The original 2600 core didn't autodetect carts. Instead it used a wildcard in the config string, and it looked at the file extension to pick a mapper. This meant that each cartridge needed to be named correctly.
The Stella Cart Detector uses a bunch of heuristics to search through the cartridge for known opcodes to detect the cart type. Another list of mappers
In C:
bool CartDetector::isProbablyCV(const ByteBuffer& image, size_t size)
{
// CV RAM access occurs at addresses $f3ff and $f400
// These signatures are attributed to the MESS project
uInt8 signature[2][3] = {
{ 0x9D, 0xFF, 0xF3 }, // STA $F3FF.X
{ 0x99, 0x00, 0xF4 } // STA $F400.Y
};
if(searchForBytes(image, size, signature[0], 3))
return true;
else
return searchForBytes(image, size, signature[1], 3);
}
bool CartDetector::searchForBytes(const uInt8* image, size_t imagesize,
const uInt8* signature, uInt32 sigsize,
uInt32 minhits)
{
uInt32 count = 0;
for(uInt32 i = 0; i < imagesize - sigsize; ++i)
{
uInt32 j;
for(j = 0; j < sigsize; ++j)
{
if(image[i + j] != signature[j])
break;
}
if(j == sigsize)
{
if(++count == minhits)
break;
i += sigsize; // skip past this signature 'window' entirely
}
}
return (count == minhits);
}in verilog, we can write a similar module, but it is going to look at a byte stream instead of reading through memory. We can do all the checks in parallel by instantiating a lot of matching modules.
we use parameters in the module to configure it and pass in the bytes we are looking for.
here is the module:
//------------------------------
// CV detector
//-------------------------------
wire hasMatchCV_0 , hasMatchCV_1;
wire hasMatchCV = hasMatchCV_0 | hasMatchCV_1;
match_bytes #(
.num_bytes(8'd3),
.pattern({ 8'h9D, 8'hFF , 8'hF3 }),
.needmatches(8'd1)
) match_bytes_CV_0(
.addr(addr),
.enable(enable),
.clk(clk),
.reset(reset),
.data(data),
.hasMatch(hasMatchCV_0)
);
match_bytes #(
.num_bytes(8'd3),
.pattern({ 8'h99, 8'h00 , 8'hF4 }),
.needmatches(8'd1)
) match_bytes_CV_1(
.addr(addr),
.enable(enable),
.clk(clk),
.reset(reset),
.data(data),
.hasMatch(hasMatchCV_1)
);
module match_bytes
(
input clk,
input reset,
input [15:0] addr,
input enable,
input [7:0] data, // data in,
output reg hasMatch
);
parameter [7:0] num_bytes;
parameter [(num_bytes*8)-1:0] pattern;
parameter [7:0] needmatches=8'b1;
reg [(num_bytes*8)-1:0] lastPattern;
reg [7:0] curMatch;
always @(posedge clk)
begin
if (enable)
begin
// use address 0 as reset since reset is high during cart loads
if (addr==16'b0)
begin
curMatch<=8'b0;
hasMatch<=0;
lastPattern<= {lastPattern[(num_bytes*8)-9:0],data};
end
else
begin
lastPattern<= {lastPattern[(num_bytes*8)-9:0],data};
if (lastPattern == pattern)
begin
curMatch<=curMatch+8'b1;
if (curMatch==(needmatches-8'b1))
hasMatch<=1;
end
end
end
if (reset) begin
curMatch <= 0;
hasMatch <= 0;
lastPattern <= '0;
end
end
endmodule: match_bytesNow that we have some code, we would like to see if it autodetects the cartridges correctly. We can use verilator to load a cartridge and feed it to our detect routine. We will create a simple top that is called from verilator, and instantiates our 2600 detector. We put in a few lines of help so that the code will print out a comma delimited string, and set a done flag when it is finished. This makes things a bit simpler.
We can pass md5sum a list of files, and it will create md5 checksums and filenames. A quick copy/replace and we can turn it into a CSV file. We use a python program to read this file and a spreadsheet we found on the web to load and check each rom, and dump out the results.