Extended Memory Interconnect
The Extended Memory Interconnect (usually given as the acronym: XMI) was a bus introduced with the VAX 6000 series, and also used in later DEC VAX computers, to cope with increased requirements, e.g bandwidth.
Overview
The XMI bus supports multiple processors, multiple memory modules, and multiple I/O adapters. It consists of:
- the electrical environment of the XMI bus,
- the protocol observed by a node on the bus,
- the backplane, and
- the logic used to implement the protocol.
The XMI is limited length, synchronous, and has centralized arbitration (individual bus grant lines run from the arbitrator to each slot in the backplane). Arbitration and data transfers use logically separate divisions of the bus, so they can occur in parallel.
It is also 'pended', which means that an operation can be left in an incomplete state, to allow other entities to use the bus. Several transactions can thus be in progress at any time, rather than forcing later operations to wait for the completion of an earlier operation before they can begin; this allows highly efficient use of the bus bandwidth.
The XMI is the primary interconnect for the VAX 6200 system.
The XMI supports:
- Quadword-, octaword-, and hexword-length reads to memory
- Quadword- and octaword-length memory writes
- Longword-length read and write operations to I/O space
The longword operations implement byte and word modes required by certain I/O devices.
The XMI has a 64 ns bus cycle. It has a bandwidth of 100 Mbytes per second; however, the usable bandwidth depends on transaction length:
| Usable XMI Bandwidth | |
| Operation | Bandwidth (Mbytes/second) |
| Longword (4 bytes) Read | 31.25 |
| Quadword (8 bytes) Read | 62.50 |
| Octaword (16 bytes) Read | 83.30 |
| Hexword (32 bytes) Read | 100.00 |
| Longword Write | 31.25 |
| Quadword Write | 62.50 |
| Octaword Write | 83.30 |
XMI Versions
The XMI bus started with the "+5V XMI-1" version which was developed to the "+3.3V XMI-2" version later on.
There was an upgrade kit option H9657-CU that takes a VAX 6000 Model 200, 300, or 400 system with an XMI-1 power system and upgrades it to a VAX 6000 Model 500 with an XMI-2 power system. The upgrade adds +3.3 volt power, an XMI-2 backplane preassembled with bus bars and cables, and an H7206-B power and logic unit.
External links
- VAX 6200 Technical User's Guide (EK-620AA-TM-001) - the XMI is covered in Chapter 2, pp. 37-91 of the PDF
- EK-650EB-UP-002 VAX 6000 XMI Conversion Manual
