1. A method for emulating calls from a user program to an operating system, said method comprising: executing a plurality of user instructions from said user program, said user instructions belonging to a first instruction set; decoding a call instruction in said user program, said call instruction calling a service routine in an operating system, wherein said call instruction in said user program is a far jump instruction; loading a pointer to a code segment, said code segment containing said service routine in said operating system, said pointer having an instruction set indicating means for indicating an instruction set for said service routine; executing service routine instructions in said code segment, decoding service routine instructions with a first instruction decoder when said instruction set indicating means indicates said first instruction set, decoding service routine instructions with a second instruction decoder when said instruction set indicating means indicates a second instruction set, said first instruction decoder for decoding only a portion of said first instruction set; returning control to said user program when a return instruction is executed in said service routine; whereby said user program containing instructions in said first instruction set calls said service routine in said operating system, said service routine having instructions from said second instruction set, said pointer to said code segment indicating if said service routine contains instructions from said second instruction set or said first instruction set.