Difference between revisions of "CPU/Memory Interconnect"

From Computer History Wiki
Jump to: navigation, search
m (typo)
(iDsCMI units: add board IDs)
Line 17: Line 17:
The CMI units present in every /750 are:
The CMI units present in every /750 are:
* [[KA750 CPU]]
* [[KA750 CPU]] - L0002-L0003-L0004-L0005/L0008
* [[MS750 Memory System]]
* [[MS750 Memory System]] - L0011/L0016/L0022
Optional units include:
Optional units include:
* [[RH750 Massbus Adapter]]
* [[RH750 Massbus Adapter]] - L0007
* [[DW750 Second Unibus Interface]]
* [[DW750 Second Unibus Interface]] - L0010
* [[CI750 Computer Interconnect Interface]]
* [[CI750 Computer Interconnect Interface]] - L0009
* [[DR750 Parallel Interface]]
* [[DR750 Parallel Interface]] - L0014
== CMI Option Slot Configuration Rules ==
== CMI Option Slot Configuration Rules ==

Latest revision as of 22:01, 3 June 2022

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) 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 processor. 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

External links