CVAX (1987)

CVAX (there was never a code name) was DEC's first CMOS VAX microprocessor, and the first microprocessor in the industry with built-in instruction and data cache. Dan Casaletto was the project manager; Paul Rubinfeld was the project lead for the CPU, Gil Wolrich for the FPA; Bob Supnik did the initial architecture and most of the microcode.

CVAX was intended to advance the MicroVAX family by providing 2.5X the performance of MicroVAX at lower power. It incorporated a number of microarchitectural advances, including an autonomous, state-machine driven instruction parser; on-chip first level cache; explicit support for an off-chip, second level cache; and a "half-folded" (register writes overlapped with register reads) pipeline. The companion floating point unit, CFPA, likewise advanced on the MicroVAX FPU by incorporating wider shifters and a better multiplier.

Starting with CVAX, DEC's microprocessors used a small custom clock driver. This driver alleviated the need to handle TTL level clocks on chip; instead, the chips could be designed around full-swing (0V to 5V) CMOS clocks. CVAX also included a full family of peripheral support chips, including the CVAX System Support Chip (CSSC), the CVAX Memory Controller (CMCTL), and the CVAX Qbus Interface Chip (CQBIC). The CVAX pin bus became a defacto standard in the chip group and was used as the interface for the Second Generation Ethernet Chip (SGEC) and the Single Host Adapter Chip (SHAC).

Both CVAX and CFPA were implemented in DEC's 2u double-metal CMOS process (CMOS1) and ran, depending on bin points, at 10Mhz-14Mhz.

Name Number Size Transistors Comments
CVAX CPU DC341 384x378 180,000 sites The CVAX CPU is a second generation VLSI VAX microprocessor. Compared to its predecessor, it offers 2.5X to 3.5X more performance at lower power. Key features include:
  • High performance
    • Macro-instruction prefetch
    • Micro-instruction pipeline
    • 1KB on chip instruction/data cache (configurable as instruction only)
    • 28 entry fully associative TLB
  • Subset VAX architecture
    • Sixteen 32b general purpose registers
    • 181 instructions
    • 21 addressing modes
    • 6 data types
  • VAX memory management
    • 4GB virtual address space
    • 1GB physical address space
    • Demand paging
    • Memory protection
    • Four privilege modes
  • Vectored multi-level interrupts (15 software, 7 hardware)
  • Improved external interface with parity protection
  • Low power
  • Single +5V supply

Power: 2.5W.

CFPA DC513 339x272 34,000 The CFPA is a high performance, single chip floating point processor for the CVAX CPU. Compared to its predecessor, it offers 2.5X to 3.5X more performance at lower power. Its key features are:
  • High performance
    • 16b per cycle multiply retirement
    • Wider range alignment/normalization hardware
    • Optimized algorithms for add/subtract, multiply, divide
  • f_, d_, and g_floating point format support
  • Full VAX floating point instruction set, including ACBf, EMODf, POLYf
  • Optimized external interface
    • Dedicated CPU to FPA control bus
    • Overlapped control and data transfers
    • Parallel reception of memory operands
  • Low power
  • Single +5V supply

Power: 2.5W.

CVAX Clock DC509 102x104 350 The CVAX clock chip provides timing control for the CVAX CPU, the CFPA, related support chips, and module level logic. Its features include:
  • Precision CMOS-level clock outputs for the CVAX CPU, CFPA, and up to two support chips
  • Precision MOS-level clock outputs for four additional CMOS or TTL loads
  • Microcycle synchronization signal
  • RESET, ERRor, and ReaDY synchronization logic
  • Low power
  • Single +5V supply

Power: 1W.

CVAX shipped at the end of 1987, in the MicroVAX 3600 systems series, and early in 1988 in the new VAX 6200 mid-range multiprocessor series. CVAX proved even more successful than MicroVAX, with $2B in systems shipped in the first twelve months. In 1988, the workstation group shipped the VAXstation 3100 and a corresponding small server, which were also successful.

