CRASH - The Online Edition
— Issue 50 Contents|
Tech Niche: Magic buttons
Tech Niche: Utilities
THERE’S a new, very fast, low-cost computer under development — a computer which will run Spectrum software. It’s the Spectrum ‘superclone’ introduced in the CRASH Christmas Special. I’ve now seen the prototype in action — and it works!
The codename of the new machine, which will cost £99.95, is SAM. It runs at almost twice the speed of a normal Spectrum, and can work with 64K to 256K of memory.
SAM will run almost all existing 48K Spectrum software, and should eventually be able to run true 128 software as well, but it’s not just a ‘Spectrum clone’ — despite a proposed price under £100, SAM is a much better computer than an Amstrad Spectrum, and has lots of unique extra features.
Cambridge-based Miles Gordon Technology (MGT), which developed SAM, plans to give the ZX market the biggest boost it’s had since the Spectrum launch six years ago.
The latest SAM prototype is hand-built on two circuit boards, with a plug-in kit keyboard. When launched it will be a small computer console with 48 keys and a magic button on the top.
All the components on the second circuit board will be squashed onto one 1,000-gate logic array. At the moment the circuit is made up from 75 complicated building block chips, put together to prove that the design works — which it certainly does!
SAM improves on the Spectrum display in several respects. For instance, you can display 85 columns of text on a monitor, or a clear 64 columns on most TVs. The display uses a new mode that gives 512 dotes across the screen, and the normal 192 lines; unfortunately it’s not the best mode for games, because you are restricted to just two colours over the whole screen.
But you can get much-improved 16-colour graphics by using another new mode. The display uses the usual 256 points and 192 lines, but the normal Spectrum attribute restriction is relaxed, so you can have any two colours in any row of eight points.
Sinclair and Amstrad Spectrums only allow two colours in any character square (block of 64 points). The new SAM mode means that graphics can use eight times the normal colour resolution, avoiding many attribute problems and allowing prettier displays.
The display can be read from any of the 16 16K pages of memory. This makes SAM much more flexible than the Spectrum 128K, which only lets you put the display in two of its eight pages.
Normally the display dots are read from a 6K area at the start of a page, and the colour information comes from 768 bytes thereafter. But in the new modes SAM has a whole extra 6K of video information, read in parallel with the original 6K but held halfway through the same 16K page.
In colour mode the extra information sets the colours for the corresponding line of eight pixels. In 85-column mode it provides eight two-colour pixels to appear after the eight that have just been read from the normal video area. In other words, groups of eight pixels are read alternately from two display areas, 8K apart.
This sounds hairy for programmers, but in fact it’s not too bad — just add 8K to move from point information to colour information, or toggle bit 13 of the address when moving one group of eight pixels to the next in 512-pixel mode.
SAM uses a souped-up version of the Spectrum Z80A processor. The Z80B in the SAM prototype runs at 6MHz — getting on for twice the speed of a normal Spectrum — and can handle 64-256K of internal RAM.
The first 64K is arranged continuously from the bottom to the top of the Spectrum’s memory map. Two 16K ROMs can be enabled independently at the the top and bottom of the map.
All 16 RAM pages — including the other 12 in a 256K system — can be freely switched into either of the 16K slots in the top half of the memory map.
You do that by writing two page numbers, each between 0 and 16, to port 229. One says which page you want in the top 16K, and the other selects the page for addresses 32768-49151. You can read the values in these ports whenever you want to know how the memory is mapped out.
Arcade games usually keep a constant speed regardless of how much is happening on the screen, by synchronising themselves to the steady output of ‘frames’ of display information. SAM could run much more complicated names than the Spectrum, because of its increased processor power, but most current titles will run on it at the normal speed, perhaps with slightly less graphic flicker.
A complete display frame is sent to the TV or monitor 50 times a second. An interrupt signal tells the program when the next frame starts. Good games may redraw the screen every one, two or three interrupts; the more work to be done, the slower the redrawing.
SAM works faster than a Spectrum, but the extra time saved is usually spent waiting for the next frame, so games don’t necessarily run faster. (Of course, this means they’re still playable.)
Processor-intensive things like utilities and languages run 50-100% faster on SAM than on the Spectrum, depending on the areas of memory used. All SAM memory runs at the same high speed, but some Spectrum RAM areas are slower than others.
I took Hewson’s Uridium down to Miles Gordon Technology’s lab and loaded it into SAM. The commercial cassette uses a high-speed loader, but the worked fine even though its code ran more quickly than normal in SAM’s memory. Presumably the loader, like most modern tape routines, automatically works out the ratio of the tape and memory speeds by analysing the leader tone.
SAM is designed to load tapes in the Spectrum format at normal or high speed. Tape files are usually written out at 2250 baud — 50% faster than Spectrum — but you can load normal commercial 1500-baud tapes too. SAM will sense the speed automatically.
Once loaded Uridium, ran at the usual speed, with a slightly wider screen than usual. (The differences inside SAM make the vertical borders narrower than on a Spectrum, though the display still fits comfortably on a TV.)
Uridium redraws everything after each three frames, or 17 times a second. That’s about the slowest rate that will fool the brain into thinking it sees smooth movement.
