The CPU, or rather microprocessor, is the heart of any computer system. The microprocessor calculates, performs logical operations and manages data flows by reading instructions from memory and then executing them. In the early days of computing the functional components of the microprocessor were separate (and physically large) units. This is when the term Central Processing Unit was coined. The modern microprocessor combines these components onto an integrated circuit etched onto a very small piece of silicon. The terms CPU, microprocessor and processor are all used interchangeably.
Microprocessors operate on binary data; that is data composed of ones and zeros. These ones and zeros correspond to electrical switches being either on or off. Just as 42 is a decimal number meaning “4 10s and 2 units”, a binary number is a series of binary digits each one representing a power of 2. In this context, a power means the number of times that a number is multiplied by itself. 10 to the power 1 ( 101 ) is 10, 10 to the power 2 ( 102 ) is 10×10, 103 is 10x10x10 and so on. Binary 0001 is decimal 1, binary 0010 is decimal 2, binary 0011 is 3, binary 0100 is 4 and so on. So, 42 decimal is 101010 binary or (2 + 8 + 32 or 21 + 23 + 25 ). Rather than using binary to represent numbers in computer programs, another base, hexadecimal is usually used.
In this base, each digital represents a power of 16. As decimal numbers only go from 0 to 9, the numbers 10 to 15 are represented as a single digit by the letters A, B, C, D, E and F. For example, hexadecimal E is decimal 14 and hexadecimal 2A is decimal 42 (two 16s + 10). Using the C programming language notation (as I do throughout this book) hexadecimal numbers are prefaced by “0x“; hexadecimal 2A is written as 0x2A .
Microprocessors can perform arithmetic operations such as add, multiply and divide and logical operations such as “is X greater than Y?”.
The processor’s execution is governed by an external clock. This clock, the system clock, generates regular clock pulses to the processor and, at each clock pulse, the processor does some work. For example, a processor could execute an instruction every clock pulse. A processor’s speed is described in terms of the rate of the system clock ticks. A 100Mhz processor will receive 100,000,000 clock ticks every second. It is misleading to describe the power of a CPU by its clock rate as different processors perform different amounts of work per clock tick. However, all things being equal, a faster clock speed means a more powerful processor. The instructions executed by the processor are very simple; for example “read the contents of memory at location X into register Y”. Registers are the microprocessor’s internal storage, used for storing data and performing operations on it. The operations performed may cause the processor to stop what it is doing and jump to another instruction somewhere else in memory. These tiny building blocks give the modern microprocessor almost limitless power as it can execute millions or even billions of instructions a second.
The instructions have to be fetched from memory as they are executed. Instructions may themselves reference data within memory and that data must be fetched from memory and saved there when appropriate.
The size, number and type of registers within a microprocessor is entirely dependent on its type. An Intel 4086 processor has a different register set to an Alpha AXP processor; for a start, the Intel’s are 32 bits wide and the Alpha AXP’s are 64 bits wide. In general, though, any given processor will have a number of general purpose registers and a smaller number of dedicated registers. Most processors have the following special purpose, dedicated, registers:
- Program Counter (PC)
- This register contains the address of
the next instruction to be executed. The contents of the PC are
automatically incremented each time an instruction is fetched.
- Stack Pointer (SP)
- Processors have to have access to large
amounts of external read/write random access memory (RAM) which facilitates
temporary storage of data. The stack is a way of easily saving and
restoring temporary values in external memory. Usually, processors
have special instructions which allow you to push values onto the
stack and to pop them off again later. The stack
works on a last in first out (LIFO) basis.
In other words, if you push two values, x and y, onto a stack and then pop
a value off of the stack then you will get back the value of y.
Some processor’s stacks grow upwards towards the top of memory whilst others grow downwards towards the bottom, or base, of memory. Some processors support both types of stacks, for example ARM.
- Processor Status (PS)
- Instructions may yield results; for example “is the content of register X greater than the content of register Y?” will yield true or false as a result. The PS register holds this and other information about the current state of the processor. For example, most processors have at least two modes of operation, kernel (or supervisor) and user. The PS register would hold information identifying the current mode.