在此基础上，每一个数字（一位的）代表16的某一个次方。一位十进制数只能用来表示0-9，然而对于十六进制数而言，十进制的数字10到15可以由单个字母A,B,C,D,E,F表示。例如，16进制的E是十进制的14，16进制的2A是十进制的42（两个16加上10）。当使用C编程语言时，我们在十六进制数前加上“0x”前缀以表示其为16进制数，于是，十六进制的2A应该写成 0x2A 。
CPU中寄存器的大小，数量和类型完全由CPU的类型决定。一个Intel 4086处理器和Alpha AXP处理器的寄存器是不一样的。一开始，Intel系列是32位的而Alpha AXP则是64位的。大体上来说，虽然任意给定的处理器有许多相同目的的寄存器和较少数量的
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 register 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 processor's support both types, 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.