Prestel - Prestel Computer Hardware and Software

Prestel Computer Hardware and Software

Prestel computers were based upon the GEC 4000 series of minicomputers with minor differences in build according to the function of the machine. IRC machines were originally GEC 4082 machines equipped with 384 kbytes of core store memory and six 70 Mbyte hard disc drives which were allocated as follows:

• One Master disc holding the “user file” for each registered user and other system control data
• One System disc holding the operating system, and the Prestel software
• Four data discs holding Prestel frames (each occupying 1 kbyte of space)

This arrangement effectively limited the size of the public service database to around 250,000 frames so in order to cope with planned growth by 1981 the IRC machines had been expanded by the addition of two further data drives.

Each IRC computer was configured with 208 ports and so was able to support 200 simultaneous Prestel users, the remaining 8 ports being used for test and control functions. Access for the ordinary user was provided via the duplex asynchronous interface provided by banks of GEC 16-port multi-channel asynchronous control units (MCACCU) known more simply as multiplexers. These devices in turn were accessed via banks of standard Post Office Modems No. 20 operating at 1200/75 bit/s, which were connected directly to the public switched telephone network (PSTN).

The single Update Centre machine was originally also a GEC 4082 machine, built with 384 kbytes of core store and the following arrangement of 70 Mbyte drives:

• One Master disc holding a user file for each registered user
• One System disc holding the operating system, and the Prestel software
• Eight data discs holding Prestel frames (each occupying 1 kbyte of space)
• Two identical transaction discs which logged changes made to the database by IPs

By 1981 this configuration had changed with memory doubled to 768 kbytes but with data discs reduced to six, corresponding to the number at the IRC machines and with just a single transaction disc. In addition to the MCACCU units required to support 1200/75 dial up access, the Update Centre machines were also connected to special modems provided to support online bulk updating by IPs. Banks of 300/300 bit/s full duplex asynchronous V21 modems supported computer to computer links for the more sophisticated IP while 1200 bit/s half duplex V23 modems supported so called intelligent editing terminals (i.e. those capable of storing a number of frames offline before uploading to the UDC). In addition twin 9-track NRZI tape decks of 800 bytes/inch capacity were provided in order to support bulk offline updates.

Although technically categorised as minicomputers, these GEC machines were physically very large by today’s standards, each occupying several standard communications cabinets, each standing 6-foot (1.8 m) high by 2 feet (0.61 m) wide. The CDC 9762 hard disc drives were housed separately in large stand-alone units, each one about the size of a domestic washing machine. (See images in the photo of the GEC Computers' Development Centre). The 70 Mbyte capacity hard discs themselves were in fact removable units, each consisting of a stack of five 14 inch platters, standing 4 inches (10 cm) high, that could be lifted in and out of the drive unit.

The GEC machines cost in excess of £200,000 each at GEC standard prices, in addition to which there were the costs of all the associated communications equipment. Putting together all of the computer and communications equipment required for a single IRC was a major undertaking and took some 15 months from order placement to commissioning.

GEC 4000 series computers were capable of running a number of operating systems but in practice Prestel machines exclusively ran OS4000 which itself was developed by GEC. This in turn supported BABBAGE, the so-called high level assembler language in which all the Prestel software for both IRC and UDC machines (and later the messaging machine) was written.

In 1987 a Prestel Admin computer was introduced which supported the user registration process: the capture of user details from the paper Prestel Application Form (PAF), the transfer of data to the relevant Prestel computer, and the printing of the Welcome letter for users. This machine, also based upon GEC 4082 equipment, was the first to be equipped with 1 Mbyte of memory which was required to support the Rapport relational database. This product from Logica was an early example of deployment of a system written in a 4GL database language which supported all features of the Prestel Admin application.