What does 386,486,&586 change about the program or how it runs?
There are certain instructions which are only available on certain processors (or later versions of them). For instance, BSWAP is a 486+ instruction, so using it on a 386 will generate an exception. Likewise, the undocumented opcode RDTSC is limited to Pentium (586) processors and above. The Assemblers use these processor switches in an attempt to make sure you don't mess up and use an invalid opcode for the currently targeted CPU. Other CPU 'switches' are: .386p .486p .586p Enables protected mode instructions .MMX Enables MMX instructions (MM0-MM7 MUST be capitalized) .K3D Enables 3D-Now! instructions