better documentation for the boot loading process and how much

sectors each part needs, fixed truncation problem
started a magic signature at the end of the image and started
to check it in stage 2 of the boot loader to avoid truncated images
in the future

6 files changed, 120 insertions, 34 deletions

diff --git a/doc/LINKS.TODO b/doc/LINKS.TODO
index 65e754f..f7a602e 100644
--- a/doc/LINKS.TODO
+++ b/doc/LINKS.TODO
@@ -4,3 +4,7 @@ http://wiki.osdev.org/Linker_Scripts
 http://wiki.osdev.org/Printing_To_Screen
 http://wiki.osdev.org/Exceptions#General_Protection_Fault
 https://en.wikipedia.org/wiki/X86_calling_conventions
+https://github.com/FlingOS/FlingOS
+http://repo.or.cz/w/lightOS.git
+https://littleosbook.github.io/
+http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
diff --git a/src/Makefile b/src/Makefile
index c983fb6..4e0755a 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -4,30 +4,47 @@ LD := ld
 
 all: image.bin
 
-image.bin: boot.bin kernel.bin
-	cat boot.bin kernel.bin > image.bin
+image.bin: boot.bin kernel.bin magic.bin
+	cat boot.bin kernel.bin > image.tmp
 	# truncate to correct number of sectors, we have
-	# 512 (boot, first stage) + N * 512 (N currenty is 3) + M * 512 (M is currently 3)
-	truncate -s 4096 image.bin
+	# 512 (boot, stage 1) + N * 512 (N currenty is 3, stage 2) = 2048 for boot.bin
+	# + M * 512 (M is currently 7) = 3144 for kernel.bin
+	# + 1 * 512 = 512 for magic.bin
+	# (M + N + 1 is the number of sectors to be read in stage 2, as stage 1
+	# loads only the first sector, so adapt NOF_LOAD_SECTORS to 10)
+	truncate -s 5632 image.tmp
+	cat image.tmp magic.bin > image.bin
 	
 boot.bin: boot.asm gdt.asm stage1_functions.asm stage2_functions.asm switch_mode.asm
 	nasm boot.asm -f bin -o boot.bin
 
-kernel.bin: kernel.o vga.o string.o
+kernel.bin: kernel.o vga.o port.o port_asm.o string.o stdlib.o
 	$(LD) -o kernel.bin -n -Ttext 0x8000 --oformat binary \
-		kernel.o vga.o string.o
+		kernel.o vga.o port.o port_asm.o string.o stdlib.o
 
+magic.bin: magic.asm
+	nasm magic.asm -f bin -o magic.bin
+	
 kernel.o: kernel.c
-	$(CC) $(CFLAGS) -O0 -c -o kernel.o kernel.c
+	$(CC) $(CFLAGS) -c -o kernel.o kernel.c
+
+port.o: port.c
+	$(CC) $(CFLAGS) -c -o port.o port.c
 
 vga.o: vga.c
-	$(CC) $(CFLAGS) -O0 -c -o vga.o vga.c
+	$(CC) $(CFLAGS) -c -o vga.o vga.c
+
+port_asm.o: port.asm
+	nasm port.asm -f elf32 -o port_asm.o
 
 string.o: string.c
-	$(CC) $(CFLAGS) -O0 -c -o string.o string.c
+	$(CC) $(CFLAGS) -c -o string.o string.c
+
+stdlib.o: stdlib.c
+	$(CC) $(CFLAGS) -c -o stdlib.o stdlib.c
 
 clean:
-	-rm -f boot.bin kernel.bin image.bin *.o
+	-rm -f boot.bin kernel.bin image.bin magic.bin *.o
 
 run-qemu: image.bin
 	qemu-system-i386 -m 32 -drive "file=image.bin,if=ide,format=raw"
diff --git a/src/boot.asm b/src/boot.asm
index 631a99a..9b130a2 100644
--- a/src/boot.asm
+++ b/src/boot.asm
@@ -5,7 +5,10 @@
 
 ; 16-bit real-mode
 	[bits 16]
-	
+
+; export a map of kernel symbols for debugging	
+	[map symbols kernel.map]
+		 
 ; initialize segment registers
 	mov ax, 0
 	mov ds, ax
@@ -18,6 +21,8 @@
 	mov [BOOT_DRIVE], dl
 	
 ; make sure we know the location of the stack by setting it on our own
+; this stack is only used in real mode in stage 1, so it's growing from
+; 0xffff down
 	mov bp, 0xFFFF
 	mov sp, bp
 	
