6502 CPU Emulation

The opcode takes one byte, and the operands are in the following bytes. Check out the byte size column here, for instance.

If you look into references like http://www.atarimax.com/jindroush.atari.org/aopc.html, you will see that each opcode has an encoding specified as:

HEX LEN TIM

The HEX is your 1-byte opcode. Immediately following it is LEN bytes of its argument. Consult the reference to see what those arguments are. The TIM data is important for emulators - it is the number of clock cycles this instruction takes to execute. You will need this to get your timing correct.

These values (LEN, TIM) are not encoded in the opcode itself. You need to store this data in your program loader/executer. It's just a big lookup table. Or you can define a mini-language to encode the data and reader.