Skywave FORTH

ZX Computing

Peter Shaw

Ray Elder

THE ZX81 GOES FORTH Peter Shaw tries an unusual variation on an old favourite. When the Ed told me about a ZX81 with a Forth ROM the same thoughts flashed across my mind as when I heard someone had linked the '81 to a 800K disc drive and an Epson printer: for what possible reason would they go to the trouble? Well, although I have the same feelings about the drive and printer, Forth on the '81 is a slightly better idea. The EPROM is either sold as a single complete unit, ie a ZX81 with a Forth EPROM fitted, or as a 'diy' conversion kit, so that you can take your existing ZX81 and exchange the BASIC ROM for Forth. Fitting the chip is quite simple. If your BASIC ROM is in a holder. All you have to do is whip one out with a screwdriver, and drop in the new one. If your BASIC ROM is soldered in then you'll probably be better off just buying the complete Forth package. David Husband, the producer of ZX81-Forth, calmly tells you to de-solder the basic ROM in one line of the manual. Taking one of those ROMs out is not very easy for one thing, and even if you do manage to get it out, you'll probably never be able to use it again and you might have damaged some of the other chips on the PCB while using the soldering iron. Once you've managed to get your EPROM in and working, what have you got? Well, it's not the Fig-Forth that I know and love, but what it loses on the swings, it gains on the roundabouts. ZX81-Forth is fast. I mean FAST, even compared to other versions Of Forth it is pretty nippy. In AUTO (FAST mode), the Forth version of a FOR-NEXT from 1 to 30000 takes less than 1 second. One major feature of Forth is that it is 'Multitasking' this means ZX81 Forth was probably developed with real-time controlling in mind. There are no graphics, as such, so the games players amongst you will be disappointed if you think this will provide a simple way of writing games without resorting to machine code. I couldn't find any bugs in the system while I was playing with the machine, this seems too good to be true! CONCLUSIONS As ZX81-Forth is based on TREE-FORTH and not the popular Fig Forth, I am not as thrilled with ZX81-Forth as I could have been. There must be a very limited market for ZX81 owners who want to run their hot-water boiler with a Forth program written on their small Sinclair. Apart from that, ZX81-Forth is a well written ROM. Addendum: Since Peter wrote this review I have had many conversations with suppliers of control units who all said that the ZX81 has taken on a new lease of life with Forth as a control unit - one telling me of a University which has installed several to monitor experiments. Although, as Peter says, perhaps the home user may have a limited, specialised use for it, I find it interesting to know of the applications for which the humble home computer is being used. If you know of any special ways in which these machines are being used, why not write in and let me know. Spectrum owners are not forgotten as David Husband also supplies a FORTH ROM unit for this computer. RE ZX81 Forth costs £25 plus £2 p&p. (VAT is not included.) it is available as an Eprom, or complete ZX81 with Eprom fitted from: David Husband, [redacted].

Your Spectrum

FORTH STAGE BOOSTER Should you ever meet a professional computercrat who smiles encouragingly at your Spectrum programming and suggests that "it's time you got yourself a real computer", show him the David Husband Forth extension. Forth is a language, but the extension could be called a modem driver, or a printer driver, or a turbocharger. And what David Husband's circuit will do, when he releases it around December, is to give Spectrum users the ability to run ten programs simultaneously. It could send messages down the phone lines, print on standard computer printers, and perform serious control operations. But best of all, say those who know Husband's Forth, it runs ten times, or a hundred times, or even more, faster than the speed of the Spectrum - while costing a mere £59 plus VAT. Husband has already been raved about for the Forth he produced - on a chip - for the ZX81. The language is at least as easy as Basic to understand, and many limes more powerful to use. And Husband's Forth is unusually clever. On the Spectrum he has found a way to use the same area of memory that Sinclair uses for the Basic - which means that he doesn't eat up Spectrum memory, as a Forth would if you had to load it from tape. But the Forth code can switch the computer back into being a Spectrum which in turn (properly programmed) can hand control back to Forth. So Basic programs can use the Forth board's Centronics port and RS232 serial port. At press-time, David Husband was still mulling over final design features, such as "shall I include a RESET button?" or "maybe we should have the BREAK function operate only with two keys down", and so on. It seems certain though that he'll offer a terminal control program on the board, and also that he'll use some of the Spectrum's memory as a pseudo-disk (making Forth 'real' and fast, instead of a FIGment of the Forth Interest Group's enthusiasm. If you want to know more about that little dig, or about Forth, but can't wait till Christmas, a ZX81 at £40 plus his Forth chip at £25 could make a great introduction. He'll even sell you a ready-converted ZX81. Details from Densham Computers Ltd, [redacted], or from Watford Electronics. Oh, it gives you a real-time clock, too. And split-screens...

