Extended Memory Interconnect
The Extended Memory Interconnect (usually given as the acronym: XMI) was a bus introduced with the VAX 6000 series of DEC computers to cope with increased requirements, e.g bandwidth.
XMI Overview
The XMI is the primary interconnect for the VAX 6200 system. The XMI supports multiple processors, multiple memory modules, and multiple I/O adapters.
Figure 2-1 shows a four-processor VAX 6200 system.
The XMI 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 bus is limited length, pended, and synchronous with centralized arbitration. Several transactions can be in progress at a given time, allowing highly efficient use of the bus bandwidth. Arbitration and data transfers can occur simultaneously.
The bus 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.
The XMI 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.
The manual EK-650EB-UP-002 VAX 6000 XMI Conversion Manual describes the upgrade procedure.
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
