Untitled_Kernel/src/start.s

;start.s
;Entrypoint for kernel, hello world!

bits 32
;Some symbols we'll need from other files...
extern kern_main
extern gdtr
extern _kernel_start
extern _kernel_end
;This will be our entrypoint function name - gotta initialize it now as global so the linker knows later.
global start
 

; Set up for C code. Practically the only requirement for C-generated assembly to work properly is alignment and the presence of a stack.
section .bss align=16
	stack_bottom:
		resb 4096 ; 4096 bytes (4kb) large stack. by skipping some amount of data (and eventually filling it with zeroes?), we've essentially just reserved space for our stack.
	;Remember, stack grows DOWNWARD! So the last thing in the section -> the highest memory address -> the very first thing on the stack!
    ;Therefore, we put a label here to represent the top of our stack for later.
    stack_top:
    ;We're gonna throw the TSS here too.
    
section .tss align=16 nobits
    tss:
        resb 104

section .bootstrap_tables nobits align=4096
    bootstrap_PD:
        resb 4096
    bootstrap_PT:
        resb 4096

;Actual code. Entry point goes here!
section .lower_text exec
	;Here it is!
    start:
        ;We made a GDT! Let's use it!
        lgdt [gdtr - 0xC0000000]
        ;Now we start by setting the code segment (CS) with a far jump...
        jmp 0x08:gdt_load


        gdt_load:
        ;Now we go ahead and dump our kernel mode data segment (0x10) into the rest of our segments.
        ;Also preserving eax because it has the bootloader's multiboot magic in it and I want to check it in main.
        mov cx, 0x10
        mov ds, cx
        mov es, cx
        mov ss, cx
        mov fs, cx
        mov gs, cx
        
        ;So, we want to set up a page table and page directory. Let's take inventory.
        ;eax - preserve, mb_magic
        ;ebx - preserve, mb_info
        ;ecx,edx,esi - can use
        ;Can technically use esp as the stack is set to stack_top later.
        
        ;ecx = our current indexing for pages, this is a 4K-aligned address
        mov ecx, 0x0
        ;edi = our pointer to the current page table entry. Array of 4byte entries. It's at, of course, where we put it earlier.
        ;Worth noting that it's actually physically at ~1M with the kernel. The pointer, however, is virtually inclined. So we need to adjust.
        mov edi, bootstrap_PT - 0xC0000000

        page_alloc_loop:
            ;If we're lower than our kernel, skip this page. If we've passed our kernel, we're done mapping.
            cmp ecx, _kernel_start
            jl next_page
            cmp ecx, _kernel_end - 0xC0000000
            jge page_alloc_done

            ;Okay, we're in kernel zone. Let's map this one.
            ;Let's make a page table entry in edx.
            ;The top bits for the page table entry are just the upper bits of the address, so we dont need to touch it.
            mov edx, ecx
            ;Set last two bits, "present" and "writeable". Later on we'll make a proper page table with proper permissions, but we're bootstrapping here.
            or edx, 0x003
            ;edi holds a pointer to the page table entry, so dereference it and throw our new entry (in edx) to it. Done!
            mov [edi], edx

        next_page:
            ;address page + 1
            add ecx, 0x1000
            ;page table entry + 1
            add edi, 4
            jmp page_alloc_loop

        page_alloc_done:
        ;By now we've mapped the kernel in a page table! Let's map VGA too, we'll need that address later.
        mov dword [bootstrap_PT - 0xC0000000 + 1023 * 4], 0x000B8000 | 0x27
        
        ;We have our page tables mapped to the phys addr of kernel + VGA. Time to add them to our page directory!
        ;First identity maps, second maps to higher half.
        mov dword [bootstrap_PD - 0xC0000000], bootstrap_PT - 0xC0000000 + 0x003
        mov dword [bootstrap_PD - 0xC0000000 + 4 * (0xC0000000 >> 22)], bootstrap_PT - 0xC0000000 + 0x003

        ;Let's set it! You do this with cr3.
        mov ecx, bootstrap_PD - 0xC0000000
        mov cr3, ecx
 
        ;Enable paging in cr0
        mov ecx, cr0
        or ecx, 0x80010000
        mov cr0, ecx
        ;Done! Leave to the higher half!
        lea ecx, setup_c_env
        jmp ecx

section .text

        setup_c_env:
        ;set up the stack. Stack grows downward on x86. We did the work earlier of defining where the top of the stack is, so just tell esp.
        mov esp, stack_top ;set the stack pointer
	    push ebx
        push eax
        ;To C-land!
        call kern_main

        ;You should never get here, but in case you do, we will just hang.
		hang:
			cli      ;Interrupts: off
			hlt      ;Halt!
			jmp hang ;just in case...