Personal Computer News

Features 4/5

Documentation 4/5

Performance 5/5

Usability 4/5

Reliability 4/5

Overall Value 4/5

David Janda 4/5

NAME: ZX81-Forth APPLICATION: Programming language SYSTEM: ZX81 with 16K RAMpack PRICE: £28.75 PUBLISHER: David Husband, [redacted] FORMAT: EPROM NEW LIFE FOR ZX81 Ted Ball examines his new Forth computer after changing the Basic for a ZX81-Forth ROM. The ZX81 is a good cheap computer for beginners, but it has many limitations and a lot of people go on to buy another computer after a few months. If you're one of the people with a ZX81 gathering dust in the back of a cupboard you now have an opportunity to give it a new lease of life as a Forth computer instead of a Basic computer by installing the ZX81-Forth ROM from David Husband. FEATURES ZX81-Forth is similar to FIGForth and Forth-79 in its fundamental features but follows neither standard exactly. It gives you the usual Reverse Polish Notation and the colon definitions you expect in Forth, but the built-in dictionary includes a mixture of words from FIGForth and Forth-79 - although not the complete vocabulary of either and it also has some non-standard words. ZX81-Forth does not give a full standard version of Forth because it is specialised for multi-tasking and there's not enough ROM space to fit in a full standard version as well as the multi-tasking facilities. Multi-tasking allows you to put in several programs that appear to run simultaneously, its main use is to control external hardware (remember the early Sinclair advertisements that suggested you could use a ZX80 for running a power station?). What actually happens is that there is a scheduling system in ZX81-Forth that cycles round all the tasks, allowing each one to run for a short time, but this is done so fast that the tasks appear to be done at once. To make the scheduling more flexible ZX81-Forth includes a real-time clock that you can use when setting up tasks, so you can specify when the task starts and how often it repeats. An important omission from ZX81-Forth is that it does not allow you to read from and write to input and output ports. You can include hex machine code in Forth words, so you can write your own Port I/O routines, or you could use memory-mapped I/O devices, but ZX81-Forth would be easier to use for controlling hardware if Port I/O routines were built in. As well as giving you Forth instead of Basic the new ROM changes several other things in the ZX81. ZX81-Forth uses the ASCII character set instead of the ZX81's old non-standard character set, and you have to type all words in full instead of having single-key entry for keywords. As well as Slow and Fast modes ZX81-Forth has another, Auto, which starts in Slow mode but switches to Fast if the program takes more than a quarter of a second to execute. There is also a different format for entering and editing programs which uses a split screen, programs being typed into the upper part of the screen and immediate commands typed into the lower part. DOCUMENTATION The documentation is a 71-page manual that gives instructions for fitting the ROM and for using ZX81-Forth. The sections on using ZX81-Forth are clearly written and include many examples to show how the various features work. Although the manual assumes you will be using a textbook to learn the language it explains its fundamentals in addition to the special features of ZX81-Forth, so you can get started without a textbook. There's a table of contents, but no index or list of Forth words and commands, so it can be difficult to find the definition of any particular word. GETTING STARTED You can buy ZX81-Forth either as a ROM chip which you fit yourself or in a ready-converted computer. The instructions for fitting the ROM are adequate for anyone with a little hardware experience, if the Basic ROM is plugged into a 28-pin socket you just replace it with the Forth ROM. But if the Basic ROM is soldered to the board it has to be unsoldered (very carefully if you want it to work afterwards) and a socket soldered in to take the Forth ROM. The manual shows what you should see on the screen when the ROM has been installed correctly, and it gives some hints on what might be wrong if the computer does not work properly. Once the new ROM is fitted you have to learn how to use ZX81-Forth, but it is not difficult with the aid of the manual and a suitable textbook. IN USE The system used for entering and editing programs is a lot more complicated than in ZX81 Basic, but it does not take long to get used to it. The main difference is that programs have to be typed directly into the upper part of the screen, without line numbers, and for editing you use the cursor keys to move the program listing around on the screen. Programs in ZX81-Forth execute at high speed, generally between 100 and 300 times the speed of ZX81 Basic, it is this that makes multi-tasking possible, and you can have about ten background tasks running before the screen display and the response to the keyboard become too slow to make it feasible to type in more. RELIABILITY I couldn't find anything wrong in the actual Forth, but there are two problems with the editor. The first is if you insert characters into a full program line you will lose the same number of characters off the right hand end of the line instead of having them move to the beginning of the next. The second problem is that it's not possible to backspace past a space in the lower part of the screen. VERDICT ZX81-Forth is non-standard, but its basic features are close enough to FIGForth and Forth-79 to make it easy to use if you are used to one of the standard forms, or to move on to a standard if you start with ZX81-Forth. However, its multi-tasking capabilities make it more useful for converting the ZX81 into a hardware controller than for learning or using Forth. The ROM is well worth the price if you have a practical application for it, or even just to get some more use out of a superannuated ZX81.

ZX Computing

GOING FORTH A ROM cartridge containing 12K of fig-FORTH, a full, RS232 interface via an 8251, and 24 bits of parallel I/O via an 8255, is now available from David Husband's company, Skywave Software. The BASIC ROM is switched out and replaced by the FORTH ROM, which contains all the standard fig-FORTH words and in addition offers a great many other features. Amongst these features are a Z80 assembler for machine code FORTH definitions; a built-in terminal to support a modem and allow access to bulletin-board systems; a machine code monitor which can operate in any number base, carry out simple one-line assembly and disassemble; RS232 and Centronics printer routines, usable from FORTH or BASIC; RAM disc simulation and support of Microdrives; and full use of colour, sound and graphics facilities from FORTH. Produced on a PTH pcb, the cartridge has a standard DB25 RS232 connector for the serial I/O and a strip cable connector for the parallel I/O. The cartridge is suitable for both 16K and 48K versions of the ZX Spectrum. Complete with an extensive manual, the price of the unit is £49.95. For further details get in touch with Skywave Software, [redacted].