Archives for the month November 2021 of Genesis8 Amstrad Page

Bug hunting on Shinobi for Amstrad CPC by Richard Aplin


Richard Aplin is the coder of the Amstrad CPC port (1989) of the arcade game Shinobi (1987). While programming it, he searched for two weeks a rare bug of Shinobi, which he is writing about it on Twitter. With his agreement, here is all the text and screens present on Twitter (screens coming a bit later : more work to do).

Back in the days of 8-bit micros, in this case the Amstrad CPC (a popular - in Europe - Z80-based home computer). I worked for a games company, I was doing a conversion [i.e. rewrite] of Sega's "Shinobi" arcade machine onto this machine.

There were all sorts of geeky, tweaky technical tricks to make it run fast and look nice (coded in assembly language), and all went well, game turned out pretty nice, ran fast and played pretty well, until one day, when nearly done, play testers reported that it occasionally - crashed on one level. Boom! Reset. It was really hard to reproduce.

Nobody could come up with anything they actually _did_ to make it crash (or not); was about one in ~20 times(IIRC), when fighting a boss.

I had nothing fancy like a logic analyzer or in-circuit-emulator or other hardware debugging tools, just a regular retail computer. So it just crashed, once in a while, when playing one specific part of the game.

Sure, just some coding bug, like any other, not uncommon. But WHERE and WHY and HOW!? ..and why so hard to reproduce? You could play for hours and no problem, but just when it seemed like it was a ghost or maybe fixed for no known reason.... BOOM reset.

This went ON and ON and I was utterly mystified. I just could _not_ induce this bug to happen more often [the key to find/fixing it], no matter what I tried, I could not find even a way to reproduce reliably, let alone the root cause.

I started doing stuff like checksumming the RAM every frame, looking for some sort of random corruption, putting all sorts of checks in there that slowed it down to a crawl, and still nothing. The bug seemingly came and went as it pleased, never in quite the same place.

Until one day I got lucky. I caught it in the act! One single byte in the middle of my program code got trashed - and this time I caught it _before_ the whole thing blew up. But how?! What on earth was causing this? [OH for a hardware logic analyzer !

Finally, _finally_, after probably two weeks of solid bug-hunting and hair-tearing I found it.

So back in those days, it was customary for the game's music to be written by someone else, and provided as a binary blob of code plus data (e.g. 4Kbytes) that you would just call once a frame, and it took care of controlling the sound chip and playing whatever music tracks.

And it turned out that the music player (I didn't have source code) had a bug in it. Not a big bug. A teeeeeny little bug. It didn't audibly affect the music at all, but one _single_ note, on one channel, in one of the tunes on one of the levels, used a wrong data byte.

And normally, when that single duff musical note played, nothing bad happened, it was a fairly harmless bug, however it caused music player to read wrong byte of RAM; not just off by one byte, but off by tens of KBytes... in fact, it ended up reading a byte of the display ram.

If I recall correctly, it ended up - at that instant - reading a single byte of the display right around where this green circle is, in the upper left corner.

And when that single bad musical note in the tune played - IF the top bit (and only the top bit) of that pixel was a 1, it would then take that as a memory address ELSEWHERE in RAM and increment that location which corrupted a single byte inside of my program code, leading to a subsequent - but not quite immediate; a couple of seconds later, when a baddie was decided to shoot at you - crash.

This was a 2D scrolling game of course, so you were constantly jumping around and doing stuff fighting ninjas- the crash only happened if ONE pixel of the display was a certain color at the instant that one single note of ONE background tune played, and it wasn't in my code.

I vividly remember finally discovering the root cause (disassembling and patching the music player, and finally catching it 'in the act', and figuring out the long chain of events..) .. To this day I have never had a bug as hard as that one.. was SO rewarding to find.

I've had a bunch of equally obscure bugs in the decades since then, but the tools got so much better - protected memory, logic analyzers, CPUs that don't just explode on contact with bad data - nothing has ever been as difficult to track down as that one. Was a great lesson which is that if you persevere for long enough you will win in the end against a computer - bugs can't hide forever. Ever since then I've known it's just a matter of time, and patience. Nowadays I relish a good bug, I rub my hands and chuckle - the game is ON! ;-).

The Basic v1.1 ROM of the Amstrad CPC 6128 unassembled by Bread80


After the unassembling of the Amstrad CPC 6128 firmware, Bread80 has done the same for the Basic v1.1 ROM also for the CPC 6128. He is writing about it on Il en parle sur its web site and Twitter.

