Part Four – The Joy Of ZX & Hex
16
Back at college, access to machine language was the final step
needed for us to begin creating some proper games at last. Soon, the cassette
buffer became too confined for serious programs, and we worked out how to force
the PET to leave a section of main memory free for us to use for code, and how
to make the cassette load and save routines work with data that wasn't lines of
BASIC code. And I learned to count again…
It turned out that because of the way data is organised inside
computers, CPUs like things arranged in clumps of powers of two, and so quite
often you needed to refer to things in clumps of twos or fours or eights or
sixteens.
Counting stuff that needs to be laid out that way is a bit
awkward using human-familiar base-10 counting. If you choose a different number
base (in this case base 16 - 16 being a power of 2) then inspecting a computer's
memory, and dealing with things like the op-codes of a machine language,
actually becomes a lot easier.
Six sides good, er, one side bad. Ten bases
bad, we mean. We think.
Base 16 is called ‘hexadecimal’, and all it means is that you
add in a few extra digits between 9 and 10. It's natural enough to use letters
of the alphabet for these extra digits, so you count
‘0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,10’. When you reach 99, instead of the next
number being 100, you count ‘A0, A1, A2’... and so on, until you reach ‘FF’, and
only then do you get to 100.
It may not seem obvious HOW that makes things easier, so here
are a couple of examples…
A computer holds data arranged in ‘bytes’, each of which
contains eight ‘bits’ of binary data. The number of discrete states that can be
held in 8 bits - and therefore the maximum number of numbers that one 8-bit byte
can hold - is given by 2 to the power 8, or 256. The number ‘256’ isn't a
particularly pleasing or round one in decimal, but in hex it's a nice round
100.
Likewise the number of bytes addressable by an 8-bit CPU like
the 6502 in the PET is 65,536. Again, an awkward number in decimal, but in hex a
nice round 10000. So, if you're inspecting computer memory, a 10x10 table of
memory contents printed in decimal would look confusing indeed, but a 16x16
table printed in hexadecimal would look much less so, and make it easier to find
numbers based on the kind of 4, 8 and 16-number boundaries that often occur
inside the machine.
You could actually ‘read’ machine language directly fairly
easily if you used hex. To the CPU, any number in a byte could be a command to
do something - and since a byte holds 256 numbers, that meant that potentially
there were 256 instructions the machine could understand.
There y’go. That oughtta explain
everything.
Consider the ‘LD A’ command I used as an example earlier - in
decimal that’s instruction 169, so if you wanted to look it up in the list of
instructions you'd have to count through to the 169th entry, which is a bit
awkward.
But if you have a table of op-codes laid out in a 16x16 grid,
and you label the rows and columns from 0 to F in hexadecimal, and look at the
op-code itself in hexadecimal, you can quickly see it’s instruction ‘A9’. You
just look in row A, column 9 and there's your instruction - easy.
Really, it is. Take my word for it. Honestly.
So, I wrote a little program (a ‘monitor’) to let me enter
numbers into memory in hexadecimal, and to look at memory in hexadecimal -
something that other machines had built in. This made entering and testing
machine language programs much easier than all that mucking about with BASIC and
POKEing values into memory by hand.
At first we used machine language sparingly - just little
routines called from a still predominantly BASIC program to assist with the
things that BASIC was slow to do, like drawing large blocks of graphics data.
This allowed for bigger objects onscreen in our games. These hybrid programs
were an ideal way for us to get used to harnessing the power of the CPU
directly. Programming in machine language was a lot more painstaking than
writing code in BASIC, and it would be a while before any of us would attempt an
entire game coded that way.
17
There was a degree of life outside college. My seventeenth
birthday rolled around and I celebrated it by going hang-gliding near
Marlborough. Gary Numan, the God of the geeks, came around touring the ‘Telekon’
album, and I went to the first music concert of my life, seeing him at the
Mayflower in Southampton.
All of us geeks were major sci-fi nuts, and so the release of
‘The Empire Strikes Back’ warranted an expedition up to London to see it at the
cinema in Leicester Square. It was excellent - I thought the Taun-Tauns were
cute, and was quite sad when Luke had to slice one of them open.
*Sniff*… Still, kept him warm.
Best of all, we got to spend time afterwards hanging out in
the numerous arcades in that part of London. There was one that was a particular
favourite of mine – the Piccadilly Arcade (now long since demolished and its
location buried underneath the Trocadero). The Piccadilly Arcade was a proper
arcade – the kind your parents warned you about. Dingy and filled with flashing
machines and dodgy characters (indeed I remember hearing on the news once that
someone got stabbed in there). But there was nothing seedy in my interest. It
was simply the fact that they had lots and lots of good games. I played Targ and
Galaxian and Space Invaders and sit-down Turbo... But there was one game in
there which got most of my attention.
