| ★ HARDWARE ★ DK'TRONICS 256K SILICONDISC ★ |
Andrew Wilton plugs In the DK Tronics 256K Silicon Disk and sees what it gets up to. DK'tronics have already made a name for themselves among Arnold users with their 64K and 256K RAM add-ons. Now they're offering extra memory in a rather different form - a ramdisc. A ramdisc is, as its name suggests, a cross between an add-on RAM and a disc drive. You can save programs on it, run CP/M on it, and generally do whatever you'd normally do with a disc drive. As far as Arnold is concerned, it is a disc drive. To us, however, there are important differences. THE PACKAGE Silicon Disc itself only works as part of a disc system - you cannot use it with a disc-less 464. It comes in two separate, similarly shaped units, one of which plugs into the other. One of these holds the actual ramdisc, and the other contains the ROMs that make the whole thing work.
As with all expansion port plug-ins, the physical side of interfacing needs at least passing consideration. The review system - a 6128 package - gave some cause for worry on this score. Though the two component units fitted together snugly, the whole assembly seemed none too securely attached to the 6128 edge connector. The units rested on the desk once plugged in - though it wasn't too clear whether this was intentional, or just a consequence of the front module's rather loose grip on the connector. Of course, physically poor connections don't always lead directly to electronic problems. By and large, the review sample behaved itself. Only once did it have serious problems -repeatedly crashing on start-up, or simply refusing to power up at all. The usual solution - turn off, adjust plug-in, turn on again took half an hour and several dozen attempts to sort the problem out. This will come as nothing new to most plug-ins users, but it's still a powerful argument in favour of Vortex-style expansion boards. AMSDOS On power-up, the firmware adds the line "Silicon Disc 1.0" to the machine start-up message. The disc is not active at this stage. Typing sdisc starts the package, implementing it as drive B if you only have a single drive system or as drive C if you have an FD1. From now on it is used in exactly the same way as a normal disc drive. The major difference from a conventional drive is in performance. The increase in speed of saving and loading is dramatic, as the table shows. The improvement in BASIC file handling is not quite of the same order, but will probably be rather more useful. After all, the longest load/save you are ever likely to do still isn't going to take more than 20 seconds with a conventional drive. There are a few other differences in its behaviour as compared to the genuine article. On cataloguing, the normal message "Drive B" is replaced by "sDisc B". This could easily go unnoticed, however, as the initial "254K free" will probably demand rather more of your attention. The extra space is very welcome, but it does have one problem. The firmware features two commands, savedisc and loaddisc, which copy the contents of the Silicon Disc respectively to and from a conventional drive. Once you have 200K or so on the ramdisc the direct transfer out. using savedisc. won't work. What you're left with is a file handling task - a selective transfer, or at best deletion - which could prove very cumbersome under AMSDOS. If this sounds to you like a job for CP/M, then read on. CP/M Implementing Silicon Disc is simple under AMSDOS, but things can be a little trickier under CP/M. While stili in AMSDOS, the external command |setcpm sets up the transient program SDISC.COM on disc. This in turn is used toset up the Silicon Disc once you have entered CP/M. This is not quite the end of the matter, however. To get SDISC to work, you have to relocate CP/M, and to do that you have to use transient commands MOVCPM and SYSGEN from within CP/M. Fortunately you don't have to be a systems hacker to do this, because you don't actually have to understand any of it. All you need to do is follow the instructions in the manual. Once you have your new, relocated CP/M you can implement Silicon Disk just by typing SDISC. There are a few points to note, though. For one thing, you can only use the Silicon Disk with this altered CP/M. For another, some utilities will only work with CP/M in its normal position. Thus you may well end up having two almost indistinguishable versions of CP/M in use - a rich source of confusion, no doubt. None of this is particularly serious stuff - after all. the instructions are quite clear about the move operation. As for the effects of the relocation, 6128 users are quite used to handling different versions of CP/M. They are. however, the ones who will suffer from Silicon Disc's most serious drawback - that it is incompatible with CP/M Plus. COMPATIBILITY? At first, this seems too awful to be true. For a product such as this, produced in a special 6128 version, to be incompatible with the main 6128 operating system is a serious matter. Immediately the Silicon Disc loses many of the major applications for which its extra speed could have been such an asset. So why the incompatibility? The problem, in a nutshell, is this - there is no CP/M Plus equivalent of MOVCPM, and without this you can't relocate CP/M in the way that Silicon Disc requires. If you try doing without and running SDISC anyway, you get a message to the effect that the disc has already been implemented.
Compatibility isn't just a problem for CP/M Plus programs, either. Of the packages that would really benefit from this sort of access speed, a good handful either crashed or failed to load. Pyradev - a big user of disc time during assembly - crashed spectacularly. Minerva's Random Access Database fared similarly, though this was rather less surprising. Most disappointing of all, Caxton's virtual-memory heavyweight Scratchpad Plus refused to load because of the small CP/M 2.2 transient program area. TECHNICALITIES The difficulty with a ramdisc comes in making a piece of RAM behave like a disc drive. The secret of this lies not in the hardware, but rather in the firmware that comes with it. The Silicon Disc itself is physically a chunk of add-on RAM and with the right additional software it should be possible to make it act like one. too. The firmware does not provide for this, however, as it has a rather harder job to do. If we want to communicate with a disc drive, we normally do so via the disc operating system - DOS for short. On the Amstrad disc systems, this means either AMSDOS or CP/M. It is very unusual to access the disk drives by any other means. If 1 the ramdisc can be read from and written to by the DOS in the same way that an additional drive would be, we will not have any problems using the vast majority of software packages with it. The trick, of course, is to make the DOS read from and write to the additional RAM while appearing, to the user at least, to be handling a disc drive. CP/M and AMSDOS were not written with this in mind, so they have to be modified. This is where the Silicon Disc firmware comes in. Two external commands are provided. Isetcpm and |sdisc, one for each operating system. Using these, the process of patching - that is, modifying - each system is quite straightforward. VERDICT There ore doubtless some tasks - spelt checking is an obvious example - for which the Silicon Disc is well suited. Against this one must set those applications for which it clearly isn't. As the comparative figures for TYPE show (see table), the speed difference on disc access is not very significant where the applications concerned have a loi else to do. My impressions of using Pocket Wordstar on fhere tended to confirm this. Of course, backing up was much quicker - but what use is RAM back-up? I suspect that normal backing up — to a conventional disc, that is - would lose you most of the time SD's extra speed gives you. In any cose, normal precautions may not be enough. In addition to occasional unreported write failures and phantom "disc R/O" errors during data handling, the system crashed during a drive-to -drive data transfer. Alarmingly, this happened during the PJP speed trials for the comparison - and the SD manual recommends PIP for this use. If the choice is between the Silicon Disc and a conventional second drive, the second drive must be the clear winner. The FD-1's flexibility, reliability and ease of use put it ahead even if you ignore CP/M Plus. As an extra for a twin-drive system, . Silicon Disk could prove worth the money - but only if yow have at least one definite speed-criticcl job for it to do. Amstrad Action #8 |
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