/dev/log for kfs-2

• /dev/log for kfs-2
• Week 1
  • Implementation of rebooting and halt
  • Reading and planning of GDT implementation
  • GDT implementation mistake
  • System hang bug
  • GDT implementation and stack printer

Week 1

Implementation of rebooting and halt

For halting, just use the hlt instruction, which stops the CPU until an interrupt is received.

For a reboot, just jump to the address 0FFFFh:0, which is the Reset Vector

Reading and planning of GDT implementation

Upon reading on some implementation methods of the GDT, I have boiled down the steps needed to execute this.

1. Set stack to a const address first / Apply a global offset for the GDT
2. Make a segment descriptor structure representation
3. Create the segment descriptors required with the appopriate flags
4. Arrange them in an array
5. Create a GDT structure which contains the pointer to the first segment descriptor and the length of the GDT
6. Load the GDT using lgdt
7. Copy the offsets of segments to the respective segment registers
8. Flush the code segment register by making a far jump

GDT implementation mistake

I did everything in asm, since my final bin is elf32, I cant use the org instruction to set the intitial address of the GDT, but I got it to launch so thats cool, but need to fix the adderss of the gdt so ill need to move everything to c

System hang bug

I managed to replicate something similar in C, however the system hangs upon setting the address to the required address 0x800.

I inspected the kernel using GDB and it turns out that the kernel is stuck because it is calling to god-knows-what address 0x7a60098b during the initialization of the IRQ handlers .

I thought this a compile error since the code instructions are generated via the compiler, but then it struck me - the instrction address of the faulty call is 0x7ff, which is 1 away from 0x800, our supposedly gdt start address.

Turns out during the initialization of GDT (which is before register irq handlers) modifies the address that points to the IRQ part of the code, causing said corruption.

As a result, we needed to allocate empty padding at the beginning of the code data so that the GDT wont overwrite exising code data.

This can be done via the linker, with just 1 line of change

GDT implementation and stack printer

With the execution in place and the fix above done, the GDT now works, and the address is now correct

I also implemented the stack printer, it just prints the contents near the GDT address.