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Reach usermode, and run some basic things on it without crashing

This will require more entries in the GDT, and probably a Task State Segment, for storing kernel register values when jumping to kernel mode from user mode.

Info about TSS: http://wiki.osdev.org/TSS
Getting to ring 3: http://wiki.osdev.org/Getting_to_Ring_3

VGA text mode is deprecated and will not run in UEFI. The VBE (or GOP, in UEFI systems) framebuffer are the way to go here.

Then, we'll be able to load this OS in UEFI environments (GRUB supports UEFI, so no urge to write a bootloader)

Things we need to have

• Working PMM (#5) and VMM (#6). The kernel heap (#7) is encouraged, but not required
• Ramdisk support, to load a console font and use it
  • Virtual filesystem, implied by the ramdisk

Design an input subsystem.

In the future it will only forward keystrokes to user mode, but for now, and for testing, its objective is to make the sufficient for a getc() ` function in the kernel, for a kernel shell.

We need

• A keyboard driver, interrupt-based. PS/2 is a good start
• A generic keyboard subsystem

The kernel heap allocator serves to do small allocations, less than the 4kb granularity of the virtual memory manager.

#5 and #6 need to be closed. The PMM and the VMM need to be working well. Not bug-free (this is impossible, I think), but bug-free enough.

It provides malloc and free-like interfaces. I will use kmalloc and kfree here, for simplicity.

kmalloc will have three variations

• allocate a virtual address, no physical address returned
• allocate a virtual address in a specific physaddr zone, physical address returned (for DMA)
• allocate a virtual address, with a specific physaddr (for MMIO)

It would allocate virtual memory only from the kernel zone. The user zone would receive 4kb pages directly and the C library would do the same thing.

kfree would free the address allocated by any of these three kmallocs.

It would be the unique interface that drivers should use.. This results in a more portable system

I want to run this in my PC 😄

• Use a graphical framebuffer ( #8 )
  Text-mode doesn't exist in UEFI, only GOP

• Support Multiboot2 if we're staying with GRUB
  Multiboot2 offer a way of retrieving that EFI function pointer. This can be useful for getting more information about the system.

Design and add syscalls. Not all, just some basic ones

Note that we need to finish some things before even starting this:

• Working usermode
• Working device manager

Design and create a physical memory manager, so we know how much memory we have, and what blocks are occupied

This will be the base for the virtual memory manager, and for virtual memory allocations, and paging

This might help with directions

Add a driver for the Intel 8254, the legacy programmable interval timer for the x86 architecture.

It's simpler, so we'll start with it.

It should provide us base for a usleep() function, and a timer for multitasking operations.

More info about it here

Do a virtual memory manager

It should have functions to:

• Allocate a virtual address from some zone, with some allocated physical address, from some other physical zone.

  • One function might return the physical address too (for things like DMA, that need a physical address but any physical from a certain zone)
  • The another do not need to return the physical address.

• Allocate a virtual address from some zone, with a specific physical address. (for things like MMIO, that need a certain, exact physical address)

(Check #5 for some clarification about physical addresses, specially this comment )

It should be divided in zones: just a zone for kernel memory and other for user memory.

It should have a function to switch the CR3 (the page directory base register (PDBR), a register where the physical address of the page directory list go), so it's easy to do it for usermode (each process will have it's own page directory).