Developers that have yet to see Intel Corp’s P5 microprocessor are in for a shock, because judging by intelligence gleaned by Computer Reseller News in Asia, the new part bears little resemblance to the commodity microprocessors that power tens of millions of personal computers. Intel has already said that the P5 includes two RISC-like pipelined integer execution units and an advanced floating point unit, plus an 80386 execution unit. But the part also has a 64-bit data bus and 36-bit address bus. The bus is clocked at 66MHz and is heavily pipelined – it can continue operating while up to two cycles remain uncompleted. The CPU is said to have some support for out-of-order execution of instructions, which requires very tricky circuitry to reassemble the stream in the right order and avoid dependency conflicts. The part has an enlarged on-chip primary write-back cache rather than the write-through cache of the 80486, which means that the CPU does not have to wait for the cache controller to copy stored data back to main memory. Up to three external caches are expected to be supported. The write-back cache may also be added on a new version of the 80486. But the feature that really marks the P5 out from its predecessors is the integrated floating point processor that is claimed to offer three to five times the performance of the one in the 80486, giving the P5 its reputation as a RISC-killer. The part is thought overall to outperform the 50MHz 80486 2.5 times on optimised code. Intel is expected to come out with its own building block chip sets that will work with both the P5 and the 50MHz 80486: one will support the EISA bus for servers, the other, for workstations, will use Intel’s new PCI bus which has a 32-bit multiplexed burst-mode architecture to provide high bandwidth for graphics, networking and compression chips. Many of the Asian designers the US trade weekly talked to were worried about the 66MHz clock frequency, because at that speed, stray radiations become a serious problem and it is unlikely that garage operations are going to be able to knock up boards using the chip that will work. Apricot Computers Ltd, which always likes to be first with Intel’s latest chip, will reportedly announce a P5-based machine in September, but with the enormous leap in complexity in the microprocessor, the other major worry has to be that it will take early users of the chip at least 18 months to find all the bugs in it and for Intel to correct them.
