VAX-11

VAX-11 registers

31 ... 23 ... 15 ... 07 06 05 04 03 02 01 00 (bit position) General registers R0 Register 0 R1 Register 1 R2 Register 2 R3 Register 3 R4 Register 4 R5 Register 5 R6 Register 6 R7 Register 7 R8 Register 8 R9 Register 9 R10 Register 10 R11 Register 11 Index registers R12 / AP Register 12 / Argument Pointer R13 / FP Register 13 / Frame Pointer R14 / SP Register 14 / Stack Pointer Program counter R15 / PC Register 15 / Program Counter Status flags   I V N Z C Processor Status Word     Floating Point Status Register

The VAX-11 was a family of minicomputers developed and manufactured by Digital Equipment Corporation (DEC) using processors implementing the VAX instruction set architecture (ISA). The VAX-11/780 was the first VAX computer.

The VAX-11/780, code-named "Star", was introduced on 25 October 1977 at DEC's Annual Meeting of Shareholders. It was the first computer to implement the VAX architecture. The VAX-11/780 central processing unit (CPU) was built from transistor-transistor logic (TTL) devices and had a 200 ns cycle time (5 MHz) and a 2 kB cache. Memory and I/O were accessed via the Synchronous Backplane Interconnect (SBI).

The VAX-11/780 supported 128 kB to 8 MB of memory (2 MB initially) through one or two memory controllers. Each memory controller supported 128 kB to 4 MB of memory. The memory was constructed from 4 or 16 kbit metal oxide semiconductor (MOS) RAM chips mounted on memory array cards. Each memory controller controlled up to 16 array cards. The memory was protected by error correcting code (ECC).

The VAX-11/780 used the Unibus and Massbus for I/O. Unibus was used for attaching peripherals and Massbus for disk and tape drives. Both buses were provided by adapters that interfaced the bus to the SBI. All systems came with one Unibus as standard, with up to four supported. Massbus was optional, with up to four supported. The VAX-11/780 also supported Computer Interconnect (CI), a proprietary network to attach disk drives and potentially share them with other VAX computers. Later, this feature was used to connect VAX computers in a VMScluster.

The VAX 11/782, code-named "Atlas", was a dual-processor VAX-11/780 introduced in 1982. Both processors shared the same MA780 multiport memory bus and the system operated asymmetrically with the primary CPU performing all I/O operations and process scheduling with the second, attached processor only used for additional computationally-intensive work. For multistream computation-intensive tasks the system delivered up to 1.8 times the performance of a VAX 11/780.

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