[[Image:Inside GX4000 cart 1.jpg|frame|right|ACID protection chip within a cartridge (bottom)]]
The '''ACID''' ('''A'''mstrad '''C'''artridge '''I'''dentification '''D'''eviceAmstrad Cartridge Identification Device) protection chip is used in the [[GX4000 cartridge|cartridges]] of the [[Plus]] series and the [[GX4000]] game console. It is labeled with "Amstrad 40908". Because this chip was a custom creation by Amstrad After Nocash decrypted the internal algorithms it is currently not possible to replace this the protection chip nor to build new cartridges, because it is not known how it workswith a complex programmable logic device (CPLD).
if PinCCLR=0 then ShiftReg=1FFFFh ;\done at falling CLK edge
PinSIN=ShiftRegBit0 ;/
;Mind that above is a software example - a hardware solution obviously
;wouldn't require CmpVal and XorVal registers - instead, hardware would
;directly deal with the PinAx (or NOT PinAx) signals.
Replacing or overwriting the EPROM in existing cartridges is easy, so the ACID doesn't prevent piracy or homebrew programming. However, it prevents commercial developers from producing unlicensed cartridges.
== Timings ==
Although the functionatlity of the ACID chip is known. The timings of the signals coming from the computer is still unknown (reverse-engineering was done on a ACID chip, without owning a CPC+/GX4000).
* CLK is known to be 4MHz, and SIN is known to be updated on falling edge of CLK, as seen on [[Media:ACID-SIN.jpg|this picture]].
* Judging from the ACID chip, /CCLR is checked on falling edge of CLK. However, juding from [[Media:ACID-CCLR.png| this picture]], /CCLR is ''changing'' on falling edge of CLK. If this is true (the picture resolution isn't perfect), then it's unclear how the ACID can interprete /CCLR in a stable way while it's changing. Possibly, it can't do that at all (ie. it might act unstable, and the CPC+ might need to retry sending /CCLR several times).
** '''WANTED:''' A picture showing CLK and /CCLR without ACID connected (at better resolution) would help!
** A picture showing CLK and /CCLR with ACID connected is [[Media:ACID Init by Grim.png|found here]].
* The /CE timing is still unknown, too. The ACID might check A0-A7 on raising or falling edge of /CE. Or, on raising or falling edge of CLK while /CE being LOW. Or at whatever other time.
** '''WANTED:''' A picture showing CLK and /CE would help!
** '''WANTED:''' Best also showing one or more of the A0-A7 signals.
<gallery>
Image:ACID Init by Grim.png|/RESET, CLK, CCLR, SIN (init sequence)
Image:ACID-SIN.jpg|CLK and SIN
Image:ACID-CCLR.png|CLK and CCLR (and some offtopic signals) (from CPC, without ACID connected)
</gallery>
== Pinout ==
<pre>
+--()--+
GND |1 16| +5V AMSTRAD 40908 A7 |2 15| A6 1L03P1003 CLK4|3 14| A5 JAPAN
NC |4 13| A4
CCLR|5 12| A3
Note: The two GND pins are interconnected with each other (inside of the chip). The NC pin seems to be always high.
