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Atari 520ST

16-bit Home Computer

Not just another home computer; more like the future of the entire domestic micro industry, not to mention the MIDI software one. Simon Trask has the details.

From the home of Space Invaders and Pacman comes a micro so powerful, it could turn the home computer market on its head. Better still, it's got MIDI on the back.

It's unfortunate, I know, but up until recently - and much to the chagrin of musicians interested in using computers for musical ends - the average home micro has sounded dismal. People have tried, some of them very hard, to extract something useful from the dreaded internal sound chip, but it's been an uphill struggle with not an awfully big reward at the top.

Come the MIDI age, though, and both these limitations have been neatly side-stepped. Suddenly, the world and his drum machine know just how good computers can be at storing and manipulating music. And as experience with MIDI has increased, so MIDI software has improved to the point where current packages are stretching today's eight-bit micros - Commodore, Beeb and so on - to their absolute limit.

The obvious next step is to write for computers that provide both greater processing power and increased memory: the new 16-bit ones, in other words. Now, the IBM PC and Apple Macintosh have both received much attention from software houses in the US of A - the Mac has even been graced with the attentions of E-mu Systems and Kurzweil. However, neither of these machines has a particularly high profile over here, and their pricing puts them firmly in the business, rather than the domestic, domain.

The machine that could well fill the above role is a new sub-£1000 16-bit micro from Atari, the people who brought you Space Invaders, Pacman and, up till now, nothing of any musical interest whatsoever. The computer in question goes by the name of 520ST, and has an impressive 512K of onboard RAM. However, a planned 128K machine (the 130ST) has been abandoned, a decision that may turn out to be unfortunate. An initial package of 520 computer, 360K (formatted) 3.5" floppy disk drive and monochrome monitor - together with BASIC and Logo programming languages - is planned to go on sale in mid-August for £750, which does sound like good value for money. It seems a later package will replace the mono monitor with a colour version, while a 1 Mbyte floppy disk drive and a 10Mbyte hard disk unit are scheduled for separate release in August and September respectively - prices for these aren't yet known. As if that weren't enough good news, the ST will also feature the GEM, or Graphics Environment Manager, an operating system developed by American company Digital Research as a rival to the sort of graphics user-environment introduced by Apple with their Lisa and Macintosh computers.

Aside from any other considerations, this machine has attracted the attention of musicians because it has an inbuilt MIDI capability - a world first outside Yamaha's CX5M, and confirmation that the computer world isn't treating music as an afterthought any more. Jack Tramiel's company aren't going to have things all their own way, though, because Commodore also have a 16-bit contender, the Amiga, waiting in the wings. That too will have MIDI tacked on to its hindquarters, but its release is still some months away; the 520ST is imminent.

Only problem is, the MIDI software that'll be available for STs isn't yet on public view. There's a possibility the Island Logic people will be hired to write some of it here in the UK, while further packages are expected in from the US in the near future. It should be worth waiting for, though, as it's rumoured to make full use of the graphics windowing and icon techniques that the aforementioned GEM environment makes possible, and which have characterised both Island Logic's previous work on eight-bit micros and the music packages currently available for the likes of the Apple Macintosh.

From the Outside

With the ST, Atari have gone for the sleek yet understated look beloved of the business micro fraternity - to whom the machine should sell in large numbers, given the right software support. The keyboard is suitably professional in feel, and an 18-key numeric keypad and cursor keypad are provided to the right of the standard QWERTY layout. Across the top of the keyboard are ten function keys, and behind these - in the same slanting pose - lies a plentiful supply of ventilation slots. The ST peripherals are designed in a matching style, and that gives the range an integrated appearance that'll no doubt be the envy of many a UK designer.

The ST has three screen modes: 320x200 pixel resolution with a maximum 16 colours on screen, 640x200 resolution with up to four colours, and 640x400 resolution which gives a monochrome picture. Display memory resides as a 32K block of main memory, and an amazing 512 colours are available for placing on the colour palette. This doubles Atari's earlier record of 256 colours, and leaves other micros standing, though early STs won't let you witness them on either the supplied mono monitor or a colour TV.

Talking of graphics capability brings us nicely back to the GEM graphics environment. Most of GEM is written in a high-level language called C, and is designed to be easily transportable across 16-bit machines, working independently of both operating system and languages. On the ST, GEM sits on top of the unfortunately-named TOS (for Tramiel Operating System), which is basically an extended CPM-68K BIOS.

Figure 1. GEM software memory map.

Figure 1 shows an overview of GEM and the various levels at which it operates. There are two ways the programmer can interface with GEM: the Virtual Device Interface (VDI) and the Application Environment Services (AES). The former provides standardised means of access to the ST's graphics capabilities, while the latter allows you access to all GEM's graphics features - like graphics handling facilities for monitoring mouse movement (Rentokil eat your heart out), translation of text data into pull-down menus, and the creation, storage and retrieval of elements such as windows and icons.

