os_tutorial

WSL2

originally from https://github.com/cfenollosa/os-tutorial

launch qemu under Windows while running WSL2

qemu-system-x86_64.exe -s -S \wsl.localhost\Debian\home\user\devel\os\build\os-image.bin -boot c

• -s,-S awaits a debugger

Connect gdb debugger to qemu(on Windows)

ip route show | grep -i default | awk '{ print $3}'

gdb -ex "target remote 172.26.112.1:1234" -ex "symbol-file build/kernel.elf"

Compile with debug symbols ex:(hello.c)

void dummy_test_entrypoint() {
}

void main() {
    char* video_memory = (char*) 0xb8000;
    *video_memory = 'X';
}

gcc -m32 -g -ffreestanding -fno-pie -c hello.c -o hello.o

objdump -d hello.o

Disassembly of section .text:

00000000 <dummy_test_entrypoint>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   90                      nop
   4:   5d                      pop    %ebp
   5:   c3                      ret

00000006 <main>:
   6:   55                      push   %ebp
   7:   89 e5                   mov    %esp,%ebp
   9:   83 ec 10                sub    $0x10,%esp
   c:   c7 45 fc 00 80 0b 00    movl   $0xb8000,-0x4(%ebp)
  13:   8b 45 fc                mov    -0x4(%ebp),%eax
  16:   c6 00 58                movb   $0x58,(%eax)
  19:   90                      nop
  1a:   c9                      leave
  1b:   c3                      ret

gdb commands

Useful GDB commands

Command	Description
run (r)	Run the program until it finishes, crashes, or hits a breakpoint
start	Run and pause at the first line of main()
next (n)	Execute the next line (step over function calls)
step (s)	Execute the next line (step into function calls)
stepi	Executes the next instcution
continue (c)	Resume execution until the next breakpoint or crash
break file.hh:N or b file.hh:N	Set a breakpoint at line N in file.hh
break func	Set a breakpoint at function func()
delete	Remove all breakpoints
print var or p var	Print the value of variable val
list (l)	Show source code around current line
info locals	Show all local variables
quit (q)	Exit GDB

---　
b main
continue
step

---
stepi
disassemble main\

set {char}0xb8000 = 0x42
set {char}0xb8000 = 'G'

whatis *video_memory
output: type = char