==InvestigationsClones ==
*An ACID replacement using a Xilinx XC9536 CPLD and some [[Verilog implementation of the ACID chip|Verilog code]] based on nocash's reverse engineering work has been built by [[Octoate]] and [[Nilquader]].* Miguel Angel (aka mcleod_ideafix) has published all the info about his implementation of the ACID in a CPLD, verilog sources included [http://amstradcpcwww.mforoszxprojects.com/305097index.php/7723493analisis-quedel-hacechip-exactamenteacid/62-analisis-eldel-chip-acid-dedel-losamstrad-cartuchoscpcgx4000]* It's possible to use the CPC+ without ACID by exploiting a timing bug in the CPC+/ GX4000-ASIC Chip. (Spanish languageDeveloped by [[Nilquader]])<gallery caption="Running CPC+ without ACID">Image:NoAcid.jpg|GX4000 running without ACIDImage:NoAcid_sch.png|Schematics</gallery>
*[http://www.vieuzordiland.fr/index.php?option=com_content&task=view&idPictures =23&Itemid=34 Vieuxzordiland's page on the matter* ACID chip decapped. In French]Many thanks to Sean Riddle!<gallery>Image:40908_Metal_Small.jpg</gallery>
*[http://cpcwiki.eu/forum/index.php/topic,410.0.html| The CPCwiki's forum thread featuring the Epic fight between the infamous ACID and NoCa$h.]== Misc ==
==The End of the Darkness : A New Hope==* [[Test Circuit used for ACID reverse engineering]]
==Investigations==
''At first was an age of Grim Darkness, Ignorance and Despair*http://amstradcpc.mforos.com/305097/7723493-que-hace-exactamente-el-chip-acid-de-los-cartuchos/ (Spanish language)
*[http://www.vieuzordiland.fr/index.php?option=com_content&task=view&id=23&Itemid=34 Vieuxzordiland''For twenty years, s page on the once glorious Amstradom struggled and was about to loose it to the might of the Commodore's legions and Nintendorks hordes, a fight It wouldn't winmatter.''In French]
''Remembering with Nostalgia old ages of glory, gay old time when the Amstradom was young and fool, as told by the Elders*[http://cpcwiki.''''Until one day the end of a Nightmarish Time was about to End as announced by an obscure and lost prophecyeu/forum/index.'' ''A Lone Man came to stand before the Foul Darkness of Ignorance.'' ''"'''Give me two daysphp/topic, and I'll bring you the light, I'll bring you hopes, for it is our destiny to stand proudly and face bravely410.0.'''" he said to the massed crowd of the Amstradom'' ''html| The Amstrad Peoples frowned in horror and fear.'' ''"'''Heresy, you know that None can stand before the ultime Might and Eldricht Arcane of the Mighty ACID, bringer of extinction, destroyer of the Worlds!'''" scanded the crowd in anger and fury.'' ''"'''You fool, we are Doomed into Oblivion, the CommodoreCPCwiki's Legions and Nintendork Hordes shall rule us all...'''" cried others.'' ''"'''Burn this Foul Heretic for giving us false hopes ! We must accept our fate as we always did !'''" a last man hystericaly screamed.'' ''Then Fano forum thread featuring the Wise accomplished his destiny and stepped one pace before the crowd.'' ''"'''I believe in you, Lone man, accept my humble sacrifice'''" he said gently, a tear in the eye.'' ''For era we will remember Him who who sacrified his beloved Cartridge for the cause.'' ''''Strenghtened by the Blood of the martyr, the Lone Man Hurled himself into the fray. ''Four days and Four nights, he fought.'' ''The Battle was Epic, sounds of fight between the bits tearing the sky, Heaven infamous ACID and Hell colour clashed in a furious destruction of blind madness.'' ''Four days and Four nights, he Braved.''''So Grand was the cause and so inspiring was His Fight.'' ''At first sceptic, the Amstradom were slowly gained to the Holy Cause of the Lone Man.'' ''Fortified by the exemple of He who fought desperately yet with no hesitation, conscious that Grand Scheme was being at work.'' ''Four days and Four night.'' ''and then the Colour Clash ended.'' ''The chanting cheerfull acclaims of Amstradom stand before the Lone Man.'' ''Accompanied by colourfull sprites, smooth scrolling rolling as DMA channels rejoyced loudly, in a rainbow of four-thouzand-and-ninety-six Colours out of Space..'' ''Victorious and Untainted.'' ''He who defeated the Vile Beast.'' ''"'''Four Days'''" he said.'' ''"'''Fours days and Four nights, the Task was even greater than I could believe. How Fool was I... but I did overcame.'''"'' ''And future generations in Amstradom would remember Him as the Saviour, the Messiah, the Bringer of Light against Darkness.'' ''He who shown us the way.'' '''''And NoCa$h was his name, the man who degfeated the ACID chip.''''' ''Twenty years and Four days.'']
''A new Era was to begin.
''''
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