In PDP-11/20 the block KA11 is termed "CPU". It is intended to be inserted into a rack. In PDP-11/05 on the other hand a similar block is termed "computer" even though it also is intended to be inserted.

It should be noted that KA11 is much smaller than the whole rack, so I wonder whether it could be used without the rack as a whole. What functions the other parts of the rack provided (except external storage)?

"Computer" in terms of PDP-11/05 is of the same size as a horizontal desktop computer case of today:

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    It may be helpful if you add exact reference (document and page) for either example.
    – Raffzahn
    Oct 24, 2023 at 22:04
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    You are doomed to confusion if you expect words to retain unfixed meanings over time. If I recall correctly, the 'computer' in the EDSAC was what we'd nowadays call the ALU.
    – dave
    Oct 25, 2023 at 1:10
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    The 'rest of the rack' includes the backplane, which is a vital part of the system. Without the backplane, the KA11 does not function, because the parts are not connected. Without the backplane, there's no Unibus, so no memory or I/O.
    – dave
    Oct 25, 2023 at 1:21
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    The second manual is for a GT40 graphics terminal, which is a device that contains a PDP-11 computer. The contents page (for example) makes it clear that the "computer" is made up from a KD11 processor, backplane, core memory, and power supply.
    – dave
    Oct 25, 2023 at 1:32
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    @another-dave not to mention, a "computer" used to be an occupation for humans, before the widespread use of machines.
    – Hackworth
    Oct 26, 2023 at 8:36

2 Answers 2


TL;DR: It Depends on Purpose.

At the base is a profound mixup of Processor and Processing and both being meant when talking about a CP(U).

  • A Processing Unit is a circuitry doing computation

  • A Processor is a device containing one or more Processing Units

Building up thereon

  • A Processor is a component in a computer (system). It's usually operational on its own but not necessary useful as such.
  • A Computer is a configuration of devices, including a Processor, into a usable system.

The differentiation between either of those can be blurry and depend a lot on time, system type, technology and manufacturer. Not to mention that designers, marketing and especially the general public tend to use terms with way greater freedom (*1).

The Good Old Days.

It was way more clear back then. Take for example a classic IBM 7030 computer. Eventually the fastest scientific computer in 1961, seen here:

enter image description here

(Picture taken from the 7030 Reference Manual p.4)

Except that the number only names the CPU cabinet(s), marked here in red. No Memory, IO or whatsoever - even things we call a bus today was separate, nicely seen on this schematic:

enter image description here

(Picture taken from the 7030 Reference Manual p.6)

(Almost) each of the boxes shown is housed in a dedicated cabinet and connected by arm sized cables. Each of those cabinets was a product on its own, with a product name and to be ordered separate. Memory for example, marked a '1', was known as IBM 7032 Each of those cabinets providing 16 KiWords of RAM.

Same goes for disk controllers, disk drives, tape controllers, tape drives and so on. All one or more dedicated boxes - heck, even the interface for that table sized console needed a cabinet of its own.

Due this quite distinguishable structure the meaning is rather synonymous: The CPU is the Processor and the Processing Unit.


With time passing units became smaller and higher integrated. Less than 5 years after the 7030 the System /360 was introduced, now components like CPU, Bus controller, I/O Processor and especially Memory became integrated into a single unit - although still occupying several cabinets, depending on machine size and configuration. Only communication controllers,I/O controllers and peripheral devices were still separate units.

While technician still saw and handled the main unit as multiple components (CPU/IOC/Memory Interface/Memory Controller/Memory), most people would call that big box in the middle just CPU.

Mainframe development does continue until today and while some old interfaces have been removed and communication interfaces have been added to the cabinet, the storage is still an external component.

With the integration of multiple processors into a single box a hard dual use of CPU became common: The cabinet at whole being called a CPU, but als talked about multiple CPU being inside.

To solve this IBM started labelling the big box a 'Processor' while the insides doing the computing were called 'Processing Unit(s)'.

Revolt of the Pygmies

About the same time as mainframes started a race to become ever more powerful while even shrinking a bit, other companies focused on a new area, computer systems not as powerful as mainframes, but so small they could fully fit into a few or only a single cabinet. To reach that the processor had to shrink first. DEC was one of them and the PDP-8 eventually their first mini computer. While the first PDP-8 (*2), still filled almost a whole Rack just for the processor (20 HE), the follow up PDP-8/s got it down to a 6 HE box (*3).

With that size (*4) a full computer system, that is CPU with memory, I/O system, console interface, disk(s), (paper) tape and other, could now be as compact as a half height rack.

enter image description here (Pictures taken from Wikipedia and van der Mark's great PDP-8/E site)

The lower half nicely shows the issue about how to define the CPU part of a processor. In DEC documentation only the 5 left most boards are considered the (KK8E) CPU, but the next two boards hole memory interface, management and sharing, items most would consider part of a CPU - likewise memory control further down.

In the end every manufacturer draws that line different, thus any naming below processor as a blox will be inconsistent.

A 360 Degree Circle ...

Not much later it became possible to integrate a very basic CPU (remember that red line?), now called 'micro-processor', still needing many external components to even start to function. But by now modern PC have taken the full circle back to what mainframes and minis have done with their CPU's.

Just look at one of today's highly integrated microprocessors like AMD Ryzen or Intel Core. They integrate CPU die(s), I/O die(s), memory controller die(s) and maybe cache die(s)in a single multi chip package. It interfaces to the outside only via memory interface, PCIe and USB. Put one on a more basic mainboard that does not add any further peripherals and the result would be quite like what a one would call a CPU back in mainframe days.

