Multics and Related Honeywell 6000 Series Front Panels

Introduction

Multics was a tremendously important operating system, from which (via Unix, which was significantly influenced by it) most modern operating systems are descended.

The first machine Multics ran on was the GE-645, using an extension to the basic architecture of the GE 600 series CPUs; after GE's computer business was sold to Honeywell, it then it ran on a number of models from the follow-on Honeywell 6000 series.

Multics was born in the days when mainframes had lots of lights and switches, and the early machines that it ran on were no exception; early Multics hardware was festooned with lights.

A number of Web sites claim to have pictures of Multics panels, but... Alas, the 6000 series included models that ran another OS, GCOS, and the CPUs (and their front panels) are not the same; Multics CPUs included an extension called the 'Appending Unit' to support both virtual memory, and Multics' single-level store architecture. A lot of pictures of 'Multics' front panels actually show panels from GCOS machines.

To make things even more complicated, in the late 1970s Honeywell came up with an alternative extension to the 6000 Series architecture, the so-called 'New System Architecture'. It added a new security architecture (using domains, instead of the 'rings' of Multics), and also support for virtual memory (which the basic 6000 series architecture did not have). A new version of GCOS, GCOS 8, which came into use starting in 1980, made use of this.

So, this Web site attempts to show actual Multics panels; and also shows some of the panels from GCOS and NSA machines which are confused with panels from Multics machines.

Overall system architecture

A 6000 Series machine consisted of three classes of units: 'active' units, 'passive' units, and peripherals; only the first two will concern us here. (Each instance of each of those was originally housed in a separate free-standing cabinet.)

'Active' units included CPUs and IOMs (I/O controllers); the 'passive' units are SCUs ('System Control Units', each of which contained a multi-port memory controller, and the memory attached to that controller). A 6000 Series system contained at least one CPU, one SCU and one IOM - and potentially many more of any or all of them (7-CPU systems existed).

(Having multiple units of each type was good for system robustness - a system could continue to operate, albeit with degraded peformance, if one unit of a particular type had to be taken offline. It was also possible to split a large system into two smaller working systems - a capability which was used at several sites.)

Ports, Ports, Ports

There were no busses which connected multiple units together; all inter-unit links were point-point, and required a separate 'port' in the units on each end. In Multics, every active unit was required to be connected to all the passive units.

So, in a system with 5 CPUs, 3 IOMs, and 6 SCUs, there would have been 30 separate CPU<->SCU links, and 18 IOM<->SCU links - and each CPU would have had to have had 6 ports (one for each SCU), and each SCU would have required 8 ports (5 for the CPUs, and 3 for the IOMs).

This is important because early GCOS machines did not support the larger configurations: e.g. CPUs and IOMs could hook up to at most 4 SCUs, so GCOS CPUs had only 4 ports, and IOMs only 2. This means that early GCOS active unit configuration panels only show a limited number of ports - making them easy to tell apart from Multics panels.

The early Multics active units needed to be able to connect up to 8 SCUs, because early SCUs contained a limited amount (256KW) of memory. Some of the later NSA active units had more ports - e.g. the NSA IOM (below) had 8 ports.

When the later SCUs, which supported much more memory (4MW) arrived, machines did not need as many SCUs. Since ports were expensive, the later active units of both systems did not have as many ports - 4 ports, enough for 4 SCUs, was plenty.

Panels, Panels, Panels

The front panels of the early units were huge; they were mounted on the back of a large swing-out door, on the front of the unit's cabinet, and they were very large - roughly 4' high, and 20" wide.

Most of the early front panels were divided into several 'sub-panels', for:

configuration
display
maintenance
test

functionality. In some cases, these took up too much room to be held on a single maximum-sized physical front panel, and some sub-panels were housed in a separate, apparently half-width, swing-out physical panel, here called a 'half-panel'.

It appears that there were basically three different generations of Honeywell machines that ran Multics:

the 6180
the Series 60 Level 68 - a re-packaging of the 6180, in shorter cabinets
the DPS-8/M

The first two have the major front panels; in the third, all the maintenance/etc sub-panel functionality had been moved to a console terminal, driven by a micro-computer, the 'Diagnostics Processor Unit', which interfaced to the various major units of the 6000 series (CPUs, SCUs and IOMs); only the smaller configuration physical sub-panels were retained on each type of unit.

(Most images below can be clicked on, to show a larger image.)

Multics CPU Panels

These panels are from Multics-specific CPU models, found only on Multics machines.

In addition to the large maintenance panel (shown below), the early CPUs held another, smaller, half-panel, the configuration panel, but we don't yet have a good image of that entire half-panel (but see the image of the MIT 6180 system, below); we do have a (poor) image of the the Configuration sub-panel of that half-panel, though (below).

We don't have an image of the maintenance panel from the later Series 60 Level 68 Multics CPU, but we do have (below) the corresponding Series 60 Level 66 GCOS CPU panel, which is identical to the 6000 series GCOS CPU panel, except that it has changed from white to black, and the light bulbs have been replaced with LEDs; the Multics CPU panel for the Level 68 probably went through the same transformation.

	Type	Comments
	H6180 CPU Maintenance panel	Note the section at the top labeled 'Maintenance' - this is actually the sub-panel for the 'Appending Unit', the special extension found only in Multics CPUs. This particular panel now belongs to the Living Computers Museum, where it has been hooked up to a Multics CPU simulator; a video of it while Multics boots can be seen here.
	6000 Series / Level 68 Multics CPU Configuration panel	In GCOS CPUs, the CPU's Configuration sub-panel is part of the main CPU physical front panel. In Multics CPUs, that space was taken up by the sub-panel for the Appending Unit - and with more ports, the Multics CPU's configuration panel was larger anyway, so it was apparently moved to a separate half-panel.
	DPS-8 Multics CPU Configuration panel	This seems to be functionally basically the same as the CPU Configuration panel above, but with a panel 'look' consistent with the other DPS-8 panels. Note, however, that it only supports 4 ports; by now, the larger SCU's were ubiquitous, so there was no need for more than 4.

