diff options
Diffstat (limited to 'bootloader.asm')
| -rw-r--r-- | bootloader.asm | 86 |
1 files changed, 64 insertions, 22 deletions
diff --git a/bootloader.asm b/bootloader.asm index bf3afb4..f42e702 100644 --- a/bootloader.asm +++ b/bootloader.asm @@ -1,6 +1,8 @@ -ORG 0x0 +ORG 0x7c00 BITS 16 +CODE_SEG equ gdt_code - gdt_start ; 0x8 +DATA_SEG equ gdt_data - gdt_start ; 0x10 _start: jmp short start nop @@ -10,42 +12,82 @@ _start: times 33 db 0 start: - ; Set CS to 0x07c0 with label step2 as its beginning offset - jmp 0x07c0:step2 + ; Set CS to 0 with label step2 as its beginning offset + jmp 0:step2 step2: ; Setup segments cli ; Disable interrupts - mov ax, 0x07c0 + mov ax, 0 mov ds, ax mov es, ax - mov ax, 0x00 mov ss, ax mov sp, 0x7c00 ; Stack segment starts right before ds ; and grows down to 0x0. This works because ; Intel implements a full descending stack sti ; Enable interrupts - jmp $ +.load_protected: + cli + lgdt[gdt_table] + mov eax, cr0 + or eax, 0x1 + mov cr0, eax + jmp CODE_SEG:start_32 + +; GDT table description +gdt_start: +gdt_null: ; First segment Descriptor is always 0 + dd 0x0 + dd 0x0 + +;Second segment descriptor... Code segment - 0x8 +gdt_code: + dw 0xffff ; Segment limit 0-15 + dw 0 ; Base address 0-15 + db 0 ; Base 16-23 + db 0x9a ; Access byte field 0b10011010 + + ; We don't have a 4 bits type, so use a byte type + ; to set both Segment limit high bits and flags + + db 11001111b ; Segment limit 16-19 + ; Flags 0-3 + db 0 ; Base address high bits 24-31 + +;Second segment descriptor... Data segment - 0x10 +gdt_data: + dw 0xffff ; Segment limit 0-15 + dw 0 ; Base address 0-15 + db 0 ; Base 16-23 + db 0x92 ; Access byte field 0b10010010 + db 11001111b ; Segment limit 16-19 + ; Flags 0-3 + db 0 ; Base address high bits 24-31 +gdt_end: + +gdt_table: + dw gdt_end - gdt_start - 1 + dd gdt_start -; Message Print routines -print: - mov bx, 0 -.loop - lodsb - cmp al, 0 - je .done - call print_char - jmp .loop -.done: - ret - -print_char: - mov ah, 0x0e - int 0x10 - ret +[BITS 32] + +; No access to BIOS from now on.... +start_32: + ; Set all segments to the same as the DATA_SEG + mov ax, DATA_SEG + mov ds, ax + mov es, ax + mov fs, ax + mov gs, ax + mov ss, ax + + ; Set the stack pointer and base pointer further in mem + mov ebp, 0x00200000 + mov esp, ebp + jmp $ ; Fill in to the end and add bootloader signature times 510 - ($ - $$) db 0 |