POWER2/P2SC Processor 

Source: Understanding IBM RS/6000 Performance and Sizing (SG24-4810-00)

The early POWER2 processor complex consists of eight semi-custom chips partitioned in the same way as the POWER: the instruction cache unit (ICU) which also processes branches, the FXU, FPU, four data cache units (DCUs), and a storage control unit (SCU). The early models of the POWER2 were announced in September 1993 and are the basis of the POWER2 Super Chip (P2SC) version. The POWER2 architecture expands the cache capacity, doubles the number of
functional units, doubles the bandwidth of most buses, and quadruples the cache to floating-point bandwidth.

The POWER2 architecture offers numerous performance gains:
· New instructions enhance floating-point bandwidth, square root, and data conversion.
· User-level instruction set is a superset of the original POWER set. Existing POWER binaries can
      run unmodified on the faster systems.
· Existing POWER binaries gain the performance benefits of the larger data cache, higher clock 
      rates, improved bandwidth, and additional functional units.
· Recompiling can maintain portability while providing gains from added compiler transformations
   . When portability is not required, a recompiled application can also exploit new instructions.

   POWER implements one floating-point, one integer, and one branch unit. It can execute four instructions per cycle (five operations).The POWER2 multichip implements dual integer, floating-point, and branch units that can execute six instructions per cycle (eight operations).

   The architecture features high-performance, floating-point storage access instructions, load quad word (128 bits) and store quad word, which support all of the addressing forms of double-precision storage references. The load quad word moves two adjacent double-precision storage operands into two adjacent floating-point registers.

   The instruction cache size is 32 KB, two-way set associative, and the data cache is either 128 KB or 256 KB, four-way set associative single line, depending on the RS/6000 model. The following are some other differences in the POWER2 multichip compared to the POWER architecture:
· Eleven new instructions (giving a total of 195 instructions)
· Different page frame table format (hash instead of inverted page frame tables)
· Page aliasing
· Support for floating-point imprecise mode
· Different interrupt mechanism
· Faster clock speed
· Doubles B/W from memory to scache (eight-word bus, 128 bytes)
· Double B/W from scache to sinteger (two single words,16 bit) and FPUs (two quad words, 32 bit)
· Different alignment requirements for the quad word data
· A special data address break-point register
· A performance monitoring facility

Figure 13. POWER2 Eight-Word System

ICU Instruction Cache Unit
FXU Fixed Point Unit
FPU Floating Point Unit
DCU Data Cache Unit
SCU Sorage Control Unit

POWER2 Super Chip
   The POWER2 Super Chip (P2SC) is a compression of the POWER2 eight-chip architecture into a single chip with increased processor speed and performance. It retains the design of its predecessor, the POWER2.
  The initial models have clock speeds of 120 MHz and 135 MHz. High-density CMOS-6S technology allows each to incorporate 15 million transistors. The most significant change is a halving of the size of the data cache and the data TLB, which now are 128 KB and 256 KB, respectively. These changes were required to fit the eight-chip processor onto a single chip.
   The P2SC delivers the processing and dual floating-point power needed for large, numeric-intensive tasks as well as the integer and transaction performance for commercial applications. The P2SC contains on-chip 32 KB instruction and 128 KB data cache and is full binary compatible with the POWER2 architecture.

Figure 14. POWER2 Super Chip Module

ICU Instruction Cache Unit
FXU Fixed Point Unit
FPU Floating Point Unit
DCU Data Cache Unit
SCU Sorage Control Unit

   The P2SC can issue and execute six instructions per cycle, two of which can be floating-point multiply-add (FMA) instructions. It supports register renaming and out-of-order execution, although only for floating-point instructions. With dual branch units, the P2SC can also execute two branches per cycle, although only one can be taken, while the other is put on hold. The data cache is triple-clocked to handle two CPU accesses (load or store) plus a cache refill (write to cache from main memory). Thus, this part of the chip operates at 500 MHz.
   The P2SC's great performance is directly associated with the inclusion of up to 2 GB of DRAM across a 256-bit-wide interface. This interface plugs the processor directly into a byte stream bus of up to 2.2 GB per second. The chip also integrates a 64-bit I/O bus for peripheral interconnection.

>Can anyone tell me when where the MicroChannel Bus bottlenecks on the RS/600 line?  

Milton Miller
For the Combo boxes (520-560 except 550L, 320-340 and 350, 920, 930), the Micro Channel and IOCC max out about 17MB/s (writes) and 19MB/s (reads).   The newer XIO IOCC mahcines (Power w/32k I-cache and POWER2 machines) max out at 77MB/s (read or write per bus) sustained.  These are based on long (4k) block transfers by Bus Master devices, smaller blocks, slower adapters, and PIO to initate transfers will lower achieved throughput.

Also, in Combo boxes PIO (loads and stores vs dma) had last priority, in XIO they are hidden under grant (highest priority).

> (which models use the bus for memory access?)?  
All the RS/6000 series (including the PowerPC boxes) have a bridge from the IO bus to a system bus which gives access to memory for DMA (none use the Micro Channel for system memory).

In systems with dual Micro Channel busses, both buses can operate at full bandwidth at the same time

 Faster XIO instead of I/O Channel Controller IOCC (Micro Channel controller)

 Frank Kraemer
>IBM ALSO has a link which IS proprietary called Serial Optical. This link hangs DIRECTLY OFF the CPU ( as opposed to all of the other network types which run via the Micro-channel ). It is this serial optical link that IBM itself uses to create parallel computing arrangements of RS/6000s. Serial Optical is only available on certain models of RS/6000. I think it is: 930,970,980,560, and 580 - maybe 540 & 550 also have it - I can't remember.

Serial Optical Link (called SOCC) IS proprietary - YES, but it's driven from the IOCC (I/O Controller Chip). The IOCC is attached to the CPU/RAM channel and drives the MicroChannel bus and the SOCC ports. It offers about twice the FDDI speed and is very very cheap, yes I know IBM and cheap are two different words ;-), but it's true. One SOCC adapter has two ports. You can have 1 SOCC adapter in all 5xx systems (2 ports) - all 9xx systems support 1 SOCC adapters (4 ports). The SOCC adapter does not need a MicroChannel slot.

   +--------+                          +--------+
   ! CPU    ! =========================! RAM    !
   +--------+         !!               +--------+
   40 MB/s MC or      !!
   80 MB/s MC      +--------+  SOCC   !--- SOCC Port 1
          +--------! IOCC   ! =======+                   (220 Mbit/s)
          !        +--------+         !--- SOCC Port 2
          +------ Micro Channel ......
          +------ Micro Channel ......
          +------ Micro Channel Adapter for FDDI (100 Mbit/s)
          +------ .....

The main disadvantage of SOCC is the point to point connection. Without any special switch you can connect only 3 boxes (5xx) together. This is a good solution for some kind of compute cluster,
but if you think of clustering more boxes the NSC switch is big money.

So the best solution is the combination of SOCC and FDDI:

            !            !             !
            X            X             X
           / \          / \           / \
          / 1 \        / 2 \         / 3 \      SOCC clusters  1,2,3
         X-----X      X-----X       X-----X
         !     !      !     !       !     !

before I stop, SOCC runs TCP/IP....

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