Composition of computer

The following figure is an abstract diagram of the hardware that makes up the computer .

• CPU： The core hardware of a computer , Responsible for the execution of （ Calculation ） Program . All hardware devices work around it .
• Bus ： There are many integrated circuits on the motherboard , be responsible for CPU High speed communication with other devices .
• Memory ： Assist the computer to complete the calculation task . Because complex tasks require complex calculation steps , The amount of calculation results produced by complex calculation steps is CPU The register cannot hold , Memory is responsible for helping CPU Storage exceeds CPU Those intermediate results of register capacity .
• Other equipment ： There are other devices on the bus , For example, the graphics card will be connected to the display 、 The disk controller will connect the hard disk 、USB The controller will connect the keyboard, mouse and so on .

CPU The composition of

CPU In fact, it is not a simple piece , It consists of three parts , Arithmetic unit 、 Data unit and control unit .

• Arithmetic unit ： In charge of calculation , For example, adding 、 Do displacement, etc . however , It doesn't know what data to count , Where should the operation result be placed . If the data calculated by the operation unit passes through the bus every time , Go to the memory and get it now , It's too slow , So there it is
• Data unit ： Be responsible for temporarily storing data and calculation results . Data unit Include CPU Internal cache and register set , The space is very small , But fast .
• control unit ： Be responsible for what operations . It's a unified command center , Its register can get the next instruction , Then execute the command . This instruction will instruct the arithmetic unit to fetch some data in the data unit , Calculate a result , Then put it somewhere in the data unit .

CPU Cooperation with memory

First , Make it clear that ,

• Each process corresponds to a program , The program is stored on the hard disk in binary form .
• Once the process runs , You will have your own independent memory space . for example ： The process in the figure A and B Memory space is isolated and discontinuous .
• The memory space of a process is divided into code segments and data segments , This is a relatively abstract division method , It's actually more complicated .
• The instruction is divided into two parts . The first part is the operand , What operation does it represent ; The latter part is the operation data .
• Instruction pointer register ： In order to CPU control unit , It stores the address of the next instruction in memory .

after , Let's see CPU Interaction with memory ,

1. First , be located CPU The instruction pointer register of the control unit will guide the control unit to continuously read instructions from the code segment , And store the instructions in the instruction register located in the control unit .
2. after , The first part of the instruction is given to the arithmetic unit , The second part is handed over to the data unit .
3. In the following , The data unit reads the data in the data segment into the data register of the data unit according to the data address , At this time, the data can participate in the operation of the operation unit .
4. When the operation is finished , The result will be temporarily stored in the data register of the data unit .
5. Final , There will be instructions to write data back to the data segment in memory .

CPU How to distinguish the processes to be executed

The above process is for a process , So many times CPU How to distinguish ？
CPU There are two registers in the control unit to distinguish which process should be executed at present , They are Instruction start address register and Data start address register .

• Instruction start address register ： Used to point to the starting address of the code segment .
• Data start address register ： Used to point to the starting address of the data segment .

When the points of two registers belong to the memory space of the same process , So what is currently executing is the instruction of this process .

Bus ——CPU Channels that interact with memory

CPU The interaction with memory mainly involves two types of data , One is the address , That is, where do I want to get the data in memory ; One is data , The real data . They are transmitted through different types of buses , They are address bus and data bus .

• Address bus ： The number of digits determines how wide the address range can be accessed . For example, there are only two , that CPU You can only recognize 00,01,10,11 Four positions , More than four positions , You can't tell . The more digits , The more locations you can access , The more memory you can manage .
• data bus ： The number of digits determines how many data can be taken in at a time . For example, there are only two , that CPU You can only get two digits from memory at a time . If you want eight , Just take it four times . The more digits , The more data you get at a time , The faster the access speed .

X86 framework

16 Bit model ——8086 processor

control unit

First look at the registers of the control unit ：

• IP register ： Instruction pointer register . Point to the location of the next instruction in the code segment .CPU According to it, the instructions will be continuously removed from the code segment in memory , Load into CPU In the instruction queue , Then give it to the arithmetic unit to execute .
• CS register ： Code segment register , It is the instruction start address register in the previous figure .
• DS register ： Segment register , Is the data start address register in the previous figure .
• SS register ： Stack segment register , The starting address of the stack segment .
• ES register
• Other registers

Data unit

First look at the registers of the control unit ：

• General registers ：8 individual 16 position , The number of digits depends on the machine word length . Namely AX、BX、CX、DX、SP、BP、SI、DI. These registers are mainly used to temporarily store data during calculation .
• AX、BX、CX、DX It can be divided into two 8 Bit register to use , Namely AH、AL、BH、BL、CH、CL、DH、DL, among H Namely High（ High position ）,L Namely Low（ Low position ） It means .

CPU How to get data from memory

The data bus has only 16 position , The address is also 16 position . The address bus has 20 position , So how to address it ？CS、DS Corresponding to the starting position , The starting position can only be calibrated , To determine the specific position in the segment, you also need the position in the segment , This is called offset . The formula is ：
The specific location in the paragraph = The starting position << 4 + Offset .
The offset of the code segment is IP In the register , The offset of the data segment is in the general register . in addition , Addition does not cause data bit overflow , because , The range of offset is determined according to the start address and end address of the segment , It's not as big as you can be . such as ： The starting address of the segment is FFFF0, The maximum segment end address is FFFFF, The range of the offset is 0000~000F, The offset can never be FFFF.
This is a kind of direct Methods , Get the segment start address from the segment register .

32 Bit model

x86 Architecture is a compatible structure ,32 The design of bit should also take into account 16 Design of bit architecture .

Data unit

• General registers ： Put the original 16 The bit register is extended to 32 position , But still keep 8 Bit and 16 Bit composition .

control unit

• IP register ：16 Bit extension to 32 position .

Control unit and original 16 Bit design incompatibility

Because the original model （16 Bit design ） Actually, it's a little nondescript , Because it's not used 16 Bit data as the starting address of a segment , I didn't press 8 Bits or 16 Bit extended form , But according to the hardware at that time , Got a top-down 20 Address of bit . In this way, you have to move four places to the left every time , This means that the starting address of the segment cannot be anywhere , It's just divisible 16 The place of .

How to solve it ？ set up a separate kitchen ！

• Segment descriptor cache device ： True segment start address .
• Some form ： Consists of segment descriptors , Each item in the table is a segment descriptor .
• CS、DS、SS、ES register ： Still 16 position , Save an item in the form .
• Segment selection sub register ：CS、DS、SS、ES Register composition .

CPU How to get data from memory

Segment select from register first select an item from the table , Then get the starting address of the segment from this item . The segment start address is initially in memory ,CPU In order to get the address faster , Will put the starting address of the segment into CPU The cache .
This is a kind of indirect Methods .

CPU Real mode and protection mode

CPU Pattern	The way of distinction	Time	remarks
Real model	Get the segment start address directly from the segment register	When the system starts up	This is compatible 16 Bit
Protected mode	Indirectly, first find an entry in the table from the segment register , Then get the starting address of the segment from an item in the table	Need more memory	Follow certain rules , Carry out a series of operational switching