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Code: Select all

ENTRY (_loader)

SECTIONS
{
    . = 0xF0100000;

    .text : AT(ADDR(.text) - 0xF0000000)
    {
        *(.text)
    }

    .rodata ALIGN (0x1000) : AT(ADDR(.rodata) - 0xF0000000)
    {
        *(.rodata)
    }

    .data ALIGN (0x1000) : AT(ADDR(.data) - 0xF0000000)
    {
        *(.data)
    }

    .bss :  AT(ADDR(.bss) - 0xF0000000)
    {
        _sbss = .;
        *(COMMON)
        *(.bss)
        _ebss = .;
    }
    
    _endofkernel = . ;
}

Code: Select all

KERNEL_VIRTUAL_BASE equ 0xF0000000   ; last 256MB of address space will be the virtual address for our kernel's code
KERNEL_PAGE_NUMBER equ (KERNEL_VIRTUAL_BASE >> 22)

Code: Select all

; Entry code for zerOS, by Wim Vander Schelden, based on many tutorials found at www.osdever.net and www.mega-tokyo.com/osfaq2
; Many thanks to bluecoder

global _loader           ; making entry point visible to linker
extern _main             ; _main is defined elsewhere

; setting up the Multiboot header - see GRUB docs for details
MODULEALIGN equ  1<<0                   ; align loaded modules on page boundaries
MEMINFO     equ  1<<1                   ; provide memory map
FLAGS       equ  MODULEALIGN | MEMINFO  ; this is the Multiboot 'flag' field
MAGIC       equ    0x1BADB002           ; 'magic number' lets bootloader find the header
CHECKSUM    equ -(MAGIC + FLAGS)        ; checksum required
KERNEL_VIRTUAL_BASE equ 0xC0000000   ; last 256MB of address space will be the virtual address for our kernel's code
KERNEL_PAGE_NUMBER equ (KERNEL_VIRTUAL_BASE >> 22)

section .data
; The following part contains the kernels page directory
kernelpagingdir:
   dd 0x00000083   ; First page: identity map the first 4MB - will be erased later
         ; bits set:
         ;   0 - page is present
         ;   1 - page is writable
         ;   7 - page is 4MB large
   times(KERNEL_PAGE_NUMBER-1) dd 0 ; All other pages, before the 0xF0000000 mark, are not present
   dd 0x00000083   ; Same as the first page, but now mapping 0xF0000000 to 0x00000000
   times(1024-KERNEL_PAGE_NUMBER-1) dd 0   ; All pages after the kernel are not present

; Note that the lines below are used for flat segmentation, every segment starts at 0 and ends at 
; the end of the systems memory. The lots of labels are just so that I would know what does what two weeks from now
GDTLIMIT    dw 40         ; The limit of the GDT: 5 entries * 8 bytes per entry -1 = 40
GDTBASE     dd NULLENTRYLIMITL   ; Start of the table

NULLENTRYLIMITL   dw 0         ; Null entry in the GDT
NULLENTRYBASEL   dw 0
NULLENTRYBASEM   db 0
NULLENTRYACCESS db 0
NULLENTRYGRANUL   db 0
NULLENTRYBASEH   db 0

KERNELCSLIMITL   dw 0xFFFF      ; Kernel code segment
KERNELCSBASEL   dw 0x0000
KERNELCSBASEM   db 0x00
KERNELCSACCESS   db 0x9A
KERNELCSGRANUL   db 0xCF
KERNELCSBASEH   db 0

KERNELDSLIMITL   dw 0xFFFF      ; Kernel data segment
KERNELDSBASEL   dw 0x0000
KERNELDSBASEM   db 0x00
KERNELDSACCESS   db 0x92
KERNELDSGRANUL   db 0xCF
KERNELDSBASEH   db 0

USERCSLIMITL   dw 0xFFFF      ; User code segment
USERCSBASEL   dw 0x0000
USERCSBASEM   db 0x00
USERCSACCESS   db 0xFA
USERCSGRANUL   db 0xCF
USERCSBASEH   db 0

USERDSLIMITL   dw 0xFFFF      ; User data segment
USERDSBASEL   dw 0x0000
USERDSBASEM   db 0x00
USERDSACCESS   db 0xF2
USERDSGRANUL   db 0xCF
USERDSBASEH   db 0

; End of segmentation code

section .text
align 4
MultiBootHeader:
   dd MAGIC
   dd FLAGS
   dd CHECKSUM

; reserve initial kernel stack space
STACKSIZE equ 0x4000          ; that's 16k.

_loader:
   mov esp, stack+STACKSIZE-KERNEL_VIRTUAL_BASE   ; set up the stack
   lgdt [GDTLIMIT-KERNEL_VIRTUAL_BASE]      ; set up the GDT
   mov cx, 0x10               ; The kernel data segment (16 byte offset into the GDT)
   mov ds, cx               ; copy it to the data segment registers
   mov es, cx
   mov fs, cx
   mov gs, cx
   mov ss, cx               ; and stack segment
   jmp 0x08:continue-KERNEL_VIRTUAL_BASE      ; Far jump into the code segment, on the next line
   continue:
   push eax               ; pass Multiboot magic number
   push ebx               ; pass Multiboot info structure
   call _main               ; call kernel
      hlt               ; halt machine should kernel return (which never should happen)

set_paging:
   

section .bss
align 32
stack:
   resb STACKSIZE      ; reserve 16k stack on a quadword boundary