FPU Added On-Die
Prior to the advent of the 486DX processor, IA32 processors did not include an on-die FPU. Rather, the end user had to add an external x87 FPU chip to the system and the processor treated it as a specialized IO device. Whenever the processor encountered a FPU instruction while fetching the current program from memory, it had to perform a series of one or more IO writes to send the instruction to the off-chip FPU to be executed. Obviously, this was very inefficient.
The 486DX processor incorporated the x87 FPU on the processor die (see Figure 19-1 on page 433) as another execution unit and all subsequent IA32 processors include the on-die FPU. The sections that follow provide a description of the FPU's register set ...