It was a big hulking sit-down cabinet with a large colour
monitor inside. As I walked past, I saw the game spell out its name in bright,
glowing letters (and in blatant plagiarisation of the ‘Star Wars’ logo)…
“Exidy EXCITEMENT Brings You... STAR
FIRE"...
…shortly followed by the attract mode, which revealed a
first-person space shooting game heavily based on Star Wars – right down to the
enemy craft having the same ball-with-wings shape as the TIE Fighters. This was
of course irresistible to someone fresh out of the cinema from just seeing
Empire Strikes Back, and I climbed eagerly into the cockpit with a Luke
Skywalkerish attitude and prepared to spend a few happy minutes sending Darth
and his chums off to oblivion.
Coo. Stand-up Star Fire cab.
Almost.
The game was primitive by modern standards, but impressive for
its time, in which first-person games of any description were rare indeed. For
control you had a throttle lever and a flight yoke, and before long you found
yourself swooping through the simple 3D starfields with ease, chasing squadrons
of TIE fighters, dodging shots, getting a ‘laser lock’ on the enemy ships and
seeing them break apart into colourful chunks as your lasers hit home.
Every now and then, a differently-shaped ship - looking a lot
like a Colonial Viper from out of Battlestar Galactica with ‘EXIDY’ written on
the side - would fly by, and if you managed to line up and shoot that ship you
would get a bonus and the words ‘GOT US!’ would appear in large, flashing
letters.
There it is. Get the bastard!
Simple though it was, Star Fire was one of the first ever
games that I found to be truly immersive. Sitting in that cockpit with that
large colour screen filling myfield of view, it felt for a while like looking
out of a craft instead of merely remote-piloting one from above, as was the norm
in most other games. I became pretty good at the game, and before long could
post up a boardful of ‘JCM’ hi-scores which would still be there on returning to
the arcade a few hours later – no mean feat for a machine in a popular arcade in
London.
18
In college, some of us would occasionally attempt versions of
popular arcade games, in our own simple way on the PET. Rup put together a
pretty decent Space Invaders, and I decided that, as my Big Project, I would
attempt to bring something of Star Fire over to the PET... and that I would
write the entire program in machine language.
One problem we had with the PET was that there was no sound
generator built in to the machine, which made designing satisfying games a bit
harder - games aren't really games without a few electronic burbles to signify
the firing of shots and the explosions of enemies.
However, we discovered a way to work around it. If you placed
an AM radio near the machine while it was working, the CPU would generate RF
noise that could be picked up as interference on the radio. And we found that if
you altered what the CPU was doing you could cause variations in that noise. If
you stuck it into a tight loop, and then caused that loop to lengthen slightly
with each iteration, then you could actually hear quite a passable ‘zap’ noise
in the interference on the radio.
Phineas Freak. Oh, the fuzz.
And so that’s how the sound effects were generated for my
first ever all-machine-code game -from a radio standing on top of the cassette
deck picking up RF interference tuned into sound effects by sculpting CPU loop
lengths!
Eventually the teacher whose name i forget but who looked a
bit like Phineas out of the Freak Brothers came up with a small hardware device
which attached to the I/O port on the PET and which could generate sound by
toggling one of the I/O bits, and that made for slightly better sound effects,
but i still have a special fondness for our AM radio hack.
19
Star Fire was to be my PET magnum opus, taking several weeks
to complete. Hand-compiling the machine code for a game of any reasonable
complexity was a painstaking task. But between us, me and the rest of the PET
geeks had amassed a fairly large body of work by this time, and we were never
short of games to play when we weren't directly engaged in creating more.
In the second year I was at sixth form, the computing
department had been moved into its own small building – a wooden hut with a few
small rooms, each of which housed one machine and one large classroom. People -
even some non-geeks - would congregate in the PET room to play the games, and,
since this was before it was really practical to save hi-scores, before long the
walls of the PET room got covered in hi-score lists as people wrote down their
personal bests on the wall in biro. This eventually came to the attention of the
Powers That Be, who were not amused and the whole lot had to be painted
over.
We also acquired a new PET - still only 8K, but with a green
screen and a proper keyboard instead of the somewhat cramped calculator-style
one on the original. On the very day it was delivered I was queueing up to get a
crack at it. I managed to be the first one on the new machine, and I duly loaded
up my game.
Star Fire. Definitely not influenced by
Star Wars.
Steering in Star Fire was accomplished using the numeric pad -
8 for down, 2 for up, other directions and diagonals derived from the keys
around the ‘5’ key – and I think it was ‘space’ to fire. In the course of a
game, the keying action could get quite hectic as you scrolled the field of
little PET-character TIE fighters around into your laser sights. I'd even put in
an Exidy-style bonus ship that would come around once in a while and which, if
shot, would refuel your ship and restore your shields (in the wonderfully weird
logic of games of that era).
