You can use the CPU
directive. From the fine manual:
The CPU directive restricts assembly to those instructions which are
available on the specified CPU.
Options are:
CPU 8086 Assemble only 8086 instruction set
CPU 186 Assemble instructions up to the 80186 instruction set
CPU 286 Assemble instructions up to the 286 instruction set
CPU 386 Assemble instructions up to the 386 instruction set
CPU 486 486 instruction set
CPU 586 Pentium instruction set
CPU PENTIUM Same as 586
CPU 686 P6 instruction set
CPU PPRO Same as 686
CPU P2 Same as 686
CPU P3 Pentium III (Katmai) instruction sets
CPU KATMAI Same as P3
CPU P4 Pentium 4 (Willamette) instruction set
CPU WILLAMETTE Same as P4
CPU PRESCOTT Prescott instruction set
CPU X64 x86-64 (x64/AMD64/Intel 64) instruction set
CPU IA64 IA64 CPU (in x86 mode) instruction set