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cfenollosacfenollosa
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lesson 23, initial commit
1 parent d271f5b commit 0134c56

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‎23-fixes/Makefile

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C_SOURCES = $(wildcard kernel/*.c drivers/*.c cpu/*.c libc/*.c)
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HEADERS = $(wildcard kernel/*.h drivers/*.h cpu/*.h libc/*.h)
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# Nice syntax for file extension replacement
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OBJ = ${C_SOURCES:.c=.o cpu/interrupt.o}
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# Change this if your cross-compiler is somewhere else
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CC = /usr/local/i386elfgcc/bin/i386-elf-gcc
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GDB = /usr/local/i386elfgcc/bin/i386-elf-gdb
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# -g: Use debugging symbols in gcc
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CFLAGS = -g -m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles -nodefaultlibs \
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-Wall -Wextra -Werror
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# First rule is run by default
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os-image.bin: boot/bootsect.bin kernel.bin
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cat $^ > os-image.bin
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# '--oformat binary' deletes all symbols as a collateral, so we don't need
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# to 'strip' them manually on this case
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kernel.bin: boot/kernel_entry.o ${OBJ}
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i386-elf-ld -o $@ -Ttext 0x1000 $^ --oformat binary
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# Used for debugging purposes
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kernel.elf: boot/kernel_entry.o ${OBJ}
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i386-elf-ld -o $@ -Ttext 0x1000 $^
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run: os-image.bin
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qemu-system-i386 -fda os-image.bin
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# Open the connection to qemu and load our kernel-object file with symbols
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debug: os-image.bin kernel.elf
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qemu-system-i386 -s -fda os-image.bin -d guest_errors,int &
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${GDB} -ex "target remote localhost:1234" -ex "symbol-file kernel.elf"
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# Generic rules for wildcards
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# To make an object, always compile from its .c
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%.o: %.c ${HEADERS}
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${CC} ${CFLAGS} -ffreestanding -c $< -o $@
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%.o: %.asm
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nasm $< -f elf -o $@
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%.bin: %.asm
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nasm $< -f bin -o $@
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clean:
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rm -rf *.bin *.dis *.o os-image.bin *.elf
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rm -rf kernel/*.o boot/*.bin drivers/*.o boot/*.o cpu/*.o libc/*.o

‎23-fixes/README.md

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*Concepts you may want to Google beforehand: XX*
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**Goal: Fix miscellaneous issues with our code**
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The OSDev wiki has a section [which describes some issues with
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JamesM's tutorial](http://wiki.osdev.org/James_Molloy%27s_Tutorial_Known_Bugs).
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Since we followed his tutorial for lessons 18-22 (interrupts through malloc), we'll
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need to make sure we fix any of the issues before moving on.
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‎23-fixes/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

‎23-fixes/boot/bootsect.asm

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; Identical to lesson 13's boot sector, but the %included files have new paths
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[org 0x7c00]
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KERNEL_OFFSET equ 0x1000 ; The same one we used when linking the kernel
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mov [BOOT_DRIVE], dl ; Remember that the BIOS sets us the boot drive in 'dl' on boot
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mov bp, 0x9000
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mov sp, bp
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mov bx, MSG_REAL_MODE
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call print
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call print_nl
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call load_kernel ; read the kernel from disk
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call switch_to_pm ; disable interrupts, load GDT, etc. Finally jumps to 'BEGIN_PM'
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jmp $ ; Never executed
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%include "boot/print.asm"
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%include "boot/print_hex.asm"
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%include "boot/disk.asm"
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%include "boot/gdt.asm"
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%include "boot/32bit_print.asm"
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%include "boot/switch_pm.asm"
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[bits 16]
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load_kernel:
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mov bx, MSG_LOAD_KERNEL
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call print
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call print_nl
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mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
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mov dh, 31 ; Our future kernel will be larger, make this big
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mov dl, [BOOT_DRIVE]
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call disk_load
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ret
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[bits 32]
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BEGIN_PM:
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mov ebx, MSG_PROT_MODE
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call print_string_pm
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call KERNEL_OFFSET ; Give control to the kernel
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jmp $ ; Stay here when the kernel returns control to us (if ever)
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BOOT_DRIVE db 0 ; It is a good idea to store it in memory because 'dl' may get overwritten
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MSG_REAL_MODE db "Started in 16-bit Real Mode", 0
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MSG_PROT_MODE db "Landed in 32-bit Protected Mode", 0
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MSG_LOAD_KERNEL db "Loading kernel into memory", 0
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MSG_RETURNED_KERNEL db "Returned from kernel. Error?", 0
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; padding
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times 510 - ($-$$) db 0
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dw 0xaa55

‎23-fixes/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

‎23-fixes/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

‎23-fixes/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 $

‎23-fixes/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

‎23-fixes/boot/print_hex.asm

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; receiving the data in 'dx'
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; For the examples we'll assume that we're called with dx=0x1234
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print_hex:
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pusha
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mov cx, 0 ; our index variable
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; Strategy: get the last char of 'dx', then convert to ASCII
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; Numeric ASCII values: '0' (ASCII 0x30) to '9' (0x39), so just add 0x30 to byte N.
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; For alphabetic characters A-F: 'A' (ASCII 0x41) to 'F' (0x46) we'll add 0x40
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; Then, move the ASCII byte to the correct position on the resulting string
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hex_loop:
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cmp cx, 4 ; loop 4 times
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je end
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; 1. convert last char of 'dx' to ascii
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mov ax, dx ; we will use 'ax' as our working register
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and ax, 0x000f ; 0x1234 -> 0x0004 by masking first three to zeros
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add al, 0x30 ; add 0x30 to N to convert it to ASCII "N"
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cmp al, 0x39 ; if > 9, add extra 8 to represent 'A' to 'F'
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jle step2
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add al, 7 ; 'A' is ASCII 65 instead of 58, so 65-58=7
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step2:
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; 2. get the correct position of the string to place our ASCII char
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; bx <- base address + string length - index of char
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mov bx, HEX_OUT + 5 ; base + length
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sub bx, cx ; our index variable
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mov [bx], al ; copy the ASCII char on 'al' to the position pointed by 'bx'
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ror dx, 4 ; 0x1234 -> 0x4123 -> 0x3412 -> 0x2341 -> 0x1234
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; increment index and loop
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add cx, 1
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jmp hex_loop
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end:
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; prepare the parameter and call the function
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; remember that print receives parameters in 'bx'
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mov bx, HEX_OUT
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call print
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popa
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ret
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HEX_OUT:
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db '0x0000',0 ; reserve memory for our new string

‎23-fixes/boot/switch_pm.asm

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[bits 16]
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switch_to_pm:
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cli ; 1. disable interrupts
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lgdt [gdt_descriptor] ; 2. load the GDT descriptor
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mov eax, cr0
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or eax, 0x1 ; 3. set 32-bit mode bit in cr0
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mov cr0, eax
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jmp CODE_SEG:init_pm ; 4. far jump by using a different segment
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[bits 32]
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init_pm: ; we are now using 32-bit instructions
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mov ax, DATA_SEG ; 5. update the segment registers
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mov ds, ax
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mov ss, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ebp, 0x90000 ; 6. update the stack right at the top of the free space
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mov esp, ebp
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call BEGIN_PM ; 7. Call a well-known label with useful code

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