... Back to the Naming Issue

With just a single processor in a chip and only one of them in a computer the term CPU could be seen as a distinct naming of a chip and it's solitary nature. People that haven't experienced he development prior would assume that as the only and fixed meaning.

Having two in a computer didn't change much, one calling that a Dual-Processor (or Dual-CPU) sounded great. It became more blurry when they started to integrate two processors into a single chip and a complete mess when systems were created using multiple chips each with multiple processing units

Does It Compute?

The central point to distinguish both might be it's usefulness. A CPU in itself may be operational, but not really useful. Imagine an IBM /360 or DEC PDP-8. Both can run without any peripherals, they can even be programmed using the blinkenlights. Not very comfy. Only by adding peripheral system, most important storage systems a useful computer system is build - with that CPU as it's core component.

It's the very same with a 1960 Mainframe, 1970 Mini or 2020 Desktop.

*1 - Also there's the usual caveat when it comes to any overlapping terminology: In real world communication people care only to differentiate if that is neccessary - otherwise it's an anything goes.

*2 - Introduced 1965, the very same year as the /360

*3 - And cut price almost in half - at cost of reducing speed to ~1/8th.

*5 - Like similar computers of the time and thruout the 70s.

  • 1
    The thing you have apparently labelled 'CPU' (on the PDP-8) iooks like just the console, with the switches and blinky-lights. It's separate from the CPU, just as your 7030 diagram shows a separate console. The PDP-8 console is in front of a drawer which contains a backplane. The CPU is several boards in the backplane; there are other boards holding memory, peripheral controllers, etc. The wiki page has a better picture of the inside.
    – dave
    Oct 25, 2023 at 1:43
  • @another-dave Err, correct me, but what other box than the marked one does hold the CPU boards? Note that the text already explains that PDP CPU's integrate, like /360 before, several components with the core processor (see the 'progress' section).
    – Raffzahn
    Oct 25, 2023 at 1:49
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    Sure, the box contains the CPU, but it looks like you're calling the console the CPU. What you meant is likely clear enough to you, but I doubt it is clear to the OP, whose very question indicates some confusion about matters.
    – dave
    Oct 25, 2023 at 1:52
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    @Anixx But your question is about the term Computer vs. CPU, not if a CPU can be put on a desk. It feels as if you're stuck in a mismatch between definition, intended use and factual use. Putting some parts of a computer on a desk doesn't make it a desktop computer. Same way as not mounting a machine intended to be used in a rack doesn't change nature when not mounted.
    – Raffzahn
    Oct 25, 2023 at 9:54
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    @Mazura LOL. Yes :))
    – Raffzahn
    Oct 26, 2023 at 3:24

The newly-posted pictures are of a PDP-11/05 computer. The computer contains a backplane.

Inserted into that backplane are some number of boards which are the KD11-B CPU, or 'processor'.

(Also insertable into that backplane are peripheral controllers).

There is no confusion. The CPU is a part of the computer. A PDP-11/05 contains a KD11, a PDP-11/05 is not itself a KD11.

The computer is packaged as a rack-mountable unit. Note the slides on the ends. Note the relatively fragile cables coming out of the rear. You are unlikely to use it outside a rack (though maybe you could MacGyver it). If you want any storage devices, say DECtape, then they will require rack space as well.

The same terminology applies to a PDP-11/20 which contains a KA11 CPU on many small boards ('flip chips' in the DEC terminology of the time). You don't insert the KA11 in a rack. The KA11 is in the system unit ('computer'). The system unit is in the rack.

The only ambiguous terminology I see is whether the word 'computer' applies strictly the system unit -- the box with the console, backplane, CPU, etc. -- or the entire rack. We'd likely use 'computer' for both, context-dependent.

As a user, I regard the whole rack as the computer, since when it's my timeslot, I have the whole thing. Hardware designers and field service engineers probably think differently.

  • It's nice that we can agree that only the whole setup makes a computer. Still, you may want to add a bold caveat that this is a description (not definition) possibly hard tied to DEC computers of a certain area, certain use case and time. At best a single example for that. It's not really one fit as generic terminology across different manufacturers and computers or even just DEC systems overtime.
    – Raffzahn
    Oct 25, 2023 at 13:44
  • Sure. it's DECish, but so was the OP's basis for the question: PDP-11/20, PDP-11/05, KA11. The title of the question is vendor-agnostic, but the body is not.
    – dave
    Oct 25, 2023 at 21:16
  • @Raffzahn - but I don't think we differ that much. The CPU is the stuff that executes the instruction set. In your example, that stuff is big enough to have its own box; in mine, the 'stuff' is a few boards in a box with other things, and the OP made the mistake of thinking that CPU (or processor) referred to the box rather than the specific boards. The terminology is pretty similar, regardless.
    – dave
    Oct 26, 2023 at 19:37
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    Well, yes. Though, I would read it as asking for a generic answer while using DEC as example as that's where he seems to have some knowledge. The whole topic isn't easy, as either of those terms has been used different depending on time and topic. Especially including calling the 'box' rightful a CPU, even though it may include more than just the processing unit. It seems kind of a common issue with seemingly obvious, simple, every day terms - they often tend to loose all meaning when looking close - unless context (and/or history) is given. That's also what I tried to convey.
    – Raffzahn
    Oct 26, 2023 at 21:43
  • And yes, We are, as usual not really far apart.
    – Raffzahn
    Oct 27, 2023 at 2:46

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