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Chapter 24

A user machine in a time-sharing system1

B. W. Lampson / W. W. Lichtenberger / M. W. Pirtle

Summary This paper describes the design of the computer seen by a machine-language programmer in a time-sharing system developed at the University of California at Berkeley. Some of the instructions in this machine are executed by the hardware, and some are implemented by software. The user, however, thinks of them all as part of his machine, a machine having extensive and unusual capabilities, many of which might be part of the hardware of a (considerably more expensive) computer.

Among the important features of the machine are the arithmetic and string manipulation instructions, the very general memory allocation and configuration mechanism, and the multiple processes which can be created by the program. Facilities are provided for communication among these processes and for the control of exceptional conditions.

The input-output system is capable of handling all of the peripheral equipment in a uniform and convenient manner through files having symbolic names. Programs can access files belonging to a number of people, but each person can protect his own files from unauthorized access by others.

Some mention is made at various points of the techniques of implementation, but the main emphasis is on the appearance of the user's machine.


A characteristic of a time-sharing system is that the computer seen by the user programming in machine language differs from that on which the system is implemented [Bright, 1964; Comfort, 1965; Forgie, 1965; McCullogh et al., 1965; Schwartz, 1964]. In fact, the user machine is defined by the combination of the time-sharing hardware running in user mode and the software which controls input-output, deals with illegal actions which may be taken by a user's program, and provides various other services. If the hardware is arranged in such a way that calls on the system have the same form as the hardware instructions of the machine [Lichtenberger and Pirtle, 1965], then the distinction becomes irrelevant to the user; he simply programs a machine with an unusual and powerful instruction set which relieves him of many of the problems of conventional machine-language programming [Lampson, 1965; McCarthy et al., 1963].

In a time-sharing system which has been developed by and for the use of members of Project Genie at the University of California at Berkeley [Lichtenberger and Pirtle, 1965], the user machine has a number of interesting characteristics. The computer in this system is an SDS 930, a 24 bit, fixed-point machine with one index register, multi-level indirect addressing, a 14 bit address field, and 32 thousand words of 1.75 m s memory in two independent modules. Figure 1 shows the basic configuration of equipment. The memory is interleaved between the two modules so that processing and drum transfers may occur simultaneously. A detailed description of the various hardware modifications of the computer and their implications for the performance of the overall system has been given in a previous paper [Lichtenberger and Pirtle, 1965].

Briefly, these modifications include the addition of monitor and user modes in which, for user mode, the execution of a class of instructions is prevented and replaced by a trap to a system routine. The protection from unauthorized access to memory has been subsumed in an address mapping scheme: both the 16384 words addressable by a user program (logical addresses) and the 32 768 words of actual core memory (physical addresses) have been divided into 2048-word pages. A set of eight six-bit hardware registers defines a map from the logical address space to the real memory by specifying the real page which is to correspond to each of the user's logical pages. Implicit in this scheme is the capability of marking each of the user's pages as unassigned or read-only, so that any attempt to access such a page improperly will result in a trap.

All memory references in user mode are mapped. In monitor mode, all memory references are normally absolute. It is possible, however, with any instruction in monitor mode, or even within a chain of indirect addressing, to specify use of the user map. Furthermore, in monitor mode the top 4096 words are mapped through two additional registers called the monitor map. The mapping process is illustrated in Fig. 2.

Another significant hardware modification is the mechanism for going between modes. Once the machine is in user mode, it can get to monitor mode under three circumstances:

1Proc. IEEE, 54, vol. 12, pp. 1766-1774, December, 1966.


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