I sat down at the new PET and began to play - noting with
admiration that there was a lot less ‘snow’ interference from accessing video
RAM than on the old machines (this was way before I'd heard of VSYNC).
I got into the rhythm of the game and wave after wave of green
TIE fighters succumbed to my lasers. Then, in the throes of manoeuvring my ship
with the numeric pad, my finger slipped off a key that I was holding down - and
with a SPANG!, the key sprang up and flew off the keyboard and halfway across
the computer room - just as Phineas Freak came in to check that the new machine
was working properly.
Fortunately, it proved easy enough to clip the key back onto
the keyboard again, and I escaped with a minor bollocking, but for a few moments
I'd been convinced that I'd broken the new machine within minutes of its
arrival.
20
One night after college, the parents were out and I was lying
on the couch idly watching the news, when a story came on which made me sit up
and take notice.
A man called Sinclair – a bit of a boffin from the looks of
him – had designed a really cheap computer, and he'd gone to a school with some
prototype machines for the pupils to try out. The news footage showed pupils
sitting in front of televisions to which were attached odd-looking little
machines, looking a bit like squashed calculators but without any LED
numbers.
Clive Sinclair. A deeply, deeply insane
genius.
The keyboard appeared to be printed on, being totally flat,
and - at the time I assumed it was some kind of TV interference - the display on
the tellies seemed to flicker whenever anyone entered a keystroke. It was
certainly no PET - but it was recognisably a computer, evidently running BASIC -
and it was to be introduced at a then stunningly low prove of £100. A price that
even I might be able to afford, given a bit of saving and cleaning of the
offices at Lansing Bagnall.
When my parents came home, I excitedly told them about the
cheap computer I'd seen on telly, and although at first they were dubious (why
did I want one of those? surely I'd get one and then it'd be unused in a
cupboard after a few weeks…) but, eventually they relented, and I even extracted
a promise that if I could save up fifty quid on my own they'd match that so I
could have the machine sooner.
Excellent stuff. At last, nearly two years after I'd first set
eyes on a computer, there was a real chance that I could get one of my own. The
idea of having your own machine that you could use exclusively and never have to
worry about the next person shoving you off to claim their time slot was
tremendously exciting. I don't think I'd keenly anticipated a purchase so much
in my life. I went on cleaning my offices at Lansing Bagnall and putting away a
few quid each week towards my goal.
Dig that crazy 2D chic.
A couple of months after the news piece on telly, the little
machine was released and adverts and articles about it started appearing in
computer magazines. Us geeks devoured these hungrily, soaking up every detail
and collectively drooling in anticipation. The machine was called the Sinclair
ZX-80, and it really was a barely minimal machine - made out of snap-together
vacuum-formed plastic, with a small membrane keyboard, and with only 1K of
memory inside (you could buy a plug-in ‘RAM pack’ to increase the memory to 4K).
But still, we were ingenious coders, and I was sure that somehow I'd be able to
make some simple games even in that tiny amount of RAM.
Demand for the machine was overwhelming the small company that
Sinclair had formed to sell it, and at first there was a six-week waiting time
between ordering one and actually receiving it. I couldn't afford to order mine
yet, but Rup managed to acquire some funds and he ordered his straight away.
Eventually, it arrived, and the same weekend I set out on my bicycle to visit
his house so that I could finally have a hands-on look at it.
We sat and geeked the evening away, writing small BASIC
programs on the ZX-80's odd little keyboard. It was strange. It insisted on
entering whole keywords with a single keypress, and there were different SHIFT
modes you had to use to get particular keywords, and some you had to type in
full (these were listed in a small sticker on the front of the machine's white
plastic lid). Not at all like the PET's screen-oriented editing, and a bit
awkward at first.
Calm yourselves. It’s three extra K.
And that flicker I'd seen on the news piece wasn't TV
interference at all, it was a result of the fact that if the little machine was
actually doing ANYTHING else apart from generating the display - running a
program or even reading a key-press - then it had to turn off the TV display.
This was a little disappointing. How were we going to make games move if the
computer could only run programs with the screen off? It also made entering code
a somewhat epilepsy-inducing experience.
None of which was enough to put me off buying one. Just the
prospect of being able to write code, at home, on my own machine, whenever I
felt like it was enough for me to forgive any of its little foibles (and it
had plenty of ‘em). I continued saving my Lansing Bagnall money...
21
Back in the real world, I carried on doing what I'd actually
came to college to do - get my A-levels and ultimately a place at University. I
can't say I shone at any of my subjects, mainly because most of my learning and
studying energy was focussed entirely on the school's computers.
