Assembly language interrupt and external device operation --06

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This series of articles refers to the fourth edition of assembly language and Assembly language programming Keynote speaker: helijian Sort out

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Shift instructions

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Example ： Logical shift instruction shl and shr

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Operating video memory data

Principle of display

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Shows the structure of the buffer

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A way of displaying information “ direct ” The way

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Describes the label of the memory unit

About label

The above writing needs to use offset To indicate the address of the data in the code segment

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Data label with colon removed

a[si]---->[si+a]

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The data label describes both the memory address and the unit length

mov al,b It's because b The memory unit of the operation is the word size , and al It's the byte size , Do not conform to the

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The more common way ： The data label in the data segment

All of the above put the data in the code segment , But generally, each section is stored separately .

If the data label is defined as data , The corresponding memory address is the memory address pointed to by the data label

a db 123
b dw 0
#c The label points to the memory unit , Saved two words of data , One is a The memory offset address pointed to by the label , The other is b The memory offset address pointed to by the label 
# here c A pointer that can be regarded as a pointer 
c dw a,b

a db 123
c dw 0
#c Indicates that it points to a memory unit , Two double word data are saved , For each doubleword data , The previous one saves the corresponding offset address , The latter one saves the corresponding segment address 
c dd a,b

When we define data labels as data , Is it particularly similar C A pointer to a pointer in a language

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Direct address table of data

Direct address table ： Solve problems by looking up tables

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The simplest solution

00101011  Is stored as a low byte in al Medium , The front and back four of them are respectively composed of 0-15 A number in , Then go to the character table to locate the character at the corresponding position 

• Because the minimum operating unit is bytes , So will ah preservation al The senior four of , Complete by shifting four bits to the right , and al Save the lower four digits , Through an and operation

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Direct address table

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Application example ： In order to speed up the operation, we use the look-up table method

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Solution

• First, use a table to record the corresponding sin value , The above uses the method of data labeling , amount to table dw ag0… in ag0 Deposit is sin0 The offset address of the corresponding value ( The pointer points to the pointer )
• Preparation : Stack the register state that will be used , then es The segment address of the additional segment register points to the video memory space
• al Storage angle ,bl Stored divisor 30, Then do the division operation , Because it is 8 Bit Division , So the chamber of commerce is kept in al in , The remainder is saved in ah in , We only care about business here
• Save vendor to bl in ,bh empty , here bl The offset address is stored in , But the unit is bytes
• bx It needs to be doubled , Because above table The unit of the table is the word , And here are bytes
• from table According to the offset address , Get the offset address of the corresponding string , Because here, the data label points to the data label , It can be seen as a pointer pointing to a pointer , The value of the pointer is the memory address it executes
• adopt bx String offset address saved in , Locate the corresponding memory address , And then output them byte by byte , Until I met 0 until
• Restore the state of the relevant register

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Direct address table of code

Use the code's direct addressing table to solve the problem

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The realization of various functions

The focus here is not on the specific implementation of each function , Instead, the address of the beginning of each subroutine is obtained by looking up the table in the main program , How this operation is done

• By means of ah Set the relative offset address of the subroutine to be executed in the table , however ah The saved offset address is in bytes , The unit of the table is the word , So there is a need for *2, Get the real offset address of the subroutine in the table

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The advantage of writing address table directly

Looking up tables can speed up , And it can make the program architecture very clear and easy to understand , It is also convenient for program expansion .

The idea of look-up table is similar to the strategy mode , Are used to solve a pile of IF...ELSE Judgmental

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Interrupt and its handling

The concept of interruption

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8086 Internal interruption of

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Interrupt handler

• because 8086CPU In the interrupt vector table of , Each interrupt program occupies four bytes , The first two bytes hold the offset address of the interrupt program code segment , The last two bytes hold the segment address of the interrupt program code segment .

And because each interrupt program occupies four bytes , therefore IP And N There are rules to follow ,IP=N4,CS=N4+2

When there is an interruption , First look up the table to get the address of the interrupt program , Then set the CS and IP To execute the interrupt program

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Case study ： In the system 0 Interrupt number

When a division error occurs , Trigger 0 Interrupt number , At this time, I will look up the table and get 0 Address of interrupt program No , Then set the CS and IP Point to the interrupt program address , Then execute the interrupt program

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Interrupt the process

It is necessary to protect the site before performing the interruption procedure , That's important , It mainly saves the status of the flag register and the address of the current program

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Write interrupt handling program

Interrupt handler and its structure

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Write interrupt handling program —— Take division error interrupt as an example

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do0 Where should the subroutine be placed ？

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Application framework

The installer is to put the program code into the memory space previously designated to store the program , Then change the interrupt vector table 0 Stored in table item No 0 The address of interrupt program No .

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do0 Implementation of the installer

Total number of bytes occupied by the program , It can be calculated by the direct difference between two address labels

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do0 Implementation of subroutines

Show characters , Write out the corresponding characters to the display buffer , Then set the interrupt vector table to do0 The entry address of the interrupt program , Write to the interrupt vector table 0 In table No

because do0 Interrupt program mov ax,4c00h int 21h Will end the program directly , return DOS System , Therefore, it will not be interrupted after the execution of the program , Return to the original program to continue .

