Debugging Tips - The LearnixOS Book

[giscus[bot]]

ch01-03-debugging-methods

https://www.learnix-os.com/ch01-03-debugging-methods.html

[Zhomart]

For anyone curious about assembler code for the "hello world" kernel, running ndisasm -b 16 target/16bit_target/release/kernel prints the following assembler code:

; Sets ECX to -11 (in two's complement), which is the string length.
; After 11 increments, ECX becomes 0.
00000000  66B9F5FFFFFF      mov ecx,0xfffffff5

; Loads a character from memory address (0x7c25 + ECX offset)
00000006  678A91257C0000    mov dl,[ecx+0x7c25]

; BIOS teletype function (prints character).
; Move the character to AL register (value is read from DL).
; Call BIOS interrupt to print the character in AL.
0000000D  B40E              mov ah,0xe
0000000F  88D0              mov al,dl
00000011  CD10              int byte 0x10

; Loop: Increment ECX.
; Jump back to line 2 if ECX is not zero.
00000013  6641              inc ecx
00000015  75EF              jnz 0x6

; Halt and return
00000017  F4                hlt
00000018  66C3              retd

; String data (ignore the asm code, just look at the raw data)
0000001A  68656C            push word 0x6c65
0000001D  6C                insb
0000001E  6F                outsw
0000001F  20776F            and [bx+0x6f],dh
00000022  726C              jc 0x90
00000024  640000            add [fs:bx+si],al
00000027  0000              add [bx+si],al
...
; Required boot sector signature.
000001FD  0055AA            add [di-0x56],dl