System / 360 - hardware

When I started the first article on the IBM System / 360, I had no idea that this would translate into a whole series of articles, so extensive and interesting was this revolutionary system. Three articles have already been published ( first , second , third ), I am writing this, and I understand that the topic itself is far from exhausted, and you can write about System / 360 for a long time. This time we will talk about peripherals and work with external devices in System / 360.

Channels

Since the peripheral devices at that time were often not very fast, so-called “channels” were used to work with them - separate processors with a reduced set of instructions designed to transfer information between the device and the main memory. The concept of channels is a bit like modern DMA. By the principle of action, the channels were divided into byte-multiplexer and selector. The first ones were designed to work with slow devices (printers, perforators, punched card readers), the second ones were for devices with higher speeds (magnetic disks, tapes, external memory cells). Practically all the System / 360 machines, except for the “non-standard” Model 20, were equipped with a byte-multiplex channel and one or more selector channels. In the simpler models, the channels were integrated, and in the top models they were made in the form of separate cabinets.
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As it is easy to understand from the name, the multiplex channel allowed to exchange data from several devices at once through one channel into the computer's RAM. Most often, the address of this channel was 0, and addresses from C0 to FF were used to address sub-channels. For example, at addresses 0C0-0C7, streamers were located, 00E / 00F: printers 1403-N, 010-013: printers 3211, 020-0BF: telecommunication devices of the 270x family. These addresses are still used in z / VM virtual machines.

Selector channels allowed to connect more high-speed devices. Usually, a control module was placed between the device and the channel, which made it possible to combine several devices of the same type and send data from them to one or several channels, in parallel or sequentially, which made it possible to vary the connection options to achieve optimal performance.

In models 85 and 195 IBM introduced a new type of channels - block-multiplex. These channels allowed the connected device to suspend the current program of the channel until the device was ready to transmit data, thus freeing the channel for other devices. Initially, these channels were designed to work with disks with a fixed head of the 2305 family.

Component Naming

IBM has developed a digital code system for marking new devices. They were marked with a 4-digit code, starting with 2. A number of old devices that already existed before System / 360 retained their markings (for example, the most famous printers of the 1403 family, which printed those same reproductions of Gioconda).

The devices were numbered as follows.

20xx: Arithmetic processors, for example IBM 2030, IBM System / 360 Model 30 central processor
21xx: Power supplies and other equipment closely related to the processor, for example the IBM 2167 configuration module
22xx: Various output devices, for example, CRT monitors IBM 2250 and IBM 2260, IBM 2203 - System / 360 Model 20 printer
23xx: Direct access storage devices, for example, IBM 2311 and IBM 2314 disk drives, or IBM 2321 data cells. Also, this labeling was used for main storage devices (IBM 2361 - large storage, IBM 2365 - processor storage)
24xx: Tape Drives, for example IBM 2401, IBM 2405 and IBM 2415
25xx: Devices for working with punch cards, for example, IBM 2501 card reader, IBM 2520 punch, IBM 2540 reader / punch and IBM 2560 multi-function card machine
26xx: Paper tape devices, for example, IBM 2671 reader
27xx: Communication equipment, for example, interactive terminals IBM 2701, IBM 2705, IBM 2741
28xx: Channels and controllers. For example, IBM 2821, IBM 2841, and IBM 2844 control module
29xx: Other devices, such as the IBM 2914 data channel switch and the IBM 2944 data channel repeater

Direct Access Storage

First of all, different disks belonged to this category. The first drives released with System / 360 were IBM 2302s and IBM 2311s.



2302 was based on an earlier model 1302 and was a hard disk with an access speed of 156 Kb / s and was delivered as model 3 with two modules of 112.79 MB each, or as model 4, with four such modules.

2311, in turn, was an updated version of IBM 1311 and allowed working with IBM 1316 removable disk packs. The theoretical storage capacity was 7.2 MB, but in practice everything depended on the format. For example, when used with the System / 360 Model 20, this drive provided only 5.4 MB of free space.

The package of disks IBM 1316 by today's standards can be considered huge. Six discs with a diameter of about 36 cm were placed on 6 pieces on a common spindle. The top and bottom surfaces of the stack did not carry any data on them, so 10 surfaces were available for recording. All read / write heads were combined into one block and moved together. The number of tracks was 203. To reduce the number of head movements, the data were recorded “vertically” on the surface of the disks from top to bottom, forming “cylinders”. The size of the sectors was variable, like on a magnetic tape.

Later, in 1966, a 2314s drive appeared, using updated 2316 disk packs with a capacity of 28 MB.

For those cases when read-write speed was important, but not capacity, drum drives were used, in which a separate head was used for each track. The capacity of the first models was about 4 MB, and the speed of operations reached 303.8 Kb / s, later, disk drives came to replace the drum drives, also with separate heads on the track. This was, for example, IBM 2305, introduced in 1970. The disks of the drive rotated at a speed of 6000 revolutions per minute, the data exchange rate reached as much as 3 Mb / s, capacities of 5 and 11 Mb were available.

Despite the small capacity and high price, such devices were in demand, for example, for placing overlays (program modules that were dynamically loaded into RAM).

An even more expensive and rare solution was the IBM 2321 Data Cell. This drive worked with the so-called "memory cells", each of which contained within 200 magnetic tapes that could be rewound and read independently. IBM 2321 allowed to establish up to 10 such "cells", providing storage to 400 million bytes. Up to 8 IBM 2321s could connect to the IBM 2841 Control Module, thus providing as much as 3 GB of storage. The access time ranged from 95 to 600 milliseconds, depending on the position of the films.

Thus, the date cell was a very good compromise between hard drives and tape drives. Compared to the IBM 2311 hard drive, the IBM 2321 could store 55 times more data, with access speeds just 7 times slower.

Since three separate drives were used in Data Cell, almost 20 liters of engine oil was poured into the car to lubricate them, which circulated in the system under pressure, which gave rise to many stories about leaks, most often false ones. Due to the abundance of short ribbon fragments in interchangeable modules, they were often compared to noodles.