Keywords:DOS Assembly Language INT 21H 
In the DOS era, we programmed using assembly language, and through the interrupt int 21H, we could perform almost all operations. That level of control over computer hardware was very fulfilling. In the WINDOWS era, assembly language has become increasingly distant from us. I have seen many introductions to assembly language that use interrupt int 80, which is just int 21H wrapped in a layer, allowing for graphical operations, but it never feels quite right. . Recently, I watched a video that introduced a fantastic free software that can run assembly language in a DOS interface. The software is so small yet powerful; it’s truly amazing. Nowadays, it’s called DOSBOX, and the interface after installation is as follows:
Below is an example of writing a character on the screen using this software. 1. Edit the program first.asm 
2. Generate macro assembly:
3. link 
4. Run
5. System software
The software is over 800k, and it even includes a debugger, it’s truly amazing. Using this software to write a program that can boot the computer is very challenging, although it has little significance in the Windows era, this is also the reason we do not have our own operating system. Using int 21H to boot the system and then entering the graphical interface, how this low-level logic is implemented is something that not many people may truly understand.