Register of assembly language by Wang Shuang

Register concept

         The register is CPU The main components in , yes CPU Parts that programmers can read and write in . The programmer changes the contents of various registers to achieve the goal of CPU The control of .

         Different CPU, The number of registers 、 The structure is different .8086CPU Yes 14 A register , Each register has a name . These registers are ：AX、BX、CX、SI、DI、SP、BP、IP、CS、SS、DS、ES、PSW.

General registers

        8086CPU All the registers of are 16 Bit , It can hold two bytes .AX、AB、CX、DX this 4 Registers are usually used to store general data , It's called a general register .

         One 16 The bit register can store a 16 A data .

        8086CPU The last generation of CPU The registers in are 8 Bit , To ensure compatibility , Based on the previous generation CPU The program can run in with a little modification 8086 above ,8086CPU Of AX、BX、CX、DX this 4 Each register can be divided into two independent 8 Bit register to use ：

• AX Can be divided into AH and AL;
• BX Can be divided into BH and BL;
• CX Can be divided into CH and CL;
• DX Can be divided into DH and DL;

         AX It's low 8 position （0 position ~7 position ） Constitute the AL register , high 8 position （8 position ~15 position ） Constitute the AH register .AH and AL Registers can be used independently 8 Bit register .

  The storage of words in registers

         For compatibility reasons .8086CPU The following two sizes of data can be processed at one time .

• byte ： Write it down as byte, One byte by 8 individual bit form , Can exist 8 Bit register .
• word ： Write it down as word, A word consists of two bytes , These two bytes become the upper byte and the lower byte of the word respectively .

        A word can have a 16 Bit register . The upper and lower bytes of this word naturally exist   The height of this register 8 Bit register and low 8 Bit register , Pictured 2.4, A font data 20000, There is AX In the register , stay AH Its low... Is stored in 8 position .AH and AL Data in , That is, it can be regarded as the height of a font data 8 Bit and low 8 position , The size of this font data is 20000, It can also be regarded as two independent byte data , Their sizes are 78 and 32.

  Several assembly instructions

Assembly instruction	control CPU Completed operation	Use the syntax of high-level language to describe
mov ax,18	take 18 Into the register AX	AX=18
mov ah,78	take 78 Into the register AH	AH=78
add ax,8	Register AX Add... To the number in 8	AX=AX+8
mov ax,bx	Register BX The data in is sent to the register AX	AX=BX
add ax,bx	take AX and BX Add the values in , The result is AX in	AX=AX+BX

        Be careful , In order to make readers with high-level language foundation better understand the meaning of instructions , Sometimes the meaning of an assembly instruction is described in two ways: text description and high-level language description . Case insensitive when writing an assembly instruction or the name of a register . Such as ：mov ax,18 and MOV AX,18 It means the same thing ;bx and BX It means the same thing .

        PS： If the data size exceeds the register size , Then the highest data will not be stored . If you will 16 Bit register AX As two 8 Bit register AH and AL, So when doing the operation , Will be AL or AH As 1 Independent 8 Bit register , It's going on add The data size exceeds 8 position , Then the highest bit is lost （ The lost here , It means that the carry value cannot be in 8 Save... In bit register , But when CPU This carry value is not really discarded .）

        In data transmission or computation , Note that the number of bits of the two operands of the instruction should be the same .

Address adders

        The address adder uses Physical address = Segment address x16+ offset The physical address is composed of segment address and offset address . for example ,8086CPU The address to access is 123C8H The memory unit of , here , The operation of the address adder is shown in the figure 2.7 Shown ：

         “ Segment address x16” A more common expression is shift left 4 position .“ Move left 4 position ” In the middle , It refers to binary bits .

Shift left digit	Binary system	Hexadecimal	Decimal system
0	10B	2H	2
1	100B	4H	4
2	1000B	8H	8
3	10000B	10H	16
4	100000B	20H	32

1. The binary form of a data is shifted left 1 position , It is equivalent to multiplying the data by 2;
2. The binary form of a data is shifted left N position , It is equivalent to multiplying the data by 2 Of N Power ;
3. Address segment x16 Arithmetic , Is to move the address segment stored in binary form to the left 4 position .

        further , It is not difficult for us to get a left shift of Hexadecyl lactone form of data , Equivalent to times 16; The decimal form of a data is shifted one bit to the left , Equivalent to times 10; One X The hexadecimal data is shifted one bit to the left , Equivalent to times X.