Advanced Micro Devices Inc and Cyrix Corp will describe their new iAPX-86-compatible Pentium-killers at the Microprocessor Forum this week, making it clear that their architectures are beginning to diverge wildly from the Intel Corp original. Advanced Micro’s first K86 chip is designed to be pin-compatible with Intel’s P54C Pentium processor and to offer a 33% performance advantage – but writers with 80486 applications will not need to recompile to mobilise the full benefits of the chip. Internally, it is described as a RISC. It is said to use 4m transistors and to have three integer units and a 32Kb cache where the Pentium has 3.4m transisitors and two integer units with two 8Kb caches. It is also said to have 32 registers where the Pentium has eight and to issue four instructions per clock cycle, where the Pentium issues a maximum of two. According to Infoworld, the K5, initially clocked at 100MHz, is a true superscalar RISC microprocessor with six parallel execution units all told and a four-instruction-issue pipeline, designed to eliminate bottlenecks inherent in Intel’s iAPX-86 architecture by converting the instruction stream into fast RISC operations, with instructions executed with a high degree of parallelism in the RISC core, after which they are sent to a reorder buffer that converts them back into iAPX-86-ordered instructions. Features include out-of-order execution, branch prediction and speculative execution to keep the pipeline filled, all of which will give Intel plenty of scope to raise fears of incompatibility. The Cyrix Corp M1 is less of a departure: it is said to have two integer computation units and one floating-point unit, but again has 32 registers, and like the Pentium issues a maximum of two instructions per cycle. It uses 3.5m transistors, and has a single 16Kb cache and is also clocked at 100MHz. It will not work with current Pentium motherboards, but according to PC Week, Opti Inc, Santa Clara is working on a building block chip set that will enable board makers to design one board that will take a Pentium, a K5 or M1 processor interchangeably.
