CPU/Memory Interconnect

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VAX-11/750 system block diagram, showing how the CMI ties together the major /750 functional units

The CPU/Memory Interconnect (often given as CMI; the original name was Comet Memory Interconnect - 'Comet' was the code-name of the /750)) is the main bus which connects the main sub-systems (such as the CPU and main memory, as well as I/O adapters) in the VAX-11/750. A sub-system connected to the CMI is referred to as a 'nexus'.

The primary functions of the CMI are to do priority arbitration for use of the bus, and carry data between the sub-systems, as well as carrying interrupts. It is also used for a variety of lesser functions, such as diagnostics, loading writeable microcode, etc.

The CMI is carried over the main /750 backplane, into which the various sub-systems plug. It is synchronous and interlocked (like the SBI of the VAX-11/780); at the analog level, it is tri-state.


The CMI consists of 45 bi-directional lines: priority arbitration, address, data, control, and status. A uni-directional clock signal line is used to synchronize the operations on the CMI. At any given point in time, one nexus is the 'master', and another (selected by it) is the 'slave' in each CMI interaction.

Arbitration is fully distributed; each nexus has a priority arbitration line which it can assert, and on each cycle, each requesting nexus looks to see if it is the highest-priority nexus for that cycle.

CMI units

The CMI units present in every /750 are:

Optional units include:

CMI Option Slot Configuration Rules

Allowable Configurations of CMI Slots 7, 8, & 9:

Slot 7 Slot 8 Slot 9
RH750 RH750 RH750
DW750 RH750 RH750
DR750 RH750 RH750
DW750 DR750 RH750
DW750 CI750 RH750
CI750 RH750 RH750


  1. Combination of DR750 and CI750 is not allowed
  2. Only one DR750 per system is allowed
  3. Only one DW750 per system is allowed
  4. Only one CI750 per system is allowed

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