C language application development
1 Introduction
I skipped a few sections earlier. One is about a rather detailed content and the other is about the link to the original teacher’s article that is broken.
I'm not very good at taking notes, so I skipped it. Some bugs were fixed and some features were added.
It’s not that important, so I’ll ignore it.
After our operating system adds a kernel interface export mechanism
Already ready to run applications as a platform
But one shortcoming of current application development is that
We can only use assembly language to develop applications. It is too tiring to develop programs in assembly language.
It would be great if C language could be used
For example, when we want to develop an application that outputs a character on the console
It would be nice if the code could be written like this in C language (app.c):
void api_putchar(int c);
void main() {
api_putchar('C');
return;
}
If the above code can be compiled into a binary file and can be loaded and executed by the system
Then developing applications on our operating systems doesn’t have to be as painful as it used to be
What we will do in this section is to study how to use C language to develop applications that run on our system
2.Code
In fact, the steps we use to develop the kernel in C language can be transferred to developing applications.
The basic logic we actually follow when developing the kernel is this
Use assembly to develop the underlying interface and then use C language to call the interface exported from the assembly language to implement business logic.
Then compile the C code into a binary file
Then use the objconv tool to decompile C language into assembly language
The next step is to combine the original code developed in assembly with the decompiled assembly code into one
Finally, an assembly compiler is used to compile the integrated assembly code into a unified binary executable file.
We use C language to develop applications and follow the above logic
First we export the kernel API interface in assembly language
Use C language to call the interface exported by assembly language to complete the writing of business logic
Then compile the C language code into binary and use objconv to disassemble it
Finally, the two assembly language codes are combined into one and compiled into an executable binary file.
Based on the above logical steps
The first step we need to do is to export api_putcharthe interface for function calls in assembly language
The implementation code is as followsapi_call.asm
[SECTION .s32]
BITS 32
call main
retf
api_putchar:
mov edx, 1
mov al, [esp + 4]
int 02Dh
ret
%include "app.asm"
Let’s look at the api_putchar part first
We mentioned earlier that all APIs in the kernel correspond to a number. The API number that outputs one character to the console is 1.
When you want to call the corresponding api, just put the corresponding number into the register edx
At the same time, submit the parameters to the specified register and finally call the 2D interrupt.
at the front of the code
We first execute the statement call main
In other words, the main function is called directly. This means that when developing a program in C language, the main entry function must be main.
All program codes developed in C language are compiled into binary files and then disassembled into assembler
The disassembled program file is app.asm
Integrate the code of app.asm into api_call.asm through the include directive
Finally, use the assembly compiler nasm to compile api_call.asm. The resulting binary file is an application developed in C language.
3. Compile and run
After compiling and running (note that the makefile is updated)
After running
Switch focus and enter hlt to run the program
The following results were printed
