Nitpick: Only the x87 line snooped the bus and mimicked prefetch as it was a coprocessor that had its opcodes inside the x86's. Specifically, any opcode beginni

Nitpick: Only the x87 line snooped the bus and mimicked prefetch as it was a coprocessor that had its opcodes inside the x86's. Specifically, any opcode beginning with 0xD8 through 0xDF is an x87 opcode. The 8089 is different, and worked more like old IBM mainframes did with I/O channels; The 8089 had a dedicated opcode set (distinct from x86) that it would execute. "Jobs" (of sorts) were created on the 8086 CPU, then handed over to the 8089. It's interesting that Intel decided to use two different methods for executing coprocessor instructions. Why not just have the 8087 work like the 8089 does and send "jobs" to execute? Embedding the instruction set inside the 8086's meant having to put in work to duplicate the prefetch queue.