In addition to all those windowing, icon, and pull-down menu facilities, the system also provides a 'mouse' controller which plugs into a joystick port. The mouse's movements across a physical plane are translated by software into screen positions, and this enables a small pointer arrow to be positioned at any icon or word on the screen. You then press a button on the mouse to select the option shown.

This system really is a quantum leap in user-friendliness, and a crucial blow against the tyranny of the QWERTY keyboard - though it's still possible to use the cursor keys or a joystick in place of the mouse if you're a traditionalist. On a more general level, I'm convinced that GEM (together with rivals such as IBM's Topview and Microsoft's Windows) represents the immediate future direction in computer graphics, for both end-users and software writers. With British firms Sinclair, Acorn and ACT also showing interest in GEM, we'll probably all be conversant with windows, mice and icons before the year is out. But Atari will be there first.

"We should see some MIDI software that's easier to use, offers more musical memory, and provides more facilities than today's packages."


Enough of what comes up on-screen. Let's delve a little deeper into the internals that make the new Atari tick. At the heart of the ST system lies an 8MHz Motorola MC68000 processor. For the bit-hungry, this has a 16-bit data bus, a 24-bit address bus and 32-bit internal architecture - with eight 32-bit data registers and eight 32-bit address registers. It's closely related to the 68008 processor used in Sinclair's QL, the main difference being that the latter has an eight-bit data bus. Other features of the 68000 are seven levels of interrupt, 14 addressing modes, five data types, memory-mapped I/O and 56 instructions.

Directly addressable memory range totals 16Mbyte, but don't get carried away with thoughts of multi-million-note MIDI sequencing programs. The 520ST has 512K of RAM onboard, which a quick spot of arithmetic will tell you is still eight times the 64K maximum memory addressable by an eight-bit processor.

Figure 2. Ataru ST System BLock Diagram.

Happy to report, the ST scores very well on interconnection facilities (see Figure 2). The side panels contain an expansion ROM slot (giving 128K extra ROM) and two joystick ports, one of which also acts as the mouse port. These ports are read by the so-called Intelligent Keyboard (or 'ikbd') controller, which is also responsible for scanning the keyboard. All 'ikbd' functions are handled by a 1 MHz HD6301VI eight-bit processor, which in turn communicates with an MC6850 ACIA.

Along the rear panel are video ports for output to both television and monitor (the latter supported by RGB and composite video outputs for colour and monochrome capability), a Centronics parallel interface port for printers, an RS232C interface for communication via modem (with data transfer rates ranging from 50 to 19200 baud), MIDI In and Out/Thru sockets (the five-pin DINs that'll be familiar to even the most computer-illiterate muso), and floppy and hard disk drive parallel interfaces.

An MK68901 Multi Function Peripheral chip supports the Centronics and RS232 interfaces, and provides interrupt control for these together with the disk drive controller, intelligent keyboard controller and MIDI (the MK68901 itself occupies the highest-but-one position in the 68000's seven interrupt priority levels).

A Western Digital floppy disk controller is provided onboard, and the good news here is that two floppy disk drives can be daisy-chained together. Both floppy and hard disk interfaces make use of DMA (Direct Memory Access) for fast data interchange. Puzzled? Well, DMA is a technique whereby data may be transferred between a peripheral and main memory without having to pass through the main processor. DMA speed for the ST hard disk is an amazing 1.33 Megabytes per second, and as mentioned earlier, the drive which Atari are planning to introduce will have a storage capability of 10 Megabytes.

But even this amount of memory, once an outlandish figure in the daydreams of computer buffs, seems small next to the capacity of the Atari CD ROM unit. This is similar to the Compact Disc systems currently available in the audio market, except that since we're talking digital information here, not analogue music, no digital-to-analogue conversion is necessary. One CD can store up to half a gigabyte of data, which you've got to admit is pretty impressive. However, the sad fact is that current laser-disc technology is capable only of reading from disc, not writing to it, so ROM is all we'll be getting on CD for the time being. Still, rumour has it that Atari are planning to put the complete Encyclopedia Britannica on one CD, which is as good a place as any to start, I suppose. The company have just exhibited a prototype CD ROM system at the summer Consumer Electronics Show in the States, and the production model should be on sale over here before the end of the year. Price has yet to be finalised.

Incidentally, Atari are not alone in this field, as Hitachi have just produced a CD ROM unit with a parallel interface for the IBM PC, and interfaces for other machines are expected later in the year, so this is evidently an emerging technology.


There are two sides to the ST's musical capabilities. Because as well as MIDI, Atari have seen fit to include the General Instruments AY-3-8910 Programmable Sound Generator, a positively aged device that's most recently seen service in Amstrad computers. The chip offers three independent channels of sound, each with a standard amplitude ADSR and each capable of generating a square wave at frequencies over a range of between 30Hz and a bat-favourite 125KHz. A single noise generator register is provided, and noise can be mixed in with the voice channels.

Sounds awfully familiar, doesn't it? Actually, I can't help feeling the AY's inclusion is a trifle redundant on a machine such as the ST, because although it may prove useful to the micro owner with a passing interest in music, the heart of the matter for the musician is the inclusion of an inbuilt MIDI interface.