Why using a commercial compiler of the Amstrad CPC basic when you can update its speed directly ?

Siemb Chronicles: Arkos the Traitor for Amstrad CPC by ESP Soft was released in September 2021


ESP Soft has released the sequel of Game Over for Amstrad CPC Siemb Chronicles: Arkos the Traitor the 13th September 2021. It has 2 levels : the fall of Arkos and Escape from Hypsis, it's a mix of platform, run and gun and shoot them up.

The game exists thanks to ESP Soft by the following people :

  • Artaburu (code)
  • Triste1942 (graphics)
  • McKlain (sound)
  • Litos (history, script)
  • Jesús Martínez del Vas (cover)
  • MiguelSky (ideas)
  • JGonza, Chemamstrad et BlackMores (tests)

DAAD Ready v0.5 by Uto to easily compile DAAD games as tape or disk for targeted computers


Si vous souhaitez créer un jeu d'aventure texte et/ou graphique alors DAAD est ce qu'il vous faut (PunyInform aussi mais sans graphiques, en pur texte).

DAAD permettant de cibler beaucoup d'ordinateurs, Uto a créé DAAD Ready qui vous permets de créer facilement des images cassettes ou disquette prêtes à l'emploi sur les ordinateurs supportés par DAAD.

Bien que DAAD ne supporte officiellement que l'anglais et l'espagnol, DAAD ready ajoute un support limité du portugais, allemand et français. Cela va permettre notamment d'avoir une version française de Tristam Island.

Vous pouvez télécharger DAAD Ready ici.

Using a PS/2 keyboard on an Amstrad PC XT (1512/1640/2086/3086)


Benediction cross ASseMbler by Krusty targetting Amstrad CPC for windows, mac and linux


Krusty (Benediction) is developping at the moment Benediction cross ASseMbler (BASM) which you can get in his github depot.

I will let Krusty present himself hit utility : these last months, I have worked on the Benediction cross ASseMbler that will be used in our next production.I have not yet tested it in real-world conditions, so ATM I have truly no idea of its efficiency/usability.

I write this message to let anyone give it some try and provide me feedback to fix potential bugs and eventually bring more features for its real official release.I guess the official release will be accompanied by a graphic version of basm for those that are not yet ready to use command line tools.

Its aim is not to replace rasm that is a really fast and good assembler. But it can be used in contexts where rasm cannot play. BTW it is not 100%compatible with rasm code.

Of course, there is no documentation ready yet, but you can get most of its possibilities in the files named good_xxx.asm in this directory of the github depot.

Among what is not (yet) available for rasm you can check the section, basic, or iterate examples.

Note that basm uses two assembling passes by default, but can stop at the first pass if there is no need to do another one or add additional ones if necessary (which makes the ifused example compatible with basm but not rasm).

Super Space Fuel Inc., an EGA game for your XT to 486 by Peter Bone


Super Space Fuel Inc. is an EGA game in 16 colors and adlib sound programmed with Turbo C on a 486. It can be played starting on 8088 but its author Peter Bone recommend to use a 386 or 486. It's a connect type game, which you can see a gameplay video by Dosgamert on Youtube.

Network, XT-IDE, adlib, 512 Kb RAM, ISA slot for Amstrad PPC 640 (internal) by Retro Theory


Retro Theory is showing a photo of an internal card for Amstrad PPC 640 which is replacing the internal modem to add :

  • LAN ethernet
  • XT-IDE with SD card
  • adlib sound
  • 512 Kb de RAM (8 Kb granularity)
  • ISA slot

WIP CPC Bullet v2 for Amstrad CPC


CPC Bullet is a game for CPCRetroDev 2021 contest, but which couldn't be accounted due to a delay in time limit for . It is programmed by Cyrille Gouret, music by Mr Lou and graphism by Titan.

You can download the WIP v2 of CPC Bullet knowing that an enhanced v3 will come (gameplay, graphism).

Gamemplay is simple, eliminate your adversary, without shooting yourself due to a bad ricochet.

A video is available of CPC Bullet on the Amstradiens channel.

PunyInform v3.2 by Fredrik Ramsberg and Johan Berntsson to write text adventure games


PunyInform v3.2 by Fredrik Ramsberg and Johan Berntsson is a library written in Inform 6 to create adventure game (pure text, no graphic support contrary to DAAD) using the Z-machine virtual machine which will run on 8bit computers (or more recent computers too). PunyInform has a parser, knowing of common verbs and a framework to write adventure games.

