Hey guys, here's an update. No source code/screenshot with this one, though, unless someone really wants it.
Since the last update, i've made a console for inputting commands, and i've created a physical memory manager (aka a page frame allocator). The physical memory manager is pretty easy, it just keeps track of 4kb blocks of physical memory - handing out free blocks and marking them as used, or vise-versa.
Right now, i'm working on the virtual memory manager. Although, more specifically, i'm working on enabling paging. By doing this, i can isolate processes and the kernel into their own virtual address spaces. It's a bit tricky though, because once i enable paging, i have no direct access to physical memory. The processor translates assembly level addresses (ie mov eax, 0xDEADC0DE; call eax <-- 0xDEADC0DE is treated as a virtual address) into a physical address.
This means that i have to compile my kernel to load into physical memory at a certain address, but the code has to use virtual addresses for things like function calls - or anything that dereferences a pointer. Because of this, i have to write a special function that enables paging without calling any other functions.
So, how am i doing it? Well, here's my idea - and so far it's working.
My kernel is loaded to the 1mb physical memory mark - 0x100000.
The virtual address of my kernel is 0xC0000000 - the 3gb mark.
So, all my kernel functions are referenced at 0xC01xxxxx...
I write a function, say vmm_init(). In my kboot.asm (which is the code that is called by GRUB), i do the following:
Code: Select all
extern vmm_init
mov ebx, vmm_init
sub ebx, 0xC0000000
call ebx
This code allows me to call my function from it's real, physical address. However, now that i'm inside the function, i can only use local (stack) variables and no other functions unless i calculate their physical address.
From here, i set my page directory up - which has 1024 page tables. 1 Page table has 1024 page table entries that specify a 4kb aligned physical address, along with some access specifier bits (kernel/user access, read/write, execute, present/not present page, etc). A single page table represents 4mb of physical to virtual memory mapping. Since there are 1024 of these page tables, they represent, in total, 4gb of memory.
Since i'm working with physical addresses while setting this up, once i enable paging, i won't be able to refer to the page directory/page table structure to modify pages while the OS executes. How do i solve this? I map the last page directory entry to the address of the page directory - and now i know i the virtual address of the page directory ^_^
Anyway, as you can see, it gets a bit complicated. I'm having a lot of fun with it though
Sorry for the random "tutorial" lol
¯\_(ツ)_/¯ It works on my machine...
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