274 lines
3.7 KiB
NASM
274 lines
3.7 KiB
NASM
DISK_ID EQU 0x81
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org 0x7C00
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bits 16
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jmp .start
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[bits 16]
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; Function: check_a20
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;
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; Purpose: to check the status of the a20 line in a completely self-contained state-preserving way.
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; The function can be modified as necessary by removing push's at the beginning and their
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; respective pop's at the end if complete self-containment is not required.
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;
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; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
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; 1 in ax if the a20 line is enabled (memory does not wrap around)
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check_a20:
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pushf
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push ds
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push es
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push di
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push si
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cli
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xor ax, ax ; ax = 0
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mov es, ax
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not ax ; ax = 0xFFFF
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mov ds, ax
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mov di, 0x0500
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mov si, 0x0510
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mov al, byte [es:di]
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push ax
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mov al, byte [ds:si]
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push ax
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mov byte [es:di], 0x00
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mov byte [ds:si], 0xFF
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cmp byte [es:di], 0xFF
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pop ax
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mov byte [ds:si], al
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pop ax
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mov byte [es:di], al
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mov ax, 0
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je check_a20__exit
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mov ax, 1
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check_a20__exit:
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pop si
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pop di
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pop es
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pop ds
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popf
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ret
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;;; Print string at bx
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puts:
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mov al, '!'
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mov ah, 0x0e
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int 0x10
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.puts_loop:
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mov al, [bx] ; load the character
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cmp al, 0 ; check for null byte
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je .end_puts_loop
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mov ah, 0x0e
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int 0x10 ; print charachter
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inc bx
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jmp .puts_loop
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.end_puts_loop:
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ret
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;;; Print bl as hex
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puthex:
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; Print high bits
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mov dl, bl
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shr dl, 4
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cmp dl, 9
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jg .puthex_high_alpha
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; It's < 10, print as digit
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mov al, dl
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add al, 0x30
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mov ah, 0x0e
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int 0x10
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jmp .puthex_low
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.puthex_high_alpha:
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; It's >= 10, print as letter
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mov al, dl
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add al, 0x57 ; 0x57 = (0x61 - 10)
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mov ah, 0x0e
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int 0x10
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.puthex_low:
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; Print low bits
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mov dl, bl
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and dl, 0b1111
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cmp dl, 9
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jg .puthex_low_alpha
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; It's < 10, print as digit
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mov al, dl
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add al, 0x30
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mov ah, 0x0e
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int 0x10
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jmp .puthex_end
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.puthex_low_alpha:
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; It's >= 10, print as letter
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mov al, dl
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add al, 0x57 ; 0x57 = (0x61 - 10)
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mov ah, 0x0e
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int 0x10
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.puthex_end:
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ret
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.start:
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; reset disk system
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mov ah, 0x00
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mov dl, DISK_ID
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int 0x13
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; print result on error
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jnc .reset_disk_no_error
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mov bl, ah
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call puthex
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.reset_disk_no_error:
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mov cx, 1 ; start at sector 1
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mov bx, 0x8000 ; write to 0x8000
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push bx
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push cx
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.read_disk_loop:
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; Read sector dl into memory
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mov ah, 0x02
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mov al, 0x01 ; number of sectors
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mov ch, 0x00 ; cylinder number, low 8 bits
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; mov cl, 0x01 ; cylinder number, high 2 bits + sector number (6 bits)
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mov dh, 0x00 ; head number
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mov dl, DISK_ID ; drive number
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int 0x13
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jc .read_disk_error
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; print result on error
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jnc .read_disk_no_error
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mov bl, ah
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call puthex
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.read_disk_no_error:
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pop cx
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pop bx
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inc cl ; next sector
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add bx, 512 ; add 512 to data start so we don't overwrite sectors
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push bx
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push cx
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jmp .read_disk_loop
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.read_disk_error:
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call check_a20
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cmp ax, 1
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je .a20_enabled
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mov bx, .str_no_A20
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call puts
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jmp .end
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.a20_enabled:
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; disable interrupts
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cli
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; reset ds
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xor ax, ax
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mov ds, ax
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; load the GDT
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lgdt [.gdt_desc]
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; switch to protected mode
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mov eax, cr0
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or eax, 1
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mov cr0, eax
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; jump to 32 bit code
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jmp 0x08:.clear_pipe
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[BITS 32]
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.clear_pipe:
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; set Data Segment and Stack segment
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mov ax, 10h
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mov ds, ax
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mov ss, ax
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; set up stack
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mov esp, 0x090000
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; load offset from ELF file
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; mind the differing endian
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mov edx, 0x8018
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mov ah, [ds:edx+2]
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mov al, [ds:edx+3]
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shl eax, 16
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mov ah, [ds:edx]
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mov al, [ds:edx+1]
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; add the beginning 0x8000
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add eax, 0x8000
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jmp eax
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.end:
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jmp .end
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.data:
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.str_no_A20:
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db "A20 not enabled"
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db 0
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.gdt:
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dd 0
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dd 0
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dw 0FFFFh
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dw 0
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db 0
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db 10011010b
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db 11001111b
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db 0
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dw 0FFFFh
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dw 0
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db 0
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db 10010010b
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db 11001111b
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db 0
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.gdt_end:
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.gdt_desc:
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dw .gdt_end - .gdt - 1
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dd .gdt
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; print padding nullbytes
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times 510 - ($ - $$) db 0
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; write magic string
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dw 0xAA55 |