The CVAX chip set was optically shrunk to DEC's 1.5u double-metal CMOS process (CMOS2). The shrink was done by the original design team. The CVAX-60 (as it was known) taped out in August, 1987, before the original chip had been released for volume production. The CFPA-60 taped out in November, 1987. CMCTL was also shrunk, to increase its operating frequency; the other CVAX supports chips did not have to be changed. Operating frequency was 16.67Mhz; power was unchanged.

Name Number Size Transistors Comments
CVAX-60 CPU DC580 308x303 180,000 Optical shrink of the CVAX CPU chip - no functional changes
CFPA-60 DC581 302x253 60,000 Optical shrink of the CFPA chip - no functional changes

CVAX-60 and CFPA-60 shipped as upgrades to all CVAX-based low end systems, starting in late 1988. They were also shipped as an upgrade to the VAX 6000 when yield issues delayed the introduction of Rigel.

In 1990, CVAX, CFPA, the clock chip, and an 8KB second level cache were combined onto a single die to create System-On-A-Chip (SOC) in DEC's 1.0u triple-metal process (CMOS3). SOC ran at 25Mhz. SOC shipped in low-end systems and workstations.

Personal Narrative

CVAX began as a microcoder's meditation on MicroVAX. In December, 1982, after six months of struggling with the microarchitecture inherited from V-11, I started a design notebook on ways to improve the efficiency and density of the microcode. I also worked on the basic pipeline, coming up with the idea of a "half-folded" pipeline as a waystation between VAX-11/780-style machines and the forthcoming VAX 8800.

As I worked on the microcode, I had realized that the key issue for the microcode was not to make it smaller -- future chips would have more room for more transistors -- but to make it simpler and faster. The microarchitecture was also influenced by the VAX 8800 and the use of state machines to drive instruction decoding. By the end of 1983 I had a reasonably complete sketch of the microword and microcode as a basis for comparison. V-11 and MicroVAX needed more than 500 PLA terms to decode its the nine different formats in its 39b microword. CVAX, in contrast, needed only 150 terms to decode the five different formats in its 41b microword. CVAX was also faster on critical sequences such as CALLx, field instructions, and instructions with more than two specifiers.

CVAX was launched as an Advanced Development project early in 1984 and was gradually staffed with resources from J-11 as that project wound down. In October 1984, I became manager of the Microprocessor Group and took CVAX with me. Serious implementation work began in the fall of 1984. CVAX taped out in July, 1986, and CFPA in September, 1986.

While CVAX was in development, the MicroVAX architectural "subset" continued to evolve. To get VAX COBOL to drop its dependence on the decimal instruction set (and thus allow MicroVAXen to work efficiently in commercial environments), I agreed to add six more instructions to the subset (CMPC3, CMPC5, LOCC, SKPC, SCANC, SPANC). This required a late rework of the CVAX microcode and introduced a latent bug that delayed shipment by two months.

CVAX included a large number of on-die drawings and logos (MicroVAX had only the designers' initials). One drawing showed a poker hand consisting of a straight flush, with an Ace of Spades up the sleave, and the motto "$ The dream is always the same..." This was a reference to the popular movie "Risky Business", only in this case the dream was to boot VMS (get to the $ prompt) on a pass 1 part. (The Ace up the sleeve was Rigel, the next design.) Another section had a big star and bigger arrow, saying "This is the last one", namely, the last transistor to be laid out. Finally, the scribe lane contained the Cyrillic motto "VAX: When you care enough to steal the very best". In 1983, an Unnamed Intelligence Agency had given me the wording, saying that they got it off a purloined VAX-11/780 that was running a Soviet SS20 missile complex. Knowing that some CVAX's would end up in the USSR, the team wanted the Russians to know that we were thinking of them.

CVAX was presented at the 1987 International Solid State Circuits Conference.

Updated 24-Feb-2008 by Bob Supnik (simh AT trailing-edge DOT com - anti-spam encoded)