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realmode.asm
65 lines (59 loc) · 2.35 KB
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realmode.asm
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bits 16
org 0x7c00
boot:
mov ax, 0x2401 ; Function 0x24 of int 0x15 returns the amount of extended memory in kilobytes
int 0x15 ; Call BIOS interrupt 0x15
mov ax, 0x3 ; Function 0x03 of int 0x10 sets video mode (80x25 text mode)
int 0x10 ; Call BIOS interrupt 0x10
cli ; Clear interrupts
lgdt [gdt_pointer] ; Load Global Descriptor Table (GDT) pointer
mov eax, cr0 ; Move the value of Control Register 0 into EAX
or eax, 0x1 ; Set the first bit of CR0 to enable protected mode
mov cr0, eax ; Move the modified value back into CR0
jmp CODE_SEG:boot2 ; Jump to code segment (in protected mode)
gdt_start:
dq 0x0 ; Null descriptor
gdt_code:
dw 0xFFFF ; Limit (0 to 0xFFFF)
dw 0x0 ; Base (0)
db 0x0 ; Base (0)
db 10011010b ; Access byte
db 11001111b ; Granularity
db 0x0 ; Base (0)
gdt_data:
dw 0xFFFF ; Limit (0 to 0xFFFF)
dw 0x0 ; Base (0)
db 0x0 ; Base (0)
db 10010010b ; Access byte
db 11001111b ; Granularity
db 0x0 ; Base (0)
gdt_end:
gdt_pointer:
dw gdt_end - gdt_start ; GDT size
dd gdt_start ; GDT base address
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start
bits 32
boot2:
mov ax, DATA_SEG ; Load data segment with data descriptor
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax ; Load stack segment with data descriptor
mov esi, hello ; Set ESI to point to the hello string
mov ebx, 0xb8000 ; Set EBX to the video memory address
.loop:
lodsb ; Load byte from SI into AL and increment SI
or al, al ; Check if AL is zero
jz halt ; If zero, jump to halt
or eax, 0x0100 ; Set attribute byte (color) to white on black
mov word [ebx], ax ; Store character and attribute at video memory address
add ebx, 2 ; Move to the next character cell in video memory
jmp .loop ; Repeat loop
halt:
cli ; Disable interrupts
hlt ; Halt execution
hello: db "Hello world!", 0 ; Define the hello string
times 510 - ($-$$) db 0 ; Fill the rest of the sector with zeros
dw 0xaa55 ; Boot signature