Editing Control unit (section)

== Integrating with the Computer ==

All modern CPUs have control logic to attach the CPU to the rest of the computer. In modern computers, this is usually a bus controller. When an instruction reads or writes memory, the control unit either controls the bus directly, or controls a bus controller. Many modern computers use the same bus interface for memory, input and output. This is called "memory-mapped I/O". To a programmer, the registers of the I/O devices appear as numbers at specific memory addresses. [[x86]] PCs use an older method, a separate I/O bus accessed by I/O instructions.

A modern CPU also tends to include an [[interrupt]] controller. It handles interrupt signals from the system bus. The control unit is the part of the computer that responds to the interrupts.

There is often a cache controller to [[cache memory]]. The cache controller and the associated cache memory is often the largest physical part of a modern, higher-performance CPU. When the memory, bus or cache is shared with other CPUs, the control logic must communicate with them to assure that no computer ever gets out-of-date old data.

Many historic computers built some type of input and output directly into the control unit. For example, many historic computers had a front panel with switches and lights directly controlled by the control unit. These let a programmer directly enter a program and debug it. In later production computers, the most common use of a front panel was to enter a small bootstrap program to read the operating system from disk. This was annoying. So, front panels were replaced by [[BIOS|bootstrap program]]s in read-only memory.

Most [[PDP-8]] models had a data bus designed to let I/O devices borrow the control unit's memory read and write logic.<ref>{{cite book |title=PDP-8L Maintenance Manual |date=1970 |publisher=Digital Equipment Corp. |location=Maynard Mass. |url=http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8l/DEC-8L-HR1B-D_8LmaintVol1.pdf |archive-url=https://web.archive.org/web/20150422211242/http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8l/DEC-8L-HR1B-D_8LmaintVol1.pdf |archive-date=2015-04-22 |url-status=live |access-date=26 December 2019}}</ref> This reduced the complexity and expense of high speed I/O controllers, e.g. for disk.

The [[Xerox Alto]] had a multitasking microprogrammable control unit that performed almost all I/O.<ref>{{cite book |title=Alto Hardware Manual |date=1976 |publisher=Xerox |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/Alto_Hardware_Manual_Aug76.pdf |archive-url=https://web.archive.org/web/20101207201936/http://bitsavers.informatik.uni-stuttgart.de/pdf/xerox/alto/Alto_Hardware_Manual_Aug76.pdf |archive-date=2010-12-07 |url-status=live}}</ref> This design provided most of the features of a modern PC with only a tiny fraction of the electronic logic. The dual-thread computer was run by the two lowest-priority microthreads. These performed calculations whenever I/O was not required. High priority microthreads provided (in decreasing priority) video, network, disk, a periodic timer, mouse, and keyboard. The microprogram did the complex logic of the I/O device, as well as the logic to integrate the device with the computer. For the actual hardware I/O, the microprogram read and wrote shift registers for most I/O, sometimes with resistor networks and transistors to shift output voltage levels (e.g. for video). To handle outside events, the microcontroller had microinterrupts to switch threads at the end of a thread's cycle, e.g. at the end of an instruction, or after a shift-register was accessed. The microprogram could be rewritten and reinstalled, which was very useful for a research computer.