"The internal chip might be useful to some, but the heart of the matter for the musician is the ST's inbuilt MIDI interface."

And the Atari doesn't stop at providing the requisite DIN sockets mentioned above. As already intimated, the ST's operating system is basically an extended CP/M-68K BIOS, and it functions through BIOS vectored calls, with parameter values passed and returned through the 68000's registers. Among the extended BIOS functions are several dealing with access to the MIDI ports. Two of these functions send and receive a byte to/from the MIDI ports (specifically, they place a byte in an output or input queue, and an automatic interrupt routine handles the appropriate transfer). Other functions are available for determining MIDI byte ready-to-send and ready-to-receive status, and for setting the MIDI buffer location and size.

At the hardware level, MIDI communication is controlled by a familiar MC6850 ACIA (Asynchronous Communications Interface Adaptor) supplied with transmit and receive clock inputs of 500kHz. As most of you will know, the MIDI data transfer rate is 31.25k Baud, and the ST generates this simply by setting the ACIA Counter Divide Select to divide by 16.

Now, all this will mean little unless you're already familiar with MIDI hardware standards. A lot of musicians aren't (some just don't want to be), so with luck, access to the Atari's MIDI will be made a lot easier by the various programming languages that will become available for the ST. Word has it the MIDI software under development in the States is being written in Forth, and as it's ridiculously easy to build up libraries of routines in this language (or dictionaries of words, to use the Forth terminology), it's not inconceivable that a set of MIDI routines could be released for use by anyone in their own programs.

Still on the software side of things, the BASIC supplied as standard with the ST is a version of Digital Research's Personal BASIC, but as Atari UK have yet to receive a command word list, it's not clear whether this has been expanded to take account of MIDI. As the man on the news bulletin said, we'll bring you more details as soon as they become available.

The main objection I have to the ST's implementation of MIDI is simply that it is just that - nothing more. Put it this way. A lot of the currently-available MIDI interface boxes that upgrade an existing micro to the MIDI standard have quite a bit more in the way of hardware than that description might imply. Some of them have line sync facilities to allow non-MIDI drum machines of various clock rates to be included in a MIDI setup; some also offer a sync-to-tape facility. Others provide more than one MIDI Out port, to ease cabling and to avoid the sort of sequencing problem you get when your synth has no MIDI Thru and you want to drive an expander or a drum machine, say. There's also the trend among designers of dedicated sequencers to provide a MIDI Out for each sequencer track, which sounds a bit ironic for a system that has the capability to transmit all its information along a single cable, but nevertheless has a good deal of practical logic to it.

The upshot of all this is that putting MIDI In and MIDI Out sockets on the rear of a computer - which is what Atari have done with the ST - doesn't necessarily make the machine an ideal centre for musical communications. In their defence, it's worth saying that this isn't really what Atari intend the ST's version of MIDI to do. But it could have been designed to do it, and if it had, it would have transformed an already wonderful computer into a modern musical powerhouse.

As things stand, we're left with Atari's suggestion that seeing as MIDI provides a high-speed serial interface capability, there's no reason why the ST's MIDI ports can't be used for local area networking. It doesn't help the musician, but it does make sense.


There's no question that the ST is a very impressive computer. So impressive, in fact, that it could revolutionise a personal computing market in which other computers have either failed (Sinclair's QL) or not yet materialised (the 68000-based offerings from Commodore and Amstrad, for instance).

The 'budget package' approach that's worked so well for Amstrad should do the same for Atari, though I still feel the decision not to go ahead with the 128K version of the ST could prove to be a mistake. Why? Because no matter how good the machine is, the £700-800 price-tag for the initial 520ST package still puts it firmly in the lower-end business machine range. It's a lot for a home user to pay, and a cheaper 128K version (with the option to upgrade to 512K, of course) could only be a good thing.

The user-friendly GEM graphics front-end is bound to be a big attraction (especially as it's on ROM), as will the availability of a large number of tried and tested language implementations, and other software packages courtesy of Atari's licensing agreement with Digital Research. If you're a programmer and you've always wanted to get into Forth, C, Pascal, Lisp or even Cobol, this could be your big opportunity. And of course, there's always 68000 assembler for those of sado-masochistic tendencies. Apparently, no less than four 68000 packages will be available from third-party suppliers, so no hiccups there.

Aside from the large amount of memory already onboard the ST, a further attraction is the prospect of an affordable 10MByte hard disk with a direct memory access speed of 1.33 Megabytes per second.

As for the music, the significant thing about the Atari is not that it has a good implementation of musical facilities (it hasn't), but that the now universal MIDI standard has been applied to a micro that's got more all-round competence than anything else available to the home user. And what we should see as a result is some MIDI software that's much easier to use, offers far more musical memory, and provides a bigger range of facilities than any package written for today's machines. That, to me anyway, is some prospect.

Further details on the 520ST from Atari UK, (Contact Details).

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jul 1985

Computer Musician

Gear in this article:

Computer > Atari > ST

Review by Simon Trask

Previous article in this issue:

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