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Commit 418bb11

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author
Carlos Fenollosa
committed
Lesson 18
1 parent 69a7c1c commit 418bb11

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17 files changed

+412
-16
lines changed

17 files changed

+412
-16
lines changed

‎14-checkpoint/boot/32bit_print.asm‎

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‎14-checkpoint/boot/32bit_print.asm‎

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[bits 32] ; using 32-bit protected mode
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; this is how constants are defined
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VIDEO_MEMORY equ 0xb8000
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WHITE_OB_BLACK equ 0x0f ; the color byte for each character
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print_string_pm:
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pusha
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mov edx, VIDEO_MEMORY
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print_string_pm_loop:
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mov al, [ebx] ; [ebx] is the address of our character
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mov ah, WHITE_OB_BLACK
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cmp al, 0 ; check if end of string
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je print_string_pm_done
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mov [edx], ax ; store character + attribute in video memory
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add ebx, 1 ; next char
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add edx, 2 ; next video memory position
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jmp print_string_pm_loop
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print_string_pm_done:
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popa
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ret

‎14-checkpoint/boot/disk.asm‎

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‎14-checkpoint/boot/disk.asm‎

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; load 'dh' sectors from drive 'dl' into ES:BX
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disk_load:
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pusha
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; reading from disk requires setting specific values in all registers
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; so we will overwrite our input parameters from 'dx'. Let's save it
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; to the stack for later use.
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push dx
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mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
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mov al, dh ; al <- number of sectors to read (0x01 .. 0x80)
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mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
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; 0x01 is our boot sector, 0x02 is the first 'available' sector
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mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
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; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
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; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
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mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)
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; [es:bx] <- pointer to buffer where the data will be stored
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; caller sets it up for us, and it is actually the standard location for int 13h
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int 0x13 ; BIOS interrupt
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jc disk_error ; if error (stored in the carry bit)
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pop dx
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cmp al, dh ; BIOS also sets 'al' to the # of sectors read. Compare it.
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jne sectors_error
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popa
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ret
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disk_error:
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mov bx, DISK_ERROR
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call print
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call print_nl
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mov dh, ah ; ah = error code, dl = disk drive that dropped the error
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call print_hex ; check out the code at http://stanislavs.org/helppc/int_13-1.html
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jmp disk_loop
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sectors_error:
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mov bx, SECTORS_ERROR
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call print
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disk_loop:
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jmp $
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DISK_ERROR: db "Disk read error", 0
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SECTORS_ERROR: db "Incorrect number of sectors read", 0

‎14-checkpoint/boot/gdt.asm‎

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‎14-checkpoint/boot/gdt.asm‎

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gdt_start: ; don't remove the labels, they're needed to compute sizes and jumps
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; the GDT starts with a null 8-byte
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dd 0x0 ; 4 byte
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dd 0x0 ; 4 byte
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; GDT for code segment. base = 0x00000000, length = 0xfffff
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; for flags, refer to os-dev.pdf document, page 36
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gdt_code:
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dw 0xffff ; segment length, bits 0-15
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dw 0x0 ; segment base, bits 0-15
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db 0x0 ; segment base, bits 16-23
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db 10011010b ; flags (8 bits)
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db 11001111b ; flags (4 bits) + segment length, bits 16-19
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db 0x0 ; segment base, bits 24-31
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; GDT for data segment. base and length identical to code segment
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; some flags changed, again, refer to os-dev.pdf
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gdt_data:
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dw 0xffff
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dw 0x0
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db 0x0
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db 10010010b
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db 11001111b
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db 0x0
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gdt_end:
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; GDT descriptor
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gdt_descriptor:
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dw gdt_end - gdt_start - 1 ; size (16 bit), always one less of its true size
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dd gdt_start ; address (32 bit)
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; define some constants for later use
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CODE_SEG equ gdt_code - gdt_start
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DATA_SEG equ gdt_data - gdt_start

‎14-checkpoint/boot/kernel_entry.asm‎

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‎14-checkpoint/boot/kernel_entry.asm‎

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[bits 32]
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[extern main] ; Define calling point. Must have same name as kernel.c 'main' function
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call main ; Calls the C function. The linker will know where it is placed in memory
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jmp $

‎14-checkpoint/boot/print.asm‎

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‎14-checkpoint/boot/print.asm‎

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print:
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pusha
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; keep this in mind:
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; while (string[i] != 0) { print string[i]; i++ }
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; the comparison for string end (null byte)
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start:
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mov al, [bx] ; 'bx' is the base address for the string
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cmp al, 0
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je done
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; the part where we print with the BIOS help
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mov ah, 0x0e
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int 0x10 ; 'al' already contains the char
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; increment pointer and do next loop
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add bx, 1
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jmp start
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done:
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popa
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ret
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print_nl:
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pusha
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mov ah, 0x0e
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mov al, 0x0a ; newline char
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int 0x10
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mov al, 0x0d ; carriage return
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int 0x10
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popa
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ret

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