Chapter IV - first procedure

4.1 The process of a source program from writing to execution

Write assembly source program

Using the text editor , Use assembly language to write assembly source program .

Compile and connect

• Compile programs in assembly language （MASM.EXE） Compile the source program in the source program file , Generate target file ;
• Reuse connection program （LINK.EXE） Connect to the target file , Generate executable files that can be run directly in the operating system .

Executable file

• Program （ Machine code translated from assembly instructions ） And data （ Defined data ）
• Related description information ( such as ： How much memory does it take , How big the program is )

Execute the program in the executable file

• Execute the program in the executable file in the operating system .
• The operating system follows the description in the executable file , Load the machine code and data in the executable file into memory , And initialize , Then from CPU Execution procedure .

4.2 Source program

• Assembly instruction
  from CPU distinguish , Corresponding machine code
  

• Pseudo instruction
  Understood by the compiler ,CPU Cannot read . Tell the compiler how to execute our code .
  
  XXX segment（ paragraph ）
  XXX ends

• segment and ends Is a pair of pseudo instructions , The function is to define a segment , The former represents the beginning , The following represents the end .

• end Is the end of an assembler .

• assume： If 、 hypothesis .

label

• A label represents an address
• codesg： Put it in segment In front of , As the name of a segment , The name of this segment is finally

DOS Program running in

• DOS Is a single task operating system .
• At the end of a program , take CPU The controller is returned , This is called ： Return of program
• To add the returned program segment at the end of the program .
• The function of these two instructions is program return .

mov ax,4c00H
int 21H
## interrupt 

End of paragraph 、 Program end 、 Program return

Grammatical and logical errors

• Grammar mistakes , Errors found by the compiler at compile time ;
• Logic error is something that a program cannot show at compile time 、 What happens at runtime .

About compiling and linking

• compile ： Assembly language is translated into machine language
• Connect ： Connect the target files together , Generate an executable file .

4.8 The principle that programs in executable files are loaded into memory and run

• stay DOS in , Programs in executable files P1 To run , There must be a running program P2, take P1 Load into memory from an executable , take CPU Give it control of ,P1 To run ; When P1 Operation completed , Should be CPU Control of the is returned to CPU So that it can continue to run the program P2
• We are DOS Run directly in 1.exe when , It's running command take 1.exe The program in is loaded into memory .
• command Set up CPU Of CS：IP The first instruction to the program （ It's the entry point of the program ）, So that the program can run .
• At the end of the program , Back to command in ,CPU Continue operation command.

The shell of the operating system

• DOS There's a program in command.com, This program is in DOS Called command interpreter in , That is to say DOS Systematic shell

4.9 Tracking of program execution process

summary ：EXE The loading process of the program in the file

• After the program is loaded ,ds The segment address of the memory area where the program is stored in , The offset address of this memory area is 0, The address of the memory area where the program is located is ：ds:0;
• The front of this memory area 256 In bytes are PSP,dos Used to communicate with programs .
• from 256 The space behind the byte is for the program .
• so, We from ds You can get PSP Section address of SA,PSP The offset address of is 0, Then the physical address is ：SA×16+0.
• because PSP Occupy 256（100H） byte , So the physical address of the program is ：
  SA×16+0+256=（SA+16）×16
  The segment address and offset address can be expressed as ：SA+10：0

command ：

• use U Command view other instructions ;
• use T Command but not execute every instruction in the program , And observe the execution result of each instruction
• here we are int21, We need to use it. P Command execution
• Come back to debug,q go back to command