Your Spectrum
Issue 7, September 1984 - 64k Spectrum
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Recent models of the ZX Spectrum have come supplied with 64K of internal memory, rather than the normal 48K. But, reports Simon Goodwin, the extra RAM is deliberately disabled so that you can't use it ...
There was a great fuss when an earlier change in the design of the computer meant that Issue 3 machines used a subtly different keyboard interface; a few programs which had worked on earlier computers ignored the keys of an Issue 3 machine. This change - minor though it was - attracted considerable flak and Sinclair Research became coy about subsequent alterations to the machine.
But the design didn't stay fixed, both for reasons of performance and - perhaps more crucially - economics. Code letters were added to the issue numbers, so that revisions could be recorded without disturbing the punters.


The first re-design was dubbed the Issue 3 Mark 4, and can be distinguished by the roman numerals 'iv' printed on the circuit board after the issue number. In this case the change involved is believed to be slight, and essentially only of interest to repair-engineers. The next and - to the best of our knowledge - latest change has been rather more fundamental.
The issue 3 Mark 5 Spectrum contains just eight 64K memory chips. Improvements in the production of large memories had already forced other manufacturers to use full-blown 64K chips in 32K computers - one half was just left idle. The economics of this arrangement didn't upset the makers much, since the price of all the large memories had fallen steadily and half- faulty chips had ended up at almost the same price as fully-working ones.
The need for an 'upgradeable' design had decreased since most users were buying 48K machines at once, rather than purchasing the 16K model and upgrading it to 48K later. At first the price difference between models was £50 - now it has fallen to £30, and the 48K computer is selling for roughly the original price of the 16K machine. It also now sells about ten times as well as the 16K version.
The two-stage memory design was costly. It was intended to allow dealers to plug in an extra 32K, but in fact it ends up creating extra assembly work - since Sinclair Research has to plug in the extra RAM before the machine can be sold. A new design was produced, using 64K chips throughout, with no facility to remove chips and move back down to 16K.
To leave room for the 16K ROM, Sinclair Research had to disable a quarter of each 64K chip. This still gave the usual 48K, but with half the previous number of components - simplifying the manufacture, reducing power-consumption and saving money. This means that some of us may have an extra 16K of RAM lurking in the depths of our 48K
The latest version of our beloved Speccy, the Issue 3 Mark 5, can be distinguished by a letter 'v' printed on the circuit board after the issue number. Perhaps more noticeable is the revised board layout, which leaves the computer with only eight RAM chips fitted, rather than the 16 used in earlier versions of the 48K Spectrum. (Usual warning. Bear in mind that you could void your guarantee if Sinclair Research realises you've taken the computer apart.)
The original Spectrum design contained 16K bytes of memory, supplied in eight chips. Each chip contained 16K bits and, since there are eight bits in a byte, everything added up. The 48K version of the computer used another eight RAM chips to provide a further 32K bytes of memory. At first (on Issue 1 Spectrums) these additional chips were fitted on an extra plug-in board inside the computer. Later, a re-design ensured that the 32K chips could be fitted directly into the main board.


The snag - from Sinclair Research's point of view - was that 32K chips were expensive, and to understand this you need to know a little about the way RAM is manufactured. For various reasons, memory chips are made on square pieces of silicon. In principle each chip is just a matrix of memory cells and, since the matrix is square, a 'double-size' chip contains four times as many cells as a regular one - the size is doubled both vertically and horizontally.
When micros first became feasible, memory chips contained 1024 cells - 1K bits. Antique collectors among you may recall the '2102' chip - eight of these were needed to produce 1K byte, since there are eight bits in a byte. It wasn't long before 4K bit chips such as the 4007 and 2114 were available. Then everything got scaled up once again, and we were left with the 4116, the 16K bit chip which was - until recently - the industry standard.
Anyhow, the sequence of standard memory sizes goes directly from 16K to 64K, and it isn't easy to get hold of 32K components. One firm which created a demand for 32K RAMs was Tandy; it wanted to put 32K of memory into the eight sockets in its Colour Computer.


