CVAX

The CVAX is a microprocessor chip set developed and fabricated by Digital Equipment Corporation (DEC) that implemented the VAX instruction set architecture (ISA). The chip set consisted of the CVAX 78034 CPU, CFPA floating-point accelerator, CVAX clock chip, and the associated support chips, the CVAX System Support Chip (CSSC), CVAX Memory Controller (CMCTL), and CVAX Q-Bus Interface Chip (CQBIC).

CVAX 78034

The CVAX 78034, also known as the MicroVAX 78034, is a second-generation single-chip VAX microprocessor. Systems featuring the CVAX chip set became available in late 1987. It is clocked at frequencies of 12.5 MHz (80 ns) in higher-end systems such as the VAX 6000 Model 200 and at 11.11 MHz (90 ns) in lower-end systems such as the MicroVAX 3500 and 3600.

The 78034 was the first VAX microprocessor to have internal caches, a 1 KB combined instruction and data stream cache. The cache is quite different as it is implemented with one-transistor DRAM, whereas the majority of microprocessors use SRAM for their internal caches. This was the first microprocessor to use one-transistor DRAM for cache. DEC chose to use DRAM for the cache to reduce the area of the cache array. The designers concluded that to implement this cache with four-transistor DRAM cells or six-transistor SRAM cells would have taken 2.4 to 3 times more area. The internal cache is located in a narrow strip along the left side of the die. An external 64 KB cache complements the 1 KB internal cache.