The ZX Spec­trum ULA: How to De­sign a Mi­cro­com­puter

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With my ZX Spec­trum Next due to ship any day now, I’ve been re­fresh­ing my mem­ory on the clever cre­ation of the orig­i­nal Sin­clair ZX Spec­trum, cour­tesy of Chris Smith’s book, The ZX Spec­trum ULA: How to De­sign a Mi­cro­com­puter.

It’s not a cof­fee ta­ble book. You’ll find no colour­ful glam­our shots of mi­cro­com­put­ers, or sim­u­lated screen­grabs with clever CRTem­u­lat­ing free alias­ing and scan­lines. In­stead, you’ll find an in-depth tech­ni­cal look at the heart of the Spec­trum. In­cred­i­bly, Smith hadn’t even hit his teens when the Spec­trum launched, and was in no way in­volved in its cre­ation. With only a lit­tle help from orig­i­nal en­gi­neer Richard Alt­wasser and oth­ers in the Spec­trum com­mu­nity, he’s worked to re­verse en­gi­neer the de­vice from first prin­ci­ples.

For any­one un­fa­mil­iar with the ULA, it’s ef­fec­tively the se­cret sauce of how Sir Clive Sin­clair was able to bring his 8-bit mi­cro­com­put­ers to mar­ket at such a low cost. De­signed by Scot­tish en­gi­neer­ing firm Fer­ranti, the Un­com­mit­ted Logic Ar­ray (ULA) was a pre­cur­sor to the mod­ern field pro­gram­mable gate ar­ray (FPGA) – an in­te­grated cir­cuit that could be mod­i­fied to act in a par­tic­u­lar way; in the case of the ULA, by per­ma­nently mask­ing off par­tic­u­lar sec­tions.

A Fer­ranti ULA was be­hind the dra­matic re­duc­tion in chip count be­tween the ZX80 and the ZX81, and it’s no sur­prise to find one in­side ev­ery ZX Spec­trum. As Smith shows in his book, though, it’s the ULA that re­ally makes the ma­chine – far more so than the Spec­trum’s Zilog Z80 pro­ces­sor or cheaper com­pat­i­ble, which could be read­ily found in any num­ber of com­pet­ing de­signs.

Be­gin­ning with a tech­ni­cal over­view of in­te­grated cir­cuits (ICs), mi­cro­com­put­ers, semi-cus­tom ICs and the Fer­ranti ULA fam­ily it­self, Smith of­fers a full break­down of ex­actly what makes the Spec­trum tick. The ZX Spec­trum ULA is a trea­sure trove of hard-won tech­ni­cal in­for­ma­tion, from de­tails of key­board ad­dress­ing and in­ter­rupts, through to the video dis­play – an iconic sys­tem that stored bit­maps and their colours sep­a­rately, al­low­ing for greater de­tail in lower mem­ory at the cost of colour clash, also known as at­tribute clash, where blocks of colour would fol­low a sprite around the play­field.

Even if your eyes glaze while at­tempt­ing to fol­low the math­e­mat­i­cal proofs con­tained in Ap­pendix D, cov­er­ing quad­ra­ture am­pli­tude mod­u­la­tion, there’s plenty of in­ter­est for the Spec­trum fan. For ex­am­ple, there’s a chap­ter on hid­den fea­tures and er­rors, cov­er­ing ‘phan­tom’ keys and the snow ef­fect, a con­tention is­sue that causes parts of the screen to white out as if there were a snow­storm in your TV.

Smith deals with an ex­tremely tech­ni­cal sub­ject, and the re­sult is a deeply tech­ni­cal book. If you’re look­ing for a trip down Nos­tal­gia Boule­vard, you’ll be bet­ter off with Chris Wilkins’ colour­ful The Story of the ZX Spec­trum in Pix­els or Bitmap Books’ Sin­clair ZX Spec­trum: A Vis­ual Com­pen­dium.

But if you have lots of tech know-how or, like my­self, fancy skim­ming through the pages and pre­tend­ing to un­der­stand half of it, you won’t find a bet­ter or more de­tailed anal­y­sis. The ZX Spec­trum ULA is avail­able from www.

ama­ for £23.18 (VAT ex­empt).

There aren’t any shiny colour plates, but the di­a­grams are clear and in­for­ma­tive

You’ll find no bet­ter ref­er­ence to the Fer­ranti magic that made the ZX Spec­trum pos­si­ble

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