By that time, me and Rup knew considerably more than any of
the teaching staff about the minutiae of how CPUs worked just through the
practical knowledge we gained from writing our games. But, unlike Rup, who was
properly on the comp.sci A-level, I would end up with no qualification to show
for it, since i wasn't on the course and wouldn't get to take the exam. I
muddled along, barely competently, in maths, physics and Eng Lit.
Aberystwyth Castle . Not great, but a
cleaner-upper in Scrabble.
I got conditional offers from a couple of Universities and was
invited along for interview. The first place I went to was the University of
Aberystwyth - a Welsh seaside town I'd visited when i was a child going on
holiday to Wales every summer. I took the train there and arranged to stay
overnight in a bed and breakfast - quite an exciting thing for me in those days,
since I hadn't been away from home much.
I remember being met at the university by a girl called Sian
Rhys, the daughter of a Welsh family who'd been our neighbours when I was a kid.
She was already a couple of years into her degree at Aber, and she kindly met me
and showed me around, and told me about the vast number of pubs in Aber, which
sounded OK to me.
About the interview itself I don't remember a lot - I am
certain that I enquired about the possibility of transferring to a computing
course, but was told that the A-levels I was going for would admit me only to
the physics course I'd specified, and that comp.sci courses were oversubscribed,
and I needed the matching A-level to get a place on one. Apart from that, it was
standard stuff. I'd get in if I got certain grades on my A-levels.
Interview over, I had the evening to myself, so I set out in
search of sustenance, and had fish and chips and, in one of the many pubs, a
couple of beers. As it was a seaside town, there was a pier, and at the foot of
the pier was an arcade. Replete and happy after my couple of libations, I set
out in search of gaming…
22
I don't remember what else was in that arcade, because one
machine captured my full attention - a big black Atari Lunar Lander vector cab.
The objective of the game was simple - control the orientation of your lunar
module with the buttons, and fire the craft's engine with a big, satisfying
throttle handle.
THAT’S a control panel. Proper fuckin’
Atari logo, too. Mumble, mumble…
The sound on the cab was turned up very loud and the machine
shook with the roar of the engine when at full thrust. You had a limited amount
of fuel with which to decelerate your craft from orbit and achieve a soft
landing on one of several plateaux amongst the lunar mountains, depicted in
glowing white vectors.
The game was tricky at first, and my first few attempts ended
with the module flying into pieces and a little message appearing telling me
that I'd destroyed a very expensive spaceship and made a large crater in the
ground. The controls were simple but gave good control once mastered, and before
long I was able to set the spacecraft down intact – at first on the widest and
lowest-scoring plateau, and eventually, gently easing it down onto the smaller,
higher-scoring, but much trickier landing sites.
I really loved the feel of the game – bleeding off the speed
with prudent firings of the rocket motor, taking advantage of freefall and
inertia to get where I wanted to go without using too much fuel, but being
certain to allow time and fuel to decelerate and land gently before the fuel ran
out. It felt really good being able to interact with this simple physical
simulation, and it planted in my mind an appreciation of such games that I still
have to this day, especially when I fire up STEem for a game of ‘Virus’ or VICE
for a bash on ‘Thrust’.
Mr. Vauxhall Viva. Great British
Ugliness.
Eventually, the college year started to come to a close and it
was time for the A-level exams. I really can't remember that much about the
taking of them - it was the usual curious exam-time mix of anxiety and freedom,
in that you had to actually sit the exams, but in between them you had no
lessons and were free to do whatever you like. In our case, this meant driving
round our usual haunts in Mole's Vauxhall Viva, or just mooching round
Basingstoke (and, of course, playing our games on the college machines).
Then, it was all over. I remember that we all had leaving
interviews with the head teacher (and I got bollocked one last time for not
wearing a tie) and then we were free – out of the educational system for the
first time in our lives. Not yet sure if we'd get to re-enter it as University
students after the summer.
And finally I got enough money together and put in my order to
Sinclair Research for my very own ZX-80.
A few weeks later, one morning, two items of mail arrived
addressed to me. My mum brought them into my bedroom, along with a nice cup of
tea. One was an envelope containing my A-level results, and the other was a
parcel from Sinclair Research, containing my ZX-80.
The contents of one of those items was to determine the path
of the next twenty-four years of my life...
Jeff Minter, June 2004.
Part One – In The Beginning…
Part Two – Colour, Sound, Poking Around…
Part Three – Welcome To The Machine…
Part Four – The Joy Of ZX & Hex.
Part Five – Fur-Ther Education.
Part Six - Higher Learning.
First published
at Way of the Rodent.
http://www.wayoftherodent.com/
|