From 0d43780f3572a6cbbe915abdf9a5e37f6c5a5a38 Mon Sep 17 00:00:00 2001 From: Mathieu Maret Date: Tue, 10 Jul 2018 14:04:51 +0200 Subject: [PATCH] Add more comment in mbr --- mbr.asm | 28 ++++++++++++++-------------- 1 file changed, 14 insertions(+), 14 deletions(-) diff --git a/mbr.asm b/mbr.asm index d82c7c4..0db3b5a 100644 --- a/mbr.asm +++ b/mbr.asm @@ -94,26 +94,26 @@ main: mov bx, copy_target;target pointer int 0x13 - cli ; clear interruption flag + cli ; disable interruption when setting GDT ; switch in 32 bits lgdt [gdt_pointer] ; switch in 32bits here mov eax, cr0 or eax,0x1; set the protected mode bit on special CPU reg cr0 mov cr0, eax - jmp CODE_SEG:boot2 + jmp CODE_SEG:boot2 ; In protected mode we need to add the segment selector ; GDT table desciption could be found http://wiki.osdev.org/Global_Descriptor_Table ; here we define the 3 64bits segment needed: null segment, code segment and data segment gdt_start: ;null segment dq 0x0 gdt_code: ;code segment - dw 0xFFFF - dw 0x0 - db 0x0 - db 10011010b - db 11001111b - db 0x0 + dw 0xFFFF ; limit [0:15] + dw 0x0 ; base [0:15] + db 0x0 ; base [16:23] + db 10011010b ; access byte: Present(1)| Priv(2) 0 ->kernel 3->userspace | 1 | Executable(1) | Direction/Conformity (1) | RW(1) | Accessed(1) + db 11001111b ; Granularity(1) | Size (1) 0-> 16bit mode 1->32protected mode | 0 | 0 | Limit [16:19] + db 0x0 ; base [24:31] gdt_data: dw 0xFFFF dw 0x0 @@ -135,12 +135,12 @@ dw 0xaa55 copy_target: bits 32 boot2: - mov ax, DATA_SEG - mov ds, ax - mov es, ax - mov fs, ax - mov gs, ax - mov ss, ax + mov ax, DATA_SEG ; set all segments to point to DATA_SEG https://en.wikipedia.org/wiki/X86_memory_segmentation + mov ds, ax ; Data segment + mov es, ax ; Extra Segment (for string operation) + mov fs, ax ; No Specific use + mov gs, ax ; No Specific use + mov ss, ax ; stack segment mov esi,hello32 mov ebx,0xb8000 ; Cannot use BIOS anymore, use VGA Text buffer instead