Jan 192012

Bryce, well known for his great hardware projects (MegaFlash ROM box, RGB to SVideo converter, MegaROM board, etc…), released a description on how to move the ACID protection chip into the CPC Plus. If you do this modification, you won’t need the ACID protection chip in the cartridge anymore and you can just connect cartridges without an ACID protection chip to the CPC Plus.
You can find the howto in the CPC Wiki.

A replacement possibility for the ACID protection chip was also released last year, too.

Apr 152011

Nilquader and I decided to release our Verilog code (based on Nocash’s decryption of the algorithm) to “emulate” the ACID protection chip of the Amstrad Plus. So, you can now find it in the ACID article of the CPCWiki and right here in this posting:

`timescale 1ns / 1ps
// Company:
// Engineer: Octoate, Nilquader
// ACID Reverse engineering by nocash
// Create Date   : 00:45:53 09/03/2010
// Design Name   : amsacid
// Module Name   : amsacid
// Project Name  :
// Target Devices: Xilinx XC9572
// Tool versions :
// Description   : Reverse engineered Amstrad 40908 "ACID" Chip
// Revision:
// Revision 0.05
// Additional Comments:
module amsacid(PinCLK, PinA, PinOE, PinCCLR, PinSIN);

input PinCLK;
input [7:0]PinA;
input PinOE;
input PinCCLR;

output [7:0]PinSIN;

wire PinCLK;

reg [16:0]ShiftReg = 17'h1FFFF;
wire [16:0]CmpVal;
wire [16:0]XorVal;

assign CmpVal = 17'h13596	^ (PinA[0] ? 17'h0000c : 0)
				^ (PinA[1] ? 17'h06000 : 0)
				^ (PinA[2] ? 17'h000c0 : 0)
				^ (PinA[3] ? 17'h00030 : 0)
				^ (PinA[4] ? 17'h18000 : 0)
				^ (PinA[5] ? 17'h00003 : 0)
				^ (PinA[6] ? 17'h00600 : 0)
				^ (PinA[7] ? 17'h01800 : 0);
assign XorVal = 17'h0C820	^ (PinA[0] ? 17'h00004 : 0)
				^ (PinA[1] ? 17'h06000 : 0)
				^ (PinA[2] ? 17'h00080 : 0)
				^ (PinA[3] ? 17'h00020 : 0)
				^ (PinA[4] ? 17'h08000 : 0)
				^ (PinA[5] ? 17'h00000 : 0)
				^ (PinA[6] ? 17'h00000 : 0)
				^ (PinA[7] ? 17'h00800 : 0);

always@(negedge PinCLK)

	if (PinCCLR) // not in reset state
		if (!PinOE && ((ShiftReg | 17'h00100) == CmpVal))
			ShiftReg <= (ShiftReg ^ XorVal) >> 1;
			ShiftReg[16] <= ShiftReg[0] ^ ShiftReg[9] ^ ShiftReg[12] ^ ShiftReg[16] ^ XorVal[0];  // hier xorval mit berüchsichtigen
			ShiftReg <= ShiftReg >> 1;
			ShiftReg[16] <= ShiftReg[0] ^ ShiftReg[9] ^ ShiftReg[12] ^ ShiftReg[16];
			ShiftReg <= 17'h1FFFF;

//assign PinSIN = ShiftReg[7:0] ^ 8'hff;
assign PinSIN = ShiftReg[7:0];
//assign PinSIN[0] = PinCLK;


But this isn’t everything about the ACID. We also found a timing problem during the investigation with a logic analyzer, which you can see here:

ACID initialisation by Grim

ACID initialisation (by Grim)

You will find glitches on the /CCLR line which show that the SIN contact should be changed… All in all it is possible to use a fast flip-flop to change the SIN signal when such a glitch occurs (see picture below).

Flip flop for ACID replacement

Flip flop for ACID replacement

You see, this is a very simple schematic, but it isn’t much cheaper than a CPLD, which emulates the full ACID and it comes with a disadvantage: it won’t work with a reset, so you always have to switch the CPC off and on again.

So, have fun with it – maybe Bryce will change his cartridge design and support this solution in the future :-).

Apr 152011

Thanks to an tweet by Dadman I saw that he updated his blog about an ACID replacement. If I understand it correctly (I had to use a translator, because my spanish isn’t good enough), mcleod_ideafix has also finished the ACID emulation for the Amstrad Plus. So we will surely see new hardware for the Plus coming Smiley mit geöffnetem Mund. You will find more information about it on http://www.zxprojects.com.

Nilquader and I wrote a ACID emulation about half a year ago on the XzentriX 2010 meeting, but we decided not to release the code until a new hardware is ready, because we wouldn’t like to see high-priced bootlegs on e*ay…

[UPDATE:] Because the release of the ACID emulation code, Nilquader and I decided to release our ACID emulation code, too. You can find the source of it here.

Feb 022011

Bryce announced the MITM (man-in-the-middle) cartridge adapter, which sits between a Plus cartridge and the Plus. With this adapter, you just need an original cartridge with an ACID protection chip which is directly connected to the CPC, but it will read the contents of the EPROM on the adapter. Maybe you know the same principle from playing Super Nintendo games from foreign countries – they used such an adapter to bypass the protection chip, too.

You can find an article about the adapter in the CPC Wiki. The schematics and layout will follow soon.


Feb 172010

I already wrote a news article about the search on how to decrypt the ACID protection of the Amstrad Plus computers. Now NoCa$h, also known for his No$cpc emulator for DOS, received a Amstrad CPC Plus cartridge by Fano and decrypted the ACID protection of the cartridge within a few days. To do this, he desoldered the ACID protection chip from the cartridge and connected it to the printer port of the PC. With a small program he sent data to the chip and tried to find a way to reproduce the signals of the chip. He found an algorithm which produces the same sequence.
With the decryption of the chip, the last big unknown mystery of the CPC is now resolved. In the future we can produce new cartridges for the CPC Plus and maybe someone will develop a reprogrammable cartridge.

You can find the pseudocode in the thread about the ACID protection in the CPCWiki forums and this information will also be included in the CPCWiki article about the ACID protection in the future, too.

;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                    ;/
;Mind that above is a software example - a hardware solution obviously
;wouldnt require CmpVal and XorVal registers - instead, hardware would
;directly deal with the PinAx (or NOT PinAx) signals.

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