Assembly learning register

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Two 、 register ( With 8086 As a prototype )

1. General registers

 With 8086cpu For example , All of its registers are 16 Bit , It can be stored 2 Bytes .
AX BX CX DX It's called a general-purpose register ,  Used to store general data 
AX Can be divided into AH and AL ( according to 16 The height of the bit )
BX  Can be divided into BH and BL
CX  Can be divided into CH and CL
DX Can be divided into DH and DL
 With AX For example   Look at the picture below 

2. The storage of words in registers

8086cpu The following can be handled at one time 2 Data of three sizes :
1. byte (byte),1byte = 8bit,  Can exist 8 Bit register 
2. word , A word consists of two bytes , The two bytes are called the upper byte and the position byte of the word respectively , Stored separately in AH and AL in 

3. Preliminary knowledge of assembly instructions

 Assembly instructions or register names are not case sensitive 

Assembly instruction format	control cpu Completed operation	Use the syntax of high-level language to describe
mov ax( Register name ),18( data )	take 18 Into the register AX	AX=18
mov ax( Register name ),bx( Register name )	take bx The data in is sent to ax in	ax=bx
add ax( Register name ),18( data )	Register ax The value of plus 18	ax = ax + 18
add ax( Register name ),bx( Register name )	take ax and bx Add the data in	ax = ax + bx

4. 16 Bit structure cpu

8086cpu yes 16 Bit structure ,16 Bit structure means :
1.  The arithmetic unit can handle at most 16 Bit data 
2.  The maximum width of the register is 16 position 
3.  The path between the register and the arithmetic unit is 16 position 

5. 8086cpu The way to give a physical address

8086cpu There is... On the outside 20 Bit address bus ,  The addressing capability reaches 1mb(2^20B = 1MB)
8086cpu The internal is 16 Bit structure , Addressing capability is 64KB(2^16B)
8086cpu Use... Internally 2 individual 16 A bit address is synthesized by an address adder 20 For physical address 
 The workflow is as follows :

 Principle of address adder :  Physical address = Segment address *16( Move left 4 position )+ offset 
1. A binary shift of data to the left n position , Equivalent to this data 2 Of n Power ,16 A into 20 position (2^16 * 2^4 = 2^20)
2.20 position +16 position  = 20 position   analogy   Ten  +  hundred   The high position is still 100 

6. The concept of paragraph

cpu The physical address is formed in this way 
 Due to the physical address  =  Segment address *16 +  offset 
 The starting address of the segment  =  Segment address *16 ( by 16 Multiple )
 The offset address is 16 position (64KB),  So the maximum length of a segment is 64KB( Left offset or right offset )

7. cs and ip

 The segment register stores the segment address ,8086 There is CS DS SS ES	4 Segment registers , With cs For example 
cs:  Code segment register 
ip: Instruction pointer register 
8086cpu Will be with  M(cs The content of )*16+N(ip The content of )  Is the starting physical address , Read instructions and execute 

8086cpu General workflow :
1.  Through the address adder cs:ip Turn into 20 Bit address .
2. ip = ip +  The length of the read instruction  ( action   register , data  3 byte ,   action   register   register  2 byte )
3.  Execution instruction   repeat 1
8086cpu Initial value setting during startup or reset  cs=FFFFH IP=0000H 

8. modify cs and ip Instructions

Programmer pairs cpu Only by modifying the value of the register

mov Instructions :  Send instructions  mov ax ,123  take 123 The value is placed in ax in ,  Most registers can be modified by this instruction 
jmp Instructions :  Format  => jmp  Segment address : offset    jmp  Some legal register 
 Such as : 1. ax = 1000H  
    2.  perform jmp 2000H:0003H  here : cs=2000H  ip=0003H ( Directly modifying cs ip Value )
	3.  Re execution  jmp ax  here ip The value of is 2000H ( Modify with the value of the register ip Value )   

 Execute the process :  Memory value =cs*16 + ip  ip = ip +  Memory unit ( byte ) 
1.  Initial value  cs=2000H IP=0000H, cs*16 + ip = 20000H,  perform B8 22 66(mov ax,6622H), ip=ip+3=0003H
2. cs*16 + ip = 20003H,  perform EA 03 00 00 10(jmp 1000:3) ip=ip+5=0008H
3. 2 take cs,ip Reset  cs=1000H ip=0003H cs*16 + ip = 10003H, perform B8 00 00(mov ax,0000),ip=ip+3=0006H
4. cs*16 + ip = 10006H  perform 8B D8(mov bx,ax), ip=ip+2=0008H
5. cs*16 + ip = 10008H  perform FF E3(jmp bx) , ip=ip+2=000AH
6. 5 take ip=bx value , 4 take ax Value is assigned to bx,3 take ax Set up 0000  therefore ip by 0000 cpu from 10000H Start reading 

9. Code segment

A code segment is a contiguous segment of a defined memory , Used to store code , You can set the cs and ip value , Enable code snippets to execute , If the code segment is 123B0H~123B9H Set up cs=123BH IP=0000H, You can make cpu Execute the code snippet