3.1. The CPU
CPU means Central Processing Unit. The CPU is the kernel of each computer. All data
(instructions and user data) is read from the CPU into the CPU registers, see Fig. 4.
Instructions (the program code) are read into the instruction register, next they are decoded
in the instruction decoder. Depending on the instruction as next following data fetches. The
data are stored in the arithmetic registers (accumulator), from there they are processed in the
ALU (Arithmetic & Logic Unit). The ALU performs all arithmetical and logical processes on
the data. The result from the ALU is written back into a CPU register. From there the data
can be written back to the memory or an I/O device.
The most important signals from a CPU are the address bus, data bus and control signals.
These signals are connecting the CPU to the memory and the I/O devices.