridge).

  ·  AlphaPC64 (aka Cabriolet), baby AT form-factor.

  Other 21164(A) systems: The only one I'm aware of that isn't simply an
  EB164 clone is a system made by DeskStation. That system is
  implemented using a memory and I/O controller proprietary to Desk
  Station. I don't know what their attitude towards Linux is.

  8.  Bytes and all that stuff

  When the Alpha architecture was introduced, it was unique amongst RISC
  architectures for eschewing 8-bit and 16-bit loads and stores. It
  supported 32-bit and 64-bit loads and stores (longword and quadword,
  in Digital's nomenclature). The co-architects (Dick Sites, Rich Witek)
  justified this decision by citing the advantages:

  1. Byte support in the cache and memory sub-system tends to slow down
     accesses for 32-bit and 64-bit quantities.

  2. Byte support makes it hard to build high-speed error-correction
     circuitry into the cache/memory sub-system.

  Alpha compensates by providing powerful instructions for manipulating
  bytes and byte groups within 64-bit registers. Standard benchmarks for
  string operations (e.g., some of the Byte benchmarks) show that Alpha
  performs very well on byte manipulation.

  The absence of byte loads and stores impacts some software semaphores
  and impacts the design of I/O sub-systems. Digital's solution to the
  I/O problem is to use some low-order address lines to specify the data
  size during I/O transfers, and to decode these as byte enables. This
  so-called Sparse Addressing wastes address space and has the
  consequence that I/O space is non-contiguous (more on the intricacies
  of Sparse Addressing when I get around to writing it). Note that I/O
  space, in this context, refers to all system resources present on the
  PCI and therefore includes both PCI memory space and PCI I/O space.

  With the 21164A introduction, the Alpha archtecture was ECO'd to
  include byte addressing. Executing these new instructions on an
  earlier CPU will cause an OPCDEC PALcode exception, so that the
  PALcode will handle the access. This will have a performance impact.
  The ramifications of this are that use of these new instructions (IMO)
  should be restricted to device drivers rather than applications code.

  These new byte load and stores mean that future support chipsets will
  be able to support contiguous I/O space.

  9.  PALcode and all that stuff

  This is a placeholder for a section explaining PALcode. I will write
  it if there is sufficient interest.

  10.  Porting

  The ability of any Alpha-based machine to run Linux is really only
  limited by your ability to get information on the gory details of its
  innards. Since there are Linux ports for the E66, EB64+ and EB164
  boards, all systems based on the 21066, 21064/APECS or 21164/ALCOR
  should run Linux with little or no modification. The major thing that
  is different between any of these motherboards is the way that they
  route interrupts. There are three sources of interrupts:

  ·  on-board devices

  ·  PCI devices

  ·  ISA devices

  All the systems use an Intel System I/O bridge (SIO) to act as a
  bridge between PCI and ISA (the main I/O bus is PCI, the ISA bus is a
  secondary bus used to support slow-speed and 'legacy' I/O devices).
  The SIO contains the traditional pair of daisy-chained 8259s.

  Some systems (e.g., the Noname) route all of their interrupts through
  the SIO and thence to the CPU. Some systems have a separate interrupt
  controller and route all PCI interrupts plus the SIO interrupt (8259
  output) through that, and all ISA interrupts through the SIO.

  Other differences between the systems include:

  ·  how many slots they have

  ·  what on-board PCI devices they have

  ·  whether they have Flash or EPROM

  11.  More Information

  All of the DS evaluation boards and motherboard designs are license-
  free and the whole documentation kit for a design costs about 0. That
  includes all the schematics, programmable parts sources, data sheets
  for CPU and support chipset. The doc kits are available from Digital
  Semiconductor distributors. I'm not suggesting that many people will
  want to rush out and buy this, but I do want to point out that the
  information is available.

  Hope that was helpful. Comments/updates/suggestions for expansion to
  Neal Crook <mailto:[email protected]>.

  12.  References

  Bill Hamburgen, Jeff Mogul, Brian Reid, Alan Eustace, Richard Swan,
  Mary Jo Doherty, and Joel Bartlett.  Characterization of Organic
  Illumination Systems.  DEC WRL, Technical Note 13, April 1989.