BJTECHnNICHE\JTECHnNICHEBTECHL Z80 processor can recognise. The last seven chapters are. to my mind, the best in the book - they deal with specific 'concepts' like sound. Interrupts, the display, and so on. The one and a half page index is totally inadequate - it doesn't mention the LDIR instruction. or the RS-232 interlace, for example. The first chapter is by far the worst, but luckily it's also the most dispensable. It's a mixture of patronising waffle and a bare minimum of 'hard facts', Memory is referred to as * boxes' and registers are called 'hands' and 'feet'. By the end of Chapter One, we know that the computer has a 'CPU' - a two Inch piece of black plastic with 40 metal legs - plus a number of 'hands' and 'feet', where two 'hands' equals one 'loot'; a 'foot' has sixteen 'toes' and a 'hand' has 8 fingers! Besides the processor, the computer contains a mysterious 'spike' called a stack, which the CPU can push with two "hands' it's a good job most readers will already know what this chapter sets out to explain. Chapter One is the only part of the book which makes specific reference to the Plus Two - other sections refer to the Spectrum 128. and it seems certain that the book was originally written for users of the original (pre-Amstrad} 128. That doesn't matter much, as the differences between the two are almost entirely cosmetic, from the point of view of the machine code programmer at least. There is one annoying section however, that witters on about 'the EAR and MIC sockets of the tape Interface', which don't appear on the Amstrad Spectrum. The second chapter Is a vast improvement - a clear explanation of the 'binary' and 'hexadecimal' numbering systems used by machine code programmers. Binary numbers are printed with a zero at the start to distinguish them from decimal; hexadecimal numbers start with an ampersand when they crop up in the text, but a hash when they appear in programs. According lo the text, some values are marked with a trailing H', but - thankfully - I couldn't find any of those. Once numbers have been dealt with, the "background' part ol the book ends with a useful chapter about the interrelation between machine code and BASIC. The next section is quite wellwritten, but very unimaginatively planned. One chapter introduces ail of the 280's' registers' and "addressing modes', then 70 humdrum pages are used to introduce every instruction ZJLOO the chipmakers own up to. There's no mention of the hundred or so 'undocumented' instructions which work, but are not part of the machine's official specification. Joe Pritchard follows the manufacturer's reference material faithfully, even though the resultant sequence ol instructions makes rt hard to wnte meaningful examples. The information is collected by category, as a proces-
sor-designer would see it. so you have to read half-way through the book, past all the complicated and esoteric instructions for shifting, binary coded decimal, logic and so forth before you find out about 'jumps' - the simple, essential machine code equivalents of IF and GO TO! If you struggle through the strange sequence you'll find that all the instructions are helpfully and accurately described, but the examples are uniformly trivial and do not build on the previous material. arbitrary as it is. To a great extent, the last sixty pages of the book are its saviour. They consist of seven essays, with examples, dealing with 'real world' subjects. The first essay, on interrupts, gets into a bit of a mess - one paragraph covers more than a page and says that interrupt mode 2 needs a 256 (not 257} byte vector table. The next paragraph runs to three quarters of page, and gets fairly confusing. The information is there - you just get a feeling that the author's grasp of it is not as firm as it might be. The data about the sound facilities seems complete, but again It is presented in the manufacturer's sequence, rather than as a tutorial. A lot of this information has appeared, in rather condensed form, in recent TECH TIPS columns, but the book Is a good source of further Information if you still don't know everything about 128 sound and RAM paging that you would like to. Joe Pritchard documents quite a few machine code routines built in to the Spectrum ROM, but it refers you to another MELBOURNE HOUSE book. I a n L o g a n ' s Com-
plete Spectrum ROM disassembly, for further information. Logan's ROM disassembly is just that - a line-by-line commentary on the code - and doesn't contain enough of an 'overview' to guide a beginner. Yet another MELBOURNE
HOUSE book. Understanding Your Spectrum, does better in this regard, but unfortunately it duplicates a lot of the information In Pritchard's book. Joe gives you the bare minimum - for instance he explains how to plot individual points on the screen, but not how to draw lines, even though there's a simple routine in the ROM to do that. He tells us how to read individual characters from the keyboard, but not how to read the joystick or check for combinations of keys vital for games! There's absolutely no information about the second 16K ROM in the Plus 2 - the 'editor' ROM - and nothing about writing to the MIDI and Serial port. These subjects deserve attention: after all, the book sets out to be specilic to the Plus Two. I've made a lot of criticisms of this book, but they should be taken in perspective Spectnjm Plus Two Machine Language for the Absolute Beginner contains lots of information, defines all the terms it uses, and is a better-than-aven introduction to a complicated ject. It's a shame that It wasn't planned and edited more carefully.
BOX OF TRICKS
SYSTEM 7's handy little reset box in all its glory .
If you've always been a bit niggled about the Spectrum's lack of 'frills* - no power switch, no indicator light, and no reset button on earty machines — SYSTEM 7 ELECTRONICS (664 Analby High Road. Hull. HU3 6UZ) have a low-priced widget that might cheer you up. Their imaginatively-titled 'Reset box' will not send a thrill down any techie's spine, but at £5.50 it's hardly a rip-off and may appeal to some people. The box measures about 50mm by 75mm, and is about 25mm
deep, not counting the bits that stick out of the top. Your Spectrum power lead plugs into one end. and a 50mm cable from the other end goes into the computer's power socket. On the top of the box there's a red power light, an on/off switch and a reset pushbutton. It's neatly and solidly put together, but the on/off switch on my unit did collapse - reasonably enough, when I was stupid enough to put it at the bottom of a holdall filled with CRASH post, and drag the lot around with me for a few days.
NEW TRANSFER PACK 3 — for spectrum tape to drive A software system to convert many TAPE based programs to your DRIVE (no matter what type) + OTHER USEFUL utilities. Most prky programs are EASILY converted to drive. Pack has at least 7 programs! INCLUDES MANY UTILITIES (e.g "vat" creator, bytes chopper/splitter. TAPE UTILITY- neede to convert baud rates.) Opus/Beta owners add CI for optional extra program. State TYPE of Spectrum & name of drive whenordering(e.g 128k Spectrum and Microdrive). £11.00 on TAPE (£12.99 on Cartridge - 48k Spectrum only) (£6 if updating) INFORMATION SHEETS - El each - approx 8 transfers of popular programs, per sheet - needs TP3. Up to No. 25 available Special offer - No s 1 to 20 only £4.50! MICROTRANS 2b - for m/drive to m/drive back-up + easy ERASE, tape to drive (inci headeriess) - does NOT convert programs. Includes MICROTAPE end RUN programs. Only £3 on tape. £4.50 on Cart. For any Spectrum.CODE COMPRESSOR - £3.50 Overseas: add £1 Europe. £2 others each product SAE (or details. NO RISK! - ALL OUR PROGRAMS CARRY OUR MONEY BACK GUARANTEE (not updates) JiLlDCLOSt WHflLfYWft
87 CRASH March 1987 >