Chapter 3 registers (memory access)

3.1 Storage of words in memory

stay 0 Start storing... At the address 20000（4E20H）：

• Be careful ：0 Unit number is the low address unit ,1 Unit number is a high address unit .

3.2 DS and [address]

• 8086CPU There is one of them. DS register , The segment address usually used to store the data to be accessed .
• Read 10000H The content of the unit

mov bx,1000H
mov ds,bx
mov al,[0]

mov al,[0]
Known mov Three transfer functions that can be completed by instructions ：

1. Put the data directly into the register ;
2. Send the contents of one register into another .
3. besides ,mov Instructions can also put the contents of a memory unit into a register .

mov Command format ：
mov Register name , Memory unit address
“[…]” Represents a memory unit ,“[…]” Medium 0 Represents the offset address of the memory unit .

• When the command is executed ,8086CPU Pick up automatically DS The data in is the segment address of the memory unit .

8086CPU It is not supported to directly send data into the segment register .

• How to send data from register to memory unit ?
• take al The data in is sent to the memory unit 10000H？
• Conclusion ：

MOV BX,1000H
MOV DS,BX
MOV [0],AL

3.3 The transmission of words

• 8086CPU One word at a time

MOV BX,1000H
MOV DS,BX
MOV AX,[0]
MOV [0],CX   ;cx Medium 16 Bit data is sent to 1000：0 It's about 

3.4 mov、add、sub Instructions

3.5 Data segment

• We can make a group of length N（N≤64K）、 Address continuous 、 The starting address is 16 A memory unit that is multiples of a memory unit is used as a memory space dedicated to storing data , This defines a data segment .
• For example, we use 123B0H~123B9H This space is used to store data ：
• Segment address ：123BH
• length ：10 byte

3.1~3.5 Section

1. When a word is stored in memory , To use two consecutive address memory units to store , The low order byte of a word is stored in a low address unit , The high byte is stored in the high address unit .
2. use mov Instruction to access memory cells , Can be in mov Only the address given in the instruction unit is offset , here , The segment address defaults to DS In the register .
3. [address] Represents an offset address of address The memory unit of .
4. When transferring font data between memory and register , High address units and high 8 Bit register 、 Low address units and low 8 Bit registers correspond to .
5. mov、add、sub Is an instruction with two operands .jmp Is an instruction that has an operation object .

3.6 Stack

• Last in, first out
• Basic operation ： Push and pull

3.7 CPU Stack mechanism provided

Push ：PUSH
Out of the stack ：POP

• 8086CPU The stack in and out of stack operations are based on words .
• 8086CPU in , There are two registers ：
• Segment register SS Store the segment address at the top of the stack
• register SP Store the offset address of the top of the stack
• Anytime ,SS：SP Point to the top element of the stack .

3.8 The problem of stack top out of bounds

• SS and SP Only the address at the top of the stack is recorded , Rely only on SS and SP It can ensure to find the top of the stack when entering and exiting the stack .
• Use when the stack is full PUSH Instruction stack , Use when the stack is empty POP Instructions are out of the stack , There will be stack top supersession problems .
• It's dangerous to overstep the top of the stack ：because Since we arrange a space as a stack , Then the space outside the stack space is likely to store data for other purposes 、 Code etc. , These data 、 The code may be in our own program , It could be from another program .

Stack and memory

• Stack space, of course, is also part of memory space , It is just a space that can be accessed in a special way

3.9 push、pop Instructions

• push register ： Stack data in a register

• pop register ： Out of the stack , Use a register to receive data from the stack

• push Segment register ： Stack data in a segment register

• pop Segment register ： Out of the stack , Use a segment register to receive data from the stack

• push Memory unit ： Put the word at a memory unit on the stack （ Stack operations are all in words ）

• pop Memory unit ： Out of the stack , Use a memory word unit to receive the stack data
  Instruction execution ,CPU To specify the address of the memory unit , Can be in push、pop The instruction gives the offset address of the memory unit , The segment address is when the instruction is executed ,CPU from ds Get in .

3.10 Stack segment

• We can take the length of N（N<=64K） A set of addresses in succession 、 The starting address is 16 Multiple of memory units , Use as a stack , This defines a stack segment .
• Treat a piece of memory as a stack segment , It's just an arrangement when we're programming ,CPU Not because of this arrangement , Is executed push、pop When we wait for the stack operation instruction, we will automatically take the stack segment we defined as Make stack space to access .