From eqns. 10 and 19, Substituting from eqn. 21,

Ji) _RC(RP[I RQ(RB+RC)

(22)

Alun Evans is a reader in the Depart-ment of Electrical & Electronic Engin-eering at the University College ofSwansea: he graduated in physics fromthe College in 1957 and subsequentlyobtained an M.Sc. degree in infor-mation engineering at the Universityof Birmingham. Following industrialexperience in Canada, he was for twoyears a lecturer at the UniversityCollege of North Wales, Bangor,before returning to Swansea in 1968. He is a regular con-tributor to scientific and engineering journals and therecipient of two premium awards by the IERE for out-standing papers.

JL) = *c(Rp±M VnV RQ(RB

- I n

(23)

Book reviewDigital system implementationGerrit A. BlaauwPrentice-Hal/, 1976, £1505, ISBN 13 2122 413

Although not mentioned in the title, the first descriptiveparagraph about this book on the dust cover says that it'emphasises the use of interactive APL'. This is perhaps anunderstatement, since the whole approach of the bookhinges on APL as a descriptive vehicle, and many prospectiveusers might well flip the pages, see sections of APL pro-gramming with its characteristic, unfamiliar symbols, andbe put off. This would be a pity, and so the first part of thisreview explains the place and value of APL in digitaldescription and simulation.

It is hard to describe APL quickly, or to justify it inwords, away from a terminal. However, the operators,functions and structure satisfy three aspects of computation.First, one can carry out normal numerical calculations and,with very little change to the sequence structure, replaceplain variables with multidimensional arrays represented byone symbol. Consequently, APL can be used as a high-levelsimulator — with very few program steps, an original areacan be checked as an algorithm before any further hardwaredesign is done.

Secondly, the language has a full range of binaryoperators, and is equally able to mix Boolean variables,vectors or matrices. Reshaping multidimensional arrays issimple. Consequently, high-level binary algorithms can bequickly checked with a minimum of writing, as a furthercheck in the design process.

Thirdly, many digital designers in the c.a.d. area are nowfirmly recommending register transfer languages as an idealmethod for implementing and simulating computer hard-ware. APL can be used as a simple register transfer language,and, if elementary functions from an a.l.u. are the onlyfunctions used in the APL program, then one gets themicroprogram or program sequences specified directly.

78

These last two advantages, plus the fact that APL usuallyoperates interpretively (as in BASIC, the program writing,editing, debugging and testing are all done in the same soft-ware environment) mean that hardware designers who havenot yet done so should consider looking at APL veryseriously.

Gerrit Blaauw was of course, one of the original IBMSystem 360 designers. He thus has a great wealth of experi-ence in machine design, as well as having had the oppor-tunity to evaluate APL at an early stage. He is currentlya Professor in Holland, so that it is unlikely that he wouldwrite a bad book. It is this reviewer's opinion that he haswritten a very good book.

The clear, progressive appendix about APL is of greatmerit. This, in the absence of a terminal, should help thenew reader to grasp many of its special properties. Otherappendixes or instruction sets, logical algebra and numbersystems follow. There are three fantastic chapters coveringall types of adders, multipliers and dividers, which showclearly how APL helps the designer to understand his hard-ware. The high- and low-level aspects are well illustrated.Practical users may consider that the coverage in thissection savours of overkill, but educators will be grateful.

Datapaths, microprogramming, memories and controlare all dealt with in great detail, although not as exhaustivelyas the arithmetic functions, and, with the current state ofl.s.i., these will probably be very useful chapters forpresent-day and future system designers.

In these days of an ever-increasing rate of change, it isvery risky to call any book a classic. However, this bookhas an excellent chance of achieving this status, and be-coming the type of reference work which the book by R.K.Richards, published in 1955, was to many of us in thoseearly years.

F.G. HEATH

ELECTRONIC CIRCUITS AND SYSTEMS, MA Y1978, Vol. 2, No. 3