Section 3 ½ Computers of Historical Significance 105

Two original features, one-level storage and extracodes, have been copied in many other machines. A one-level store is common to most new time-shared or multiprogrammed computers.

The extracode feature allows ordinary machine operation codes to be used to call subroutines. Commonly used complex instructions (such as sin, cos, and monitor calls) can be written in a common operating system accessible to all users. Initially these subroutines were stored in a read-only memory.

The ISP is straightforward and extremely nice. The extracode idea appears in the SDS 900 series and was used in the SDS 940 system for defining common-user instructions. The IBM System! 360 SVC (supervisor call) instruction is an adaptation of the extracode.

Atlas was about the earliest computer to be designed with a software operating system and the idea of user machine in mind. The operating system has been nicely described [Kilburn et al., 1961] and evaluated [Morris et al., 1967].

In a letter to the authors of this book, F. H. Sumner makes the following comments on Atlas.

The initial ideas and the preliminary research on the Atlas computer system started in the Department of Computer Science of the University of Manchester in 1956. The team, under the direction of Professor T. Kilburn, was later supplemented by several members of the ICT. Computer Research Department, and the prototype machine was working in the department by the Autumn of 1961. The first production model became operational in January 1963.

The significant features of the system can be summarised as:

1 The provision of a virtual address field greater than the real address space.

2 The implementation of a 'one-level" store using a mixture of core store and drum store.

3 The interrupt system and the method of peripheral control.

4 The realisation at the design stage that there would he a complex operating system and the provision in the hardware of specific features to assist such an operating system.

The method of peripheral control permitted the attachment of a large number of on-line peripherals with rapid responses and entry into the operating system for a peripheral requiring attention. This, together with the multiprogramming features, makes the design ideal for the attachment of keyboards for the provision of multi-