PunyInform is based on the Inform 6 library written by Graham Nelson. Its goal is to make easily adventure games in Inform 6, with a manual describing the differences between the official library and PunyInform..

Games using PunyInform can be compiled in z3, z5 and z8 format (z3 being the best format for 8bit computers, other formats have more features). Compared to the Inform 6 library, it means that there is no support for the Glulx virtual machine but z3 format is important as Inform 6 doesnt support it.

To compile games written with PunyInform, you should use the Inform 6 compiler maintained by David Kinder. Binaries are available on if-archive. PunyInform needs Inform v6.35 (or more).

They are tutorials to write adventure game with PunyInform (end of the page) and all the documentation including a 8 page cheat sheet (quick reference)..

To try your game after compilation, you can use WinFrotz by David Kinder, to create map easily you can use Trizbort.

And finally, to create an Amstrad CPC and PCW disk image, you will have to use the Puddle BuildTools.

Halloween 2021 4mb Sample demo for Amstrad CPC by Markus Hohmann (Devilmarkus)


For Halloween Markus Hohmann (DevilMarkus) has released an Amstrad CPC demo : Halloween 2021 4mb Sample, first they are some noises then the music starts later around 2 minutes 40 seconds.

To create the sample here is his method : I'm using a volume table I calculated for the logarithmic conversion 256 volumes down to 16. I also use nibbles for stereo playback (1 byte contains stereo informations). The imported .mp3 is pre-gained to 100db and I use some filters on the imported sample.

The original source is a musical remake of the game Cauldron II by SoundLogic which you can find also on Youtube.

Amstrad CPC score contest on Amstree (John LOBO)


Border 0 organizes a score contest, whose rules are below :

1.Use the WinAPE v2.0 Beta 2 emulator
2.Use the following settings :

MEMORY: RAM: 128K / ROM Lower: OS6128 / Upper 0: BASIC1-1 / Upper 7: AMSDOS
3.Load the following DSK of Amsthree
4.Save a SNR session of your game :
4.1.Save ONLY one game per session
4.2.Start at the menu screen
4.3.End at the score menu
5.Send your SNR session with a tag or your name BEFORE the 31th January 2020 to :
Nothing to win except the respect of other who will be able to see the results on the mentionned page, looking at the best SNR on CPC-Power and a video on Youtube.

Classic Adventurer issue 9 is out by Mark Hardisty, about adventure games


Classic Adventurer is a newspaper about adventure games by Mark Hardistywhich last issue number nine is out as a free download or in printed format.

This issue has for example an article about the GAC (Graphic Adventure Creator) utility.

The Wrath of the MCPC by LambdaMikel with joystick and LambdaSpeak (CPCRetroDev 2021)


Michael Wessel has released for the CPCRetroDev 2021 contest a game in basic 1.0 : The Wrath of the MCPC.

He just released a new version with joystick support and compatible with LambdaSpeak vocal synthesis which you get with his sound cards : LambdaSpeak 1.95, LambdaSpeak III and LambdaSpeak FutureSoft Edition (LS-FS). You can see a video of the vocal synthesis on Youtube.

Amstrad Action 50 by DarkSteph, soon available as a free PDF


A Hundred more issues of AA50 by DarkSteph are being shipped, and a little bit before christmas you will be able to put a USB key with a free PDF of Amstrad Action 50 in front of your Xmas tree. The quality of impression of the paper issue is great, I have mine at least.

WIP Hyperdrive by Reidrac for Amstrad CPC


Reidrac (Juan Martinez) is actually working on two projects : a RPG (Ultima like) and Hyperdrive which is a shoot them up, which reminds me of of the excellent Light Force.

You can check Reidrac's Twitter for more informations too.

I invite you to list the last posts of Reidrac's blog about several subjects including to create the games with cartridges in mind, thanks to the Plus2CPC, upcoming Play2CPC, DES and Dandanator.

Play2CPC aka Plus2CPC II by Abalore and ToTO for Amstrad CPC(+)


After the Plus2CPC by Abalore (see on CPCWiki), here is the Play2CPC which is a mix between the Plus2CPC by Abalore (adding a cartridge for all CPC even CPC+ on expansion port) and the PlayCity. The Play2CPC adds three AY channels and one noise generator (6 channels and 2 noise generators for the PlayCity) and the possibility to modify the content of the pseudo cartridge. For more informations, I invite you to read CPCWiki (link above).

For more news, Go to home page