This is where things get complicated. Despite the square shape of chips the cells are usually arranged on the silicon in two rectangular groups of 8K, 32K or whatever. The chip industry has a very high failure rate, especially on new types of component, so many 64K chips were rejected because they were 'flawed' -
... You could even run the CP/M operating system on a Spectrum. That would upset a few manufacturers!

some of the cells didn't work.
Tandy was able to test each half of the 64K chips separately, and sift out the ones which were half-working - hey presto, a 32K RAM chip. Meanwhile, in the UK, Dragon Data had to put up with double the amount of soldering and the extra circuit-space needed to provide 32K of memory in two 16K lumps.
A similar situation must have faced Sinclair Research when the 48K Spectrum was designed. Rather than try to cram an extra 16 chips into the already- crowded computer, it chose to use eight 32K chips.
An alternative solution appears in the Acorn Electron, which uses four 64K bit chips and special hardware to read 32K bytes from them in two steps. This is cheap and simple, but it slows down the computer since each chip must be read twice to fetch a single byte.


So the first three issues of the Spectrum used a combination of eight 16K chips and eight 32K ones. The latest machines depart from that combination, but Sinclair Research has been very quiet about the alteration.

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sandwich toasters. The snag is that you can't use the extra memory, even though it's powered up, tested and in full working order. There's apparently no difference in performance between a 48K Spectrum and a 64K Mark 5 machine.


The Spectrum uses a Z80 processor, which can only address - or control - 64K of memory directly. The computer contains 16K of vital ROM without which it would completely ignore the user, so we're only left with an absolute maximum of 48K. This 64K limit is rather like having a postman who can only remember house numbers of up to three digits - bad news if you live at number 1175, since your post ends up at number 175 (I once had a house in Birmingham where precisely this happened).
You can get around the problem by having a new postman who can handle longer numbers (that is, a new processor - such as the QL's 68008 - which can address 1024K). Alternatively, you
could duplicate some addresses and teach the postman to choose between the duplicates, according to where they had been previously. In computers, this technique is called 'paging': it's used in the ZX81 add-on boards made by Memotech.


So far it may seem as if there are no real advantages in owning a Mark 5 Spectrum. This is true, unless you're interested in voiding your warranty and digging around inside the computer. Hardware modifications will certainly be needed to bring the extra 16K under software control, but they'll be quite simple - certainly less complicated than the 'SoftROM' project presented in issue 2 of YS. That design allowed users to change the contents of the ROM area by putting 16K of RAM in its place; perhaps Mike Lord will come up with a revised project for Mark 5 Spectrum owners?
Once you've got your extra RAM under software control, the possibilities are endless. The extra space can be used for machine code or other languages (so long as you don't destroy crucial information such as the font, and the code used to handle the keyboard and display). You could just use the memory to correct the bugs in the Spectrum ROM, or as a neat way of patching in extra routines of your own. If you can re-locate the Spectrum screen display and re-write the device drivers (which handle the keys,
display, printer and so on) - you could even run the CP/M operating system on a Spectrum. That would upset a few manufacturers!
Another approach would be to use paging to provide a choice between two 16K 'banks' of memory; an OUT instruction could be used to swap the banks. With care this would allow you to use 64K of RAM as well as the 16K ROM, but commercial software wouldn't use the facility and you'd have to adapt your own programs to select different banks as required. Your Spectrum would behave as if it had 16K ROM, 32K of normal RAM plus two 16K RAM lumps which used the same addresses and could only be accessed one at a time.


There may be other reasons why Sinclair Research has kept quiet about the redesign of the Spectrum. It could be a sign that the 16K Spectrum will soon be discontinued - certainly the upgrade procedure from a 16K to 48K machine will have to change, since there'll be no room f6r the 16K chips once the 64K ones are fitted. It's also possible that the retail price of the Spectrum could be cut further, now that the new design is on the market. The production cost of a Spectrum should drop if only eight memories are used.
If you find any secret quirks of the Mark 5 Spectrum, YS would like to hear about it. And if you've just bought a Mark 6 machine ...
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