Implementation of an operating system: protection mode

Today, I learned the protection mode and finally completed my own protection mode based on the content in the book and the video from the big gray wolf teacher.

[BITS 16]; indicates the following 16 BITS

Org 07c00h; this is because the system loads the code at 7C00.

Jmp start

Gdt_table_start:

Gdt_null:

Dd 0 h

Dd 0 h; Intel requires that the first descriptor in the descriptor table must be an empty descriptor.

Gdt_data_addr equ $-gdt_table_start

Gdt_data:

Dw 07FFh; the value of the data segment. The size of a Data Segment and code segment must be 8 Mb.

Dw 0 h; Segment Base Address 0 ~ 18-digit

Db 0 h; 19 ~ of the base address of the segment ~ 23-bit

Db 10010010b; the first 1 indicates P, which indicates whether the descriptor is valid for address conversion. The valid value is 1.

; The second 1 indicates DT, indicating the system segment or storage segment, and 1 indicates the system segment

; The third 1, together with 0010, indicates the type, that is, the readable and writable data segment or the readable and executable code segment.

; The sixth byte of the segment attribute

Db 111000000b; the first 1 indicates that the data segment is 4 GB. If it is 0, it indicates 64 K; the Code segment is 32 bits; if it is 0, it indicates 16 bits.

The second 1 indicates that the segment boundary granularity is byte, and 0 indicates 4 K.

; The seventh byte of the segment attribute

Db 0 h; base address

Gdt_code_addr equ $-gdt_table_start

Gdt_code:

Dw 07FFh; the value of the code segment. The size of a Data Segment and code segment must be 8 Mb.

Dw 1 h; Segment Base Address 0 ~ 18-digit

Db 80 h; 19 ~ of the base address of the segment ~ 23-bit

Db 10011010b; the first 1 indicates P, which indicates whether the descriptor is valid for address conversion. The valid value is 1.

; The second 1 indicates DT, indicating the system segment or storage segment, and 1 indicates the system segment

; The third 1, together with 1010, indicates the type, that is, the readable and writable data segment or the readable and executable code segment.

; The sixth byte of the segment attribute

Db 111000000b; the first 1 indicates that the data segment is 4 GB. If it is 0, it indicates 64 K.

; The second 1 indicates that the code segment is 32 bits, and 0 indicates 16 bits

; The seventh byte of the segment attribute

Db 0 h; base address

Gdt_video_addr equ $-gdt_table_start

Gdt_video:

Dw 01FFh; the Code segment's boundary value, which can be any value

Dw 8000 h; 0 ~ of the base address of the segment ~ 18-digit

Db 0Bh; 19 ~ of the base address of the segment ~ 23-bit

Db 10010010b; the first 1 indicates P, which indicates whether the descriptor is valid for address conversion. The valid value is 1.

; The second 1 indicates DT, indicating the system segment or storage segment, and 1 indicates the system segment

; The third 1, together with 1010, indicates the type, that is, the readable and writable data segment or the readable and executable code segment.

; The sixth byte of the segment attribute

Db 111000000b; the first 1 indicates that the data segment is 4 GB. If it is 0, it indicates 64 K.

; The second 1 indicates that the code segment is 32 bits, and 0 indicates 16 bits

; The seventh byte of the segment attribute

Db 0 h; base address

Gdt_table_end:

 

Gdtr_addr:

Dw gdt_table_end-gdt_table_start-1; segment descriptor table length, GDTR register is 48 bits

Dd gdt_table_start; base address of the segment descriptor table

 

Start:

; Initialize the data segment descriptor

Xor eax, eax

Mov eax, data_32

Mov word [gdt_data + 2], ax

Shr eax, 16

Mov byte [gdt_data + 4], al

Mov byte [gdt_data + 7], ah

; Initialize the code segment descriptor

Xor eax, eax

Mov eax, code_32

Mov word [gdt_code + 2], ax

Shr eax, 16

Mov byte [gdt_code + 4], al

Mov byte [gdt_code + 7], ah

 

Closing the interrupt vector table is equivalent to deleting the previous 16-bit interrupt vector table to prepare for loading the 32-bit interrupt vector table.

Cli

 

Load the size and base address of the GDT description table into the gdtr register.

Lgdt [gdtr_addr]

 

Open the A20 address line to access the address memory that exceeds 1 MB

In al, 92 h

Or al, 00000010b

Out 92 h, al

 

; Enable the protection mode to allow

Mov eax, cr0

Or eax, 1

Mov cr0, eax

 

Jump to the 32-bit address in Protected Mode

Jmp gdt_code_addr: 0

 

[BITS 32]

Data_32:

Dd "my OS! "

Code_32:

Mov ax, gdt_data_addr

Mov ds, ax

Mov ax, gdt_video_addr

Mov gs, ax

Mov edi, (80*20 + 30) * 2; display data in column 20th of Row 3 on the screen my OS!

Mov ah, 0ch; black background red letter

Mov bx, 0

Mov cx, 6

S:

Mov al, [ds: bx]

Mov [gs: edi], al

Mov [gs: edi + 1], ah

Inc bx

Add EDI, 2

Loop s

JMP $

Times 510-($-$) db 0

DW 0aa55h