DAT panel

This is the display panel from the DAT box
(Dynamic Address Translation unit.)
This piece of hardware along with a few exta CPU instructions
transformed the 360 Model 65 computer to the 360 Model 67.

The 360/67 computer supported Virtual Memory.
Virtual Memory in these days is taken for granted
you do not even know you are using it.
But, in those days it was revolutionary.

Before this time people who used computers submitted their programs and data
on paper tape or punch cards to computer operators then went away,
returning hours later or next day to get the results.

About this time the idea arose that people could interact directly
with a computer via a typewriter like device called a 'terminal', and run their programs interactively.
To do this their programs and data had to be in RAM (Random Access Memory)
which was very expensive. But interactive users made wildly different demands
of the computer, at one moment initiating a compilation of their latest program
that required much of the CPU and RAM, then when the program error messages were
printed they would make no demands of the computer, and the telephone might ring
or they would make a cup of tea. So their program and data were not needed in RAM

In addition to the users intermittent demand for the computers resources the programs being run use RAM in greatly different ways.
The main computation loops get exercised a lot, whilst initialisation, termination and error code perhaps little.

To address these different usages of the computer resources virtual memory was invented.
The 24bit address of the computer could address 16MByte but the actual RAM was less (1MByte). The virtual memory was on an external device.
Both it and RAM was formatted in 2KByte pages. Programs were written as if in virtual memory and stored as such on the external device when loaded.
When the program began execution pages from the device were read and allocated to pages in RAM and page tables (in RAM) updated
to record the real address of each virtual address page. As the program was executed instructions were fetched to be executed by the CPU
that contained virtual addresses. These had to be translated to real addresses and that could involve access to the page tables in RAM.
This would really slow the computer down since the speed of a computer is governed by the speed of its RAM.

This is where the DAT box came into play. It had eight ASSOCIATIVE REGISTERS that contained the last eight page table translations.
As a program executed it was highly likely that the next instruction or item of data was on the same page.
When a virtual address translation began a hardware lookup was made via the DAT box and if the real page address was in a register
no access was required to the page tables and the CPU could proceed unimpeded.

A translation rate in the high 90% via the DAT box meant VM (Virtual Memory) became a practical reality.
If you want the full grizzly details they can be found in:
http://en.wikipedia.org/wiki/IBM_System/360_Model_67

Mus.Cat. NEWUC:2003.74	Mnfctr: IBM	Date: 1967	Form No: A27-2719-0	File No: S360-01
Comp: Computer Manual	Height: 279 mm	Width: 215 mm	Depth: 6 mm	Weight: 154 g

IBM System/360 Model 67 Functional Characteristics

This is the rear of the panel
with its 16(?) 24 wire ribbon cables
each of which is manually split at each end
and manually soldered to connectors and indicator sockets.
Each is individually labelled
you have to go to Fullsize to see.

This is one of the reasons why
computers cost so much in those days.