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Summary

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Step break

from Debug Medium t Order to speak ……

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Single step interrupt process and processing

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application ： Interrupt does not respond

If you are executing to ss Register transfer data instructions , There is an interruption , that CPU The associated interrupt register value is set to 0, Interrupt generation is not allowed , So the next instruction will continue , After executing the next instruction , Then enter the interrupt

So the writing on the right is wrong , Because if it's written like this ,mov ss,ax and mov ax,0 Will be executed continuously together , and mov sp,10 The instruction will be executed in a single step

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from int An interrupt caused by an instruction

int n The interruption caused

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Write interrupt routines that are called by the provider

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Example ： interrupt 7ch Interrupt routine for

In execution int n Before the interruption , Will put the current CS and IP Register state stack , There are also flag register states on the stack , Then interrupt the execution of the routine , And then the related states are pushed out of the stack , Perform on-site restore .

If relevant registers are used in the interrupt program , It is also necessary to save the corresponding register status before use , Resume after the program ends

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BIOS and DOS Interrupt handling

BIOS—— Basic I / O system

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BIOS Interrupt call example

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What are they? BIOS interrupt , How to use it? ？

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The powerful function of assembly , also DOS interrupt ！

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int 21HDOS Application of interrupt routine

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BIOS and DOS Interrupt routine installation process

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Read and write port

Use ports to access peripherals ： Take phonation as an example

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CPU Neighbors of

CPU These ports control the behavior of various chips , These ports are essentially registers , But it is the register related to the corresponding chip , No cpu Internal slave register , These chips read the values of these registers , Know what you should do

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Read and write port

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Port read / write process demonstration

in al, 20h 
out 21h, al

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I/O Port allocation

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Examples of read / write instructions for ports

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operation CMOS RAM chip

CMOS RAM chip

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Port operation example ： extract CMOS RAM Time information stored in

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Displays the current month in the middle of the screen

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Peripheral connection and interruption

CPU Communicate with external devices through ports “ Connect ”

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External interruption ： An interrupt caused by an event occurring on an external device

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External interrupt processing

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PC Computer keyboard processing process

PC Computer keyboard processing process

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The scan code of the key on the keyboard ( General code )

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PC Computer keyboard processing process —— Cause an interrupt

The control keys and switch keys are stored by the keyboard status bytes , Each bit of this byte represents the state of a key

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PC Computer keyboard processing process —— Execute interrupt routine

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Input ‘a’ Processing of

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Custom keyboard input processing

PC Computer keyboard processing process (int 9 Interrupt routines )

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Realization ： Show... In turn ’a’~‘z’(v0.2)

The design of empty loop gives an idea :dx and ax Both are assigned a maximum value , And then put ax The value of the register is reduced to 0 end , When it's over dx The value of is also reduced to 0

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Realization ： Show... In turn ”a”~”z”(v0.4)

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Press down Esc Key to change the color of the display

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Realization : Press down Esc Key to change the color of the display (v1.0)

In the polling display a–z In the process of , Press any key , If an interrupt is triggered , First, it will call its original interrupt routine , After the execution of the original interrupt routine , If you press yes ESC Key will also change the color of the currently displayed font , If it's another key , There are no additional functions

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Methods of rewriting interrupt routines

Rewrite interrupt routine - With int 9 For example

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Implementation method

• To install a new program, first calculate int9 and int9end Byte difference between data labels , Equal to the size of the program , Then cycle through all bytes to 0:204h It's about
• Save the original interrupt address to 2:200 Unit
• take int9 The interrupt routine of the interrupt call becomes the address of the interrupt routine we have written , namely 0:204h, And this process should not be interrupted by maskable interrupts , adopt cli Setup completed , After the end , Allow to be interrupted , adopt STI complete
• Program return

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Respond to peripherals with interrupts

How to operate external equipment ？

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The processing of keyboard input int 9h Break and int 16h interrupt

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demonstration ： Input A、B、C、D、E、Shift_A、A It caused (int 9)“ action ”

The above is after input ABCDE After , Status of keyboard input buffer , Enter SHIFT_A

At this point, we press one more A

Shift The flag is restore

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demonstration int 16h

use int 16h Read out use int 9h Deposit deferred Data of flushing area

Keyboard buffer implementation

• common 16 word
• Use a ring queue
• Storable 15 One click Key scan code

Read a word from the keyboard buffer in turn , Put in the data buffer register , And then put in AX in ,AH Store scan code ,AL Deposit ASCII

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call int 16h Read the keyboard input from the keyboard buffer

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Application example ： Change the screen color

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application ： Input of string

The problem to be solved

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The processing of the program

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Subroutines ： The stack of characters 、 Out of stack and display

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Realize the putting of character stack 、 Out of stack and display

When there are several subfunctions in a program , Generally do not use one by one judgment , The table lookup method is used to determine which sub function should be called at present

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Read write disk

How to operate the disk ？

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BIOS Disk direct services provided ——int 13h

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use BIOS int 13h Read the disk

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use BIOS int 13h Write to the disk

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DOS Interrupt support for disk files ——int 21H

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Let computers “ Sing a song ”

External devices and how they are controlled ？

Programs can access peripherals directly , It is accessed through a port

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And " The computer sings " Related hardware and control

Let the computer sing , Need and 8253 and 8255 Dealing with chips , and CPU Want to control these two chips , It needs to be operated through the ports provided by the corresponding two chips

Because of the unified addressing , So we can easily locate the addresses of these two ports , And then through in and out Instructions write data to operate on it , Get the data we need from the corresponding port .

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“ translate ” music

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Performance program

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