The ACID (Amstrad Cartridge Identification Device) protection chip is used in the 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 it is currently not possible to replace this chip nor to build new cartridges, because it is not known how it works.
Technical
The 16pin ACID chip (in the cartridge) is sending a serial data stream to the 160pin ASIC chip (in the computer). The data stream is generated by some shift/xor logic, and is also affected by the EPROM address/enable signals. If the data stream isn't correct, then the ASIC does scatter access to RAM, making the cartridge unusable.
;ACID reverse-engineered 13-16 February 2010 by nocash (Martin Korth) ;below is repeated on every CLK cycle... CmpVal=13596h, XorVal=0c820h if PinA0=1 then CmpVal=CmpVal XOR 0000ch, XorVal=XorVal XOR 00004h if PinA1=1 then CmpVal=CmpVal XOR 06000h, XorVal=XorVal XOR 06000h if PinA2=1 then CmpVal=CmpVal XOR 000c0h, XorVal=XorVal XOR 00080h if PinA3=1 then CmpVal=CmpVal XOR 00030h, XorVal=XorVal XOR 00020h if PinA4=1 then CmpVal=CmpVal XOR 18000h, XorVal=XorVal XOR 08000h if PinA5=1 then CmpVal=CmpVal XOR 00003h, XorVal=XorVal XOR 00000h if PinA6=1 then CmpVal=CmpVal XOR 00600h, XorVal=XorVal XOR 00000h if PinA7=1 then CmpVal=CmpVal XOR 01800h, XorVal=XorVal XOR 00800h if PinCE=0 AND (ShiftReg OR 100h)=CmpVal then ShiftReg=ShiftReg XOR XorVal NewBit=ShiftRegBit0 XOR ShiftRegBit9 XOR ShiftRegBit12 XOR ShiftRegBit16 ShiftReg=(ShiftReg SHR 1) + (NewBit SHL 16) Wait for falling edge on PinCLK if PinCCLR=0 then ShiftReg=1FFFFh ;\done at falling CLK edge PinSIN=ShiftRegBit0 ;/
Pinout
+--()--+
GND |1 16| +5V AMSTRAD 40908
A7 |2 15| A6 1L03P1003
CLK4|3 14| A5 JAPAN
NC |4 13| A4
CCLR|5 12| A3
SIN |6 11| A2
/CE |7 10| A1
GND |8 9| A0
+------+
| Pin number | Name | Direction | Description |
| 1 | GND | - | Ground |
| 2 | A7 | Input | EPROM Address A7 |
| 3 | CLK4 | Input | Clock (4 MHz) |
| 4 | NC | - | Not connected |
| 5 | CCLR | Input | Reset ACID |
| 6 | SIN | Output | Serial Data |
| 7 | /CE | Input | EPROM Chip enable |
| 8 | GND | - | Ground |
| 9 | A0 | Input | EPROM Address A0 |
| 10 | A1 | Input | EPROM Address A1 |
| 11 | A2 | Input | EPROM Address A2 |
| 12 | A3 | Input | EPROM Address A3 |
| 13 | A4 | Input | EPROM Address A4 |
| 14 | A5 | Input | EPROM Address A5 |
| 15 | A6 | Input | EPROM Address A6 |
| 16 | +5V | - | Vcc |
Note: The two GND pins are interconnected with each other (inside of the chip). The NC pin seems to be always high.
Investigations
The End of the Darkness : A New Hope
At first was an age of Grim Darkness, Ignorance and Despair.
For twenty years, 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 win.
Remembering with Nostalgia old ages of glory, gay old time when the Amstradom was young and fool, as told by the Elders. Until one day the end of a Nightmarish Time was about to End as announced by an obscure and lost prophecy.
A Lone Man came to stand before the Foul Darkness of Ignorance.
"Give me two days, and I'll bring you the light, I'll bring you hopes, for it is our destiny to stand proudly and face bravely." he said to the massed crowd of the Amstradom
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 Commodore'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 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 the bits tearing the sky, Heaven 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. '