Falk Joensson's
Learning Programming with Eas
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How the Computer Works:07. The Control Unit

The core component, the CPU, consists itself of various sub-components, all integrated in one microchip. Because of its high density of micro-electronic cicuits and the high-speed clock, the CPU of a computer can get very hot, risking to burn its own fragile circuitry — therefore it needs cooling, at least by a ventilation fan. At the core of the CPU lies the control unit, cicuitry that works with the input, output and control lines/registers and the program pointer. As a "line of code" is read from the program memory, the command register as well as one or more operand registers are set from it, which is called "fetching a command". In the next phase of the control unit, the command will be executed. The command register is a set of command lines that through circuitry define if an operand register will be used as a memory input value (constant) or the address (pointer/variable) of a reading or writing operation. It also sets the group of an address, sort of a postal code to point to the "region" in the computer where the address is found. The CPU works with several internal groups as well as with I/O ports (external groups). The RAM is but one address group that a program can read from and write to. Other groups are I/O ports to and from I/O devices, including such simple ones as the power-on LED and the beeper, as well as special CPU sub-processor groups such as the mathematical powerhorses arithmetic-logical unit (ALU) and floating-point unit (FPU). To add two Bytes, both values are written into the operand registers of the ALU, and then the add-command reads the pre-calculated result. Both operands work like address coordinates, pointing x,y or column,row to an address in the ALU lookup table at the "addition" offset. In other words, both operands taken together and extended by the command offset are the actual address into the ALU's ROM, in this example. (Not all results are stored as ROM lookup tables, some are more efficiently generated on the fly by Boolean circuitry.) A very clever, and essential, trick is that also the program pointer can be written to, so that programs don't simply always run from start to end in a fixed linear progression and then they end, but they can loop and skip lines of code, depending on programmed conditions. And of course also the program memory itself can be written to as a destination group, so that programs can be loaded and run on demand.
07. The Control Unit
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