@@ -34,10 +39,13 @@
 ; print we are going to load stage 2 of the boot blocks
 	mov si, MESSAGE_LOADING_STAGE_2
 	call print_string
-	
+
+; load stage 2 and the kernel together to 0x7e00 (directly
+; after the boot sector)	
 	mov dl, [BOOT_DRIVE]
 	call read_from_disk
-				
+	
+; jump over variables and subroutines of stage 1 and execute stage 2
 	jmp stage2
 	
 %include "stage1_functions.asm"
@@ -67,13 +75,16 @@ times 510-($-$$) db 0
 dw 0xaa55
 
 stage2:
+; print a message that we are indeed in stage 2 now
 	mov si, MESSAGE_STAGE_2_LOADED
 	call print_string
 
-; remember current position on screen
+; remember current position on screen, otherwise we loose this
+; information after switching to protected mode 
 	call current_row
 	mov [CURSOR_Y], dh
 
+; enter protected mode
 	call switch_to_protected_mode
 	
 ; we should never get here, but just in case
@@ -88,43 +99,39 @@ MESSAGE_STAGE_2_LOADED:
 
 BEGIN_PROTECTED_MODE:
 
-	mov edx, 1
-_loop_test:
-	mov si, MESSAGE_INITIALIZING
-	call pm_print_string
-	call pm_print_newline
-	dec edx
-	cmp edx, 0
-	jnz _loop_test
-_end_of_test:
-
+; we switched to protected mode
 	mov si, MESSAGE_PROTECTED_MODE
 	call pm_print_string
 	call pm_print_newline
-	
+
+; check sanity of kernel by searching for MAGIC string at a given
+; position
+	call check_magic
+
+; print a message before we call the C level kernel	
 	mov si, MESSAGE_CALL_C_ENTRY
 	call pm_print_string
 	call pm_print_newline
 	
 ; call our kernel
 	call c_entry
-	
+
+; "kernel halted" message, when we terminate the C kernel
 	mov si, MESSAGE_HALTED
 	call pm_print_string
 	call pm_print_newline
 
 ; end of C, disable interupts again, NMIs can still happen
+; make sure we loop endlessly here but without burning too
+; much CPU
 	cli
 _halt_loop:
 	hlt
 	jmp _halt_loop
-
+	
 MESSAGE_PROTECTED_MODE:
 	db "Switched to 32-bit Protected Mode", 0
 
-MESSAGE_INITIALIZING:
-	db "Initializing the Foo machine..", 0
-
 MESSAGE_CALL_C_ENTRY:
 	db "Calling C entry function", 0
 	
@@ -133,7 +140,9 @@ MESSAGE_HALTED:
 
 %include "stage2_functions.asm"
 
-; make sure we have full sectors
+; make sure we have full sectors, stage one is 512 bytes, so we
+; have to will up 3 sectors
 times 2048-($-$$) db 0
 
+; position is 0x8400 now for the C entry
 c_entry:
diff --git a/src/magic.asm b/src/magic.asm
new file mode 100644
index 0000000..c8c580e
--- /dev/null
+++ b/src/magic.asm
@@ -0,0 +1,6 @@
+; pad rest of sector with zeroes so we get 512 bytes in the end
+times 512-11-($-$$) db 0
+
+; the magic string we search for in stage 2 to ensure we don't read
+; a truncated kernel image
+db "ABAOSMAGIC", 0
diff --git a/src/stage1_functions.asm b/src/stage1_functions.asm
index b32d23d..c43da4d 100644
--- a/src/stage1_functions.asm
+++ b/src/stage1_functions.asm
@@ -1,9 +1,8 @@
 ; number of sectors to be read
-; this is 512 (the bootloader stage1) +
 ; 3 * 512 for bootloader stage2 and the kernel code 
 ; (note: the first sector gets loaded by the BIOS, so
 ; subtract 1 here!)
-NOF_LOAD_SECTORS equ 7
+NOF_LOAD_SECTORS equ 11
 
 ; IN bx: begin of memory area to dump
 ; IN ax: number of words to dump
@@ -19,7 +18,7 @@ dump_range:
 	jnz dump_range
 	ret
 
-; IN bx: hex value to print	
+; IN dx: hex value to print	
 print_hex:
 	push bx
 	push si
diff --git a/src/stage2_functions.asm b/src/stage2_functions.asm
index 018cf6a..d692ed2 100644
--- a/src/stage2_functions.asm
+++ b/src/stage2_functions.asm
@@ -55,6 +55,34 @@ pm_print_char:
 	call inc_cursor
 	ret
 
+; IN dx: hex value to print	
+pm_print_hex:
+	push bx
+	push si
+	mov si, HEX_TEMPLATE
+	mov bx, dx
+	