ESC lets you send instructions to an external coprocessor on the system bus. On the 80386 and earlier processors, this was usually a floating-point chip called a numeric (or math) coprocessor. The 8086 and 8088 work with an Intel 8087 math coprocessor; the 80286 and 80386 work with an 80287 or 80387 coprocessor. The 80486 DX has a math coprocessor, called the Floating-Point Unit (FPU), on the chip.
Note: In order to synchronize with the 8087 math coprocessor, WAIT instructions must precede all ESC instructions. The 80286+ has automatic instruction synchronization, hence WAITs are not needed.
Opcode The opcode for ESC begins with the 5-bit sequence "11011b". This sequence indicates that the rest of the opcode is an instruction meant for a coprocessor, as opposed to an instruction meant for the CPU.