SCU Panels

These panels could be found on either Multics or GCOS machines; as far as is known, the SCUs were identical for both.

	Type	Comments
	256KW SCU Configuration and Maintenance panel
	4MW SCU Configuration and Maintenance panel	This is the later, larger capacity SCU; it apparently did not have the full maintenance panel (as above), just this smaller configuration/maintenance panel.
	DPS-8 SCU Configuration panel	This is the SCU Configuration sub-panel for the DPS-8 machines; it has a panel 'look' consistent with the other DPS-8 panels.
	DPS-8 SCU Syndrome panel	This is an ancillary sub-panel to the DPS-8 SCU Configuration sub-panel above, which displays memory error information.

IOM Panels

The Multics and GCOS IOM front panels were mostly similar, apparently with only small differences in the Configuration sub-panel.

	Type	Comments
	GCOS IOM Configuration, Maintenance and Test panel	These panels were also probably mounted on the back of a large swing-out doors, on the front of the IOM cabinet, and the same size as the early SCUs. Note the limited number of ports.
	NSA IOM Maintenance and Test panel	The NSA IOM Maintenance and Test panel appears to be indentical to the GCOS one above, but it is missing the Configuration sub-panel of that, which has been moved to a separate half-panel (below). It is believed that the Multics IOM Maintenance and Test panel is identical to this, but confirmation is currently unavailable.
	NSA IOM Configuration and Bootload panel	The NSA IOM Configuration panel is identical to the original GCOS one, but it has a lot more ports (i.e. can be used in a system with more SCUs, i.e. more memory), so would no longer fit in the same panel, and had to be moved. It looks much the same as the Multics one below, but it is missing the two sub-panels at the top of the Multics panel.
	DPS-8 GCOS IOM Configuration and Bootload panel	This seems to be functionally basically the same as the NSA and Multics IOM Configuration panels (above and below), but with a panel 'look' consistent with the other DPS-8 panels. Note that it only has 4 ports, standard for DPS-8 machines with both Multics and GCOS.

	Multics IOM Configuration and Bootload panel	Like the CPU Configuration, these panels (which supported 8 ports, i.e. connection to 8 SCUs), and were thus apparently were too large to go where the Configuration sub-panel went on a GCOS IOM front panel (above), so it was moved to a separate half-panel, much like the CPU's. One can be seen at the far right of the image of the MIT H6180 Multics system, below.
	DPS-8 Multics IOM Configuration and Bootload panel	This is the basically the same as the DPS-8 GCOS IOM Configuration panel above. It is not clear with the minor differences between this one, and the 'GCOS' IOM panel represent merely different temporal versions of a universal DPS-8 IOM panel, or if the Multics and GCOS IOM panels were slightly different.

Non-Multics CPU Panels

These panels are from CPU units which could be found only on GCOS machines.

	Type	Comments
	6000 Series GCOS CPU Configuration and Maintenance panel	This panel is the same size as the Multics CPU Maintenance panel, and like those, was probably mounted on the back of a large swing-out door on the front of the CPU cabinet, Note that the large section at the top for the Multics Appending Unit is not there; that space holds the Configuration sub-panel on these CPUs.
	Level 66 CPU Configuration and Maintenance panel	Identical to the previous one, except for the colour, and lights.
	Level 66 VU Configuration and Maintenance panel	The 'Virtual Unit' (VU) was the NSA counterpart to the Multics Appending Unit (AU); apparently Series 60 CPUs could optionally have either an AU or VU added to them. (The VU was standard on the later DPS-8 CPUs.) It too apparently occupied its own half-panel.

Other Panels

These panels are from other units in the 6K line.

	Type	Comments
	6000 Series Microprogrammed Peripheral Controller panel	Originally, all disk drives, tape drives, etc were connected to IOMs via mass storage and magnetic tape processors, respectively; those were eventually replaced by this 'Microprogrammed Peripheral Controller'.

Multics Systems

Finally, some images of complete Multics systems - or, at least, as much of them as will fit into a single image!

	Type	Comments
	H6180 Multics System at CISL	This image shows part of the dual-processor H6180 Multics system at Honeywell CISL. The unit on the far left is a 256KW SCU; to its right, there is a 6180 CPU. Note the smaller panel just to the right of the CPU's maintenance panel (mostly edge-on in this view); this is the 6180 CPU configuration half-panel (which contains things moved from the top of the 60xx CPU maintenance panel to make room for the Appending Unit sub-panel). The panel visible in the distance at the right-hand edge of this image is a Multics-type IOM's Configuration half-panel.
	DPS-8/M Multics system at the University of Mainz	This shows the Mainz Multics system; that system included (from the left) an SCU, a CPU, another SCU, another CPU, and an IOM.

Acknowledgements

Thanks to Jeff Kaylin, of the Living Computers Museum + Labs, for the image of the 6180 CPU Maintenance panel; and to Tom Van Vleck, maintainer of the Multicans Web site, for the image of the CISL 6180 machine.

Some of the images here are from a number of Internet sites which host Multics/GCOS panel images:

Thanks, and I hope they don't mind me using copies of their images!

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Last updated: 27/October/2024