How to know the instruction set in the development of an emulator?

When developing an emulator, one needs to know the instruction set of the platfor开发者_JAVA技巧m. Take as an example the development of a Playstation 2 emulator. How it is possible to know the instruction set of the Playstation 2 to code an emulator? Do they reverse engineer SDKS to know the instructions? Or they disassemble the hardware?

With the use of assemblers that generate executables for the target platform it is possible to create and debug some programs information to discover how the instructions are assembled in machine code. That could help discovering the instruction set.