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NVAX

The NVAX is a CMOS microprocessor developed and produced by Digital Equipment Corporation (DEC) that implemented the VAX instruction set architecture (ISA). A variant of the NVAX, the NVAX+, differed in the bus interface and external cache supported, but was otherwise identical in regards to microarchitecture. The NVAX+ was designed to have the same bus as the DECchip 21064, allowing drop-in replacement.

Description
NVAX contained 1.3 million transistors on a die measuring 16.2 by 14.6 mm in size (236.52 mm²). The die was fabricated in Digital's fourth-generation CMOS process, CMOS-4, a 0.75 μm process with three layers of aluminium interconnect. The NVAX is packaged in a 339-pin pin grid array. The NVAX was offered at a variety of clock speeds, 83.3 MHz (12 ns), 71 MHz (14 ns) and 62.5 MHz (16 ns), while the NVAX+ is clocked at a frequency of 90.9 MHz (11 ns). The NVAX offered about 25 VAX Unit of Performance (VUPs). The NVAX+, introduced at the same time, was identical in terms of the processor design but used a different bus, cache system and its external connection was a 431-pin array. These were identical to those on the Alpha, allowing an NVAX+ machine to be upgraded to an Alpha simply by changing the CPU. These changes also allowed it to operate with slightly higher performance, and the NVAX+ ran at roughly 35 VUPs. This was only slightly less than the VAX 9000 mainframe's roughly 40 VUPs. In 1994, the NVAX++ (also known as NV5) was introduced in VAX 7000 Model 7x0 and VAX 10000 Model 7x0 systems. It operated at 133 MHz (7.5 ns) and was fabricated in Digital's fifth-generation CMOS process, CMOS-5, a 0.50 μm process. It improved performance to 50 VUPs. In 1996, a 170.9 MHz NV5 was introduced, used in the VAX 7000/10000 Model 8x0. == Microarchitecture ==
Microarchitecture
The NVAX is partitioned into five semi-autonomous units, the I-box, E-box, F-box, M-box and C-box. The NVAX is macropipelined. Multiple VAX macroinstructions are processed in parallel by autonomous units, which have their own micropipelines. The I-box fetches and decodes VAX instructions. It also contains the 2 KB direct-mapped virtual instruction cache (VIC) and the 512-entry by 4-bit branch history table. The I-box aimed to fetch eight bytes of instruction data from the VIC during every cycle. The E-box executes most non-floating-point instructions. It is controlled by microcode from a 1,600-word control store with the capability to patch 20 words. The F-box executes floating-point instructions as well as 32-bit integer multiply instructions. It has a four-stage floating-point and integer multiply pipeline and a non-pipelined floating-point divider. == References ==
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