Hints & Tips for the ZX81 by Andrew Hewson is excellent for the average ZX81 user who has little, or no knowledge of Z80 Machine Code programming, but would like to learn. There are sections on saving space in programs, understanding the Display File, converting ZX80 programs for use on the ZX81, how to let two or more programs access one lot of data and finally, 26 pages on Machine Code programming.
The book begins by showing the reader how to save a considerable amount of RAM by carefully designing programs. With so much memory used for system variables, this chapter is essential reading for ZX81 users with only 1K RAM.
The second chapter deals with the Display File—showing how TAB, AT, PLOT and UNPLOT work. This chapter also details how the Display File can be used as memory.
The next section discusses the similarities and differences between the ZX80 and ZX81 and how to convert programs between the two machines. Two conversion charts are given—one for converting character codes from ZX80 to ZX81, the other compares the ZX80 system variables with their ZX81 equivalents.
Chapter four demonstrates how two or more programs can share one lot of data by transferring the data into RAMTOP before loading the next program. This technique also allows for data to be ‘NEW-proofed’ in a way in which the data can be subsequently retrieved and processed by another program.
The final chapter explains to the complete novice how to write, load, edit, save, and debug Z80 Machine Language programs. The reader is first introduced to the concept of bits, bytes, addresses and hexadecimal numbering.
The author goes on to describe the Z80A registers, their relative uses, and the Z80A instruction set. Obviously, a book of this size cannot provide a full examination of the machine code of the Z80A microprocessor, but it does introduce the beginner to many basic concepts.
Hints & Tips for the ZX81 ends with thirteen programs for the basic, unexpanded ZX81. These programs include a useful Hex Loader/Printer, a Line Renumber program which neatens programs but does not include GOTO or GOSUB renumber, five games, a machine code clock program, a statistics program which generates the mean, standard deviation, intercept and slope of a given set of data.
Hints & Tips is clearly typeset, but is only bound with a couple of staples. It is a very good value for the money and will introduce many to the complex world of machine code programming.
Hints & Tips for the ZX81 by Andrew Hewson from: Hewson Consultants, 7 Grahame Close, Blewbury, Oxfordshire, OX11 9QE England. 76 pp. softbound; £4.95.