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Announcement date: October 1977
Codename: Star
OS support (VMS): VMS V1.0
CPU Details
CPU name (VMS): KA780
Number of processors: 1
CPU technology: Bipolar Schottky
CPU cycle time: 200ns [1]
Instruction-buffer: 8 bytes [1]
Translation-buffer: 128 entries [1]
Control store: 4K 99-bit words [1]
Gate delay: 3ns
User Writable Control Store: 2K 99-bit words [1]
Cache: 8KB [1]
Compatibility mode: Yes [1]
Console processor: LSI-11 [1]
Console device: RX01 [1]
Minimum memory: 2MB
Maximum memory: 64MB
Physical address lines: 30
Memory checking: 8-bit ECC/longword [1]
Max I/O throughput: 5.0MB/s
MASSBUS: 4 @ 2.0MB/s
UNIBUS: 4 @ 1.5MB/s
LAN support: optional
VUPs: 1.0 [2]
A 'typical' VAX-11/780 marketing image

The VAX-11/780 was the first member to ship of the VAX family of larger computers. The VAX series was conceived as the successor to the successful PDP-11 series of minicomputers; in order to make the VAX seem more PDP-11 friendly, they retained the -11 moniker for the first VAXen, and provided the ability to execute PDP-11 object code - hence the -11/780 designation.

One of the main design goals was to increase the address space available, in fact the 'VAX' acronym originally stood for Virtual Address eXtension).[3] The first VAX-11/780 systems shipped with one quarter of a megabyte of main memory, built from 4Kb chips.[3]

The 11/780 didn't have just firmware; instead, like the KL10, it loaded microcode from secondary storage (a floppy disk), under the control of a front end (a PDP-11/03).

It was announced on October 25th, 1977 at DEC's Annual Meeting of Shareholders.[4] The VAX-11/780 was given the codename "Star" and its operating system, VAX/VMS, was codenamed "Starlet".[4] VAX/VMS Version V1.0 shipped in 1978, along with the first revenue-ship 11/780s.

There was also the VAX-11/785, which was an upgraded version.


The CPU was the KA780 CPU. It could take an optional floating point accelerator, the FP780 Floating-Point Accelerator; the floating point instructions were performed by the microcode when it was not installed.

The CPU and the other major subsystems in the VAX-11/780:

were connected via a new bus, the Synchronous Backplane Interconnect (SBI). A multi-port memory, the MA780 Multiport Memory Option, allowed creation of multi-processor systems.

Dual-CPU experiment

"In 1981, [George H. Goble] wired together the backplanes of two DEC VAX-11/780's and made the first multi-CPU Unix computer, preceding DEC's dual processor VAX-11/782. The operating system was based on the 4.2 BSD kernel, and the modifications thus made eventually made it into the 4.3 BSD Unix release. At the beginning of the 4.3 BSD user manuals, Bill Joy wrote a special note of thanks to GHG for being courageous enough to put the multi-CPU kernel into a production environment before anyone else did. (However, the frequent crashes for a while inspired the writing of many humorous text files by the Purdue University Electrical Engineering student body, such as 'The VAX had a Blowout', to be sung to the tune of London Bridge is Falling Down). The development of the Dual-CPU Unix system was the subject of Goble's Master's thesis."

DEC eventually produced a productized version, the VAX-11/782.

Operating Systems

An actual VAX-11/780 system; the CPU is in the double cabinet on the left

The 11/780 was to ship with VMS 1.0.


SIMH is the only known emulator for the 11/780. SIMH can address up to 128MB of RAM emulating an 11/780. Also the following peripherals are emulated:

  • CPU VAX-11/780 CPU
  • TLB translation buffer
  • SBI system bus controller
  • MCTL0,MTCL1 memory controllers, MS780C with 4MB memory each, or MS780E with 8MB-64MB each
  • UBA DW780 Unibus adapter
  • MBA0,MBA1 RH780 Massbus adapters
  • TODR time-of-day clock
  • TMR interval timer
  • TTI,TTO console terminal
  • RX console RX01 floppy disk
  • DZ DZ11 8-line terminal multiplexer (up to 4)
  • CR CR11 card reader
  • LPT LP11 line printer
  • RP RP04/05/06/07, RM02/03/05/80 Massbus disks, up to eight drives
  • HK RK611/RK06(7) cartridge disk controller with eight drives
  • RL RL11/RL01(2) cartridge disk controller with four drives
  • RQ UDA50 MSCP controller with four drives
  • RQB second UDA50 MSCP controller with four drives
  • RQC third UDA50 MSCP controller with four drives
  • RQD fourth UDA50 MSCP controller with four drives
  • RY RX211 floppy disk controller with two drives
  • TS TS11 magnetic tape controller with one drive
  • TQ TUK50 TMSCP magnetic tape controller with four drives
  • TU TM03 tape formatter with eight TE16/TU45/TU77 drives
  • XU DEUNA/DELUA Ethernet controller
  • XUB second DEUNA/DELUA Ethernet controller

More information about SIMH's VAX 11/780 emulation can be found here.

Related documents

VAX 11/780 Architecture Handbook Vol. 1 1977-78. EB-07466-20
VAX 11/780 Hardware Handbook 1979-80. EB-17835-18
VAX-11/780 Data Path Description. AA-H307B-TE
VAX-11/780 Diagnostic System Technical Description. EK-DS780-TD-001
VAX-11/780 Diagnostic System User's Guide. EK-DS780-UG.002
KC780 Family Remote Diagnosis Options Technical Manual. KC780, KC782 and KC785. EK-KC780-TM-007
VAX/11/780 Microprogramming Tools User's Guide. AA-H306B-TE
VAX-11/780 Software Installation Guide. AA-M545B-TE
CI750 CI780 Computer Interconnects Maintenance Advisory. EP-CIVAX-RM-001
Guide to VAX-11/780 System Troubleshooting. EY-2217E-SG-0001


[1] VAX Hardware Handbook Volume 1 - 1986.
[2] Systems & Options Catalog. European Edition. Spring 1990.
[3] VAX Architecture Reference Manual. Timothy E. Leonard. 1987. EY-3459E-DP. ISBN 0-932376-86-X.
[4] Nothing Stops It! VAX Open VMS At 20.

See also

External links