chapter  14

CPUs

WithA. Padgett Peterson

The two basic divisions between processors today are those between Complex Instruction Set Computers (CISC) and Reduced Instruction Set Processor processors. CISCs are the traditional form in which the manufacturer decides the use of the processor, designs the microcode for all of the instructions the processor will use, and implements that microcode on the chip itself, optimizing the data paths for speed. Any discussion of CPU speed must be related to the speed at which the memory can deliver instructions to the processor. Optimally, the processor would request a memory location and on the same cycle it would be able to access that information. With the PC-AT and its 80286 processor, the standard bus width became 16 bits and accounted for most of the performance increase since clock speed was increased only slightly to 6 MHz. The introduction of the 386 made Windows possible by curing a problem in the 286—the inability to switch from virtual to real mode without rebooting.