The SAM display was flicker-free, and colours were solid and unwavering on both the TV an monitor screen, with no sign of the colour fringe that sometimes appear at the edge of Spectrum graphics. (These fringes are caused by timing mistakes in the Spectrum ULA, particularly on early machines; SAM has no such trouble because does not contain any Sinclair parts. To be fair to the Spectrum’s original designers, though, MGT has much more powerful chips to play with.) Compatibility was not quite perfect — the 48K version of Elite loaded, but crashed after the Lenslok test. (This may be a particularly fussy program, as it does the same in 48K mode on a 128.) SAM seemed to cope fine with other 48K Spectrum games, such as JetPac and Gauntlet, and is definitely more often compatible with them than is Amstrad’s +3.
SAM can handle up to 256K of internal memory, and in theory has facilities to use another 512K outside the computer. But the memory-management system is not the same as Sinclair’s, so games specifically written for the 128 will require small but fiddly alterations.
It will be a while before SAM is compatible with 128 games, but true 128K titles are still rare. Software houses already writing for the 128 should find it easy to make new programs compatible.
Eventually SAM should be able to run many of the half-finished programs that were developed for the Spectrum but never released because the Sinclair machine couldn’t run them at a sensible speed. I’m sure a few software houses have such titles up their sleeves, just waiting for extra processor power!
When Miles Gordon Technology was set up in 1986 to build SAM, there were several groups designing Spectrum clones. At first MGT was just two people — Bruce Gordon, a Scots hardware-designer with more than 20 years experience designing computers and interfaces, and Alan Miles, a former teacher and manager.
In the mid-70s Gordon ran his own firm making word processors. When micros arrived he switched to Spectrum developments, selling the Gordon Microframe — the first incarnation of the Disciple interface.
Alan Miles spend the 70s in teaching and publishing. He joined the micro industry in 1983, selling thousands of computers into the Middle East.
MGT was born after the shake-out of 1986 when Sinclair Research sold out to Amstrad, giving Amstrad rights to manufacture the Spectrum. MGT started work on SAM very quickly, hut early efforts were concentrated on the Disciple, a multipurpose Spectrum interface. The Disciple is till on sale, and includes several features that are next now part of SAM.
Late last year Bruce Gordon designed the Plus D, a £50 disk and printer interface which MGT sells directly (it was reviewed in last month’s CRASH). Now the profits from PlusD sales are financing the development of SAM.
The SAM prototype doesn’t have an edge connector, so I wasn’t able to test it by plugging in Spectrum hardware add-one. But the designer, Bruce Gordon, has already designed add-ons such as the Plus D and Disciple disk systems, so he has a good idea of how the edge connector works.
Gordon doesn’t guarantee compatibility, but says that the same signals are available, with the exception of the +12V and -5V supplies. SAM only uses port addresses reserved by Sinclair, so most third-party gadgets should work. (Tech Niche will report back on this aspect as soon as MGT has added an edge connector.)
SAM has quite an array of built-in ports. There are two network ports, as on the Disciple, which allow several computers to share a disk and printer. But serial and parallel printer ports are extras — they’re not built into SAM as they are into the +3. Unlike the 128’s, SAM’s MIDI port lets you read musical information as well as write it. It’s also compatible with DADI, the new standard for Domestic Appliance Digital Interfaces — maybe you really will be able to control a power station with your micro!
The cassette port uses a single socket for both input and output, but you can put two plugs on the tape-recorder end if you don’t want to have to plug and unplug things as you load and save. This arrangement works with any tape recorder that suits the Spectrum — tape loading seemed reliable when I tried it, even with turbo loaders.
There are separate outputs for monitors, video composite monitors, UHF TV and line-level sound.
There is one 9-pin joystick port, simulating keys 6-0, and the logic contains pulse counters so that the same port will work with a mouse, as on the Atari ST.
The last connector lets you plug in a light pen. Registers on the logic array tell you exactly where the TV display is being scanned at any instant; they freeze when the light pen detects the spot on the screen, so you can tell exactly where it was pointing. If you haven’t got a light pen, or don’t want cramp, you can use the registers to avoid graphics flicker.
The sound chip in the SAM prototype is the same as in the 128, but it uses different port addresses, so it doesn’t recognise three-part music in games designed to use the 128’s AY-3-8912 chip. This could be fixed, but at the moment SAM’s designers would prefer to use a better chip.
Their favourite is the Yamaha SAA1988, a stereo sound chip with an eight-octave range. It can play up to six notes and two ‘noises’ at once, through independent stereo channels. It outperforms the AY-3-8912, but still has a ‘computer organ’ sound. I’d be more impressed if they’d put a simple 8-bit digital-to-analogue converter on the board, like the output stage of a SpecDrum or Sampler, as explained elsewhere in this Tech Niche special.
Whatever the sound chip, single-channel 48K sound effects will work on SAM just as normal. Sound is replayed through the TV speaker. Like a 128, SAM does not contain a loudspeaker — or even a quiet one!
The disk system for SAM is still under wraps, though the interface is already designed into the logic array.
The system is based on MGT’s Plus D, reviewed in last month’s CRASH, but uses new quad-density 3.5-inch disk drives. These use normal disks but pack 1560K into each one, reading and writing twice as fast as the Plus D and Swift Disc and about five times faster than Amstrad’s +3. MGT intend to sell this drive as an optional add-on, at the same price as the computer: £99.95.