MicroVAX 3800/3900

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MicroVAX 3800/3900
Announcement date: 10 April 1989
Codename: Mayfair III
OS support (VMS): V5.1
CPU Details
CPU name (VMS): KA655
CPU name (console): KA655
CPU module: KA655
Module: M7625
Number of processors: 1
SID: 0A000005
XSID: 01530301
CPU chip: CVAX
FPU chip: CFPA [1]
CPU technology: CMOS
CPU cycle time: 60ns [1]
CPU clock: 33MHz [1]
Instruction-buffer: 12 bytes [1]
Translation-buffer: 28 entries [1]
Cache: 1KB [1]
Backup cache: 64KB (120ns) [1]
Compatibility mode: No
Console processor: CPU
Console device: None
Firmware version: V5.3
Minimum memory: 16MB
Maximum memory: 64MB
Memory checking: 7-bit ECC/longword [1]
Q-bus: 1 @ 3.3MB/s
LAN support: optional
VUPs: 3.8 [2]
SPECmarks: 4.0

The MicroVAX 3800 and MicroVAX 3900 are very similar, the only distinction being the enclosure and accompanying disk drives: the 3800 was available in the BA213, and the 3900 in the H9644 cabinet. The MicroVAX 3900 includes an RA90.

They both used the KA655 CPU, so both are built around the QBUS, which was used only for I/O; main memory used the KA650 Main Memory System, using a special bus carried on over the back connectors as well as the CD interconnect of the QBUS backplane. (The address space of the QBUS was limited to 4MB, and the data section is only 16 bits wide.)

There were special VAXserver versions, for usage in Local Area VAXclusters, named VAXserver 3800 and VAXserver 3900. The difference between these and the MicroVAXen is in the system ROM on the CPU board, which prevents multi-user access to these computers.

The MicroVAX 3900 is the original model of VAX emulated by SIMH. (It now also emulates a VAX-11/780.) It can run 4.3 BSD, VMS 5.1-1 to 7.3, and NetBSD, and OpenVMS 7.3 under Debian on a Tinkerboard.


[1] KA655 CPU Module Technical Manual. EK-KA655-TM-001
[3] VAX Systems Performance Summary Featuring VAX 6000 Model 500. EC-N0377-46

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