36 Part 1 The structure of computers
function of various programs and subprograms. They may show Mp memory occupancy in a multiprogrammed environment. Some other time scales of particular interest are the instruction(s), short instruction sequences or subprograms, and the program times. The first two time scales are influenced predominantly by the hardware, and the latter time scale is influenced by software and the external environment.
The resource allocation diagrams also can describe the utilization of the C's resources over time (e.g., throughout the instruction-interpretation process) and provide a basis for more detailed analysis and design.
The design problem at the PMS-ISP interface is mainly one of resources scheduling.
1 A fixed set of operations have to be performed on the jobs (here, a job is an instruction).
2 Each instruction may create a few other small but definitive subjobs.
3 There can be a fixed set of operators which handle various parts of the operations.
4 Jobs (or instructions) enter P sequentially.
We may ask:
1 How many operators of each type do we have?
2 What is the scheduling policy for assigning instructions to the operators?
3 How many instructions can be in P at one time, and in what order must the processing be performed? How are the jobs interlocked?
We do not attempt to answer the above questions but intend only to show the relationship of the various parts which define the problem. ISP implies a certain structure (conversely, PMS behavior is specified in terms of the ISP language). A particular ISP structure and a program denote a certain path through a state space as specified by a state diagram. Finally, the physical resources (in PMS) are constrained to operate according to the state diagram as expressed by using a resources allocation diagram. The resource allocation diagram can then be used to evaluate the structure's performance (in PMS) at a higher level (e.g.. the number of instructions/second it executes).
Summary
The ISP descriptions of computers are usually given as an appendix to a chapter. We organize the description into the following units:
The above description format conveys a rather narrow-minded view of the ISP structure of computer systems. However, almost all present computers fit easily into such a format. We do not presume to say whether it will suffice for future ISPs.
With the introduction given here and with the definitions and example in the Appendix at the end of the book, it should be possible to understand all the PMS diagrams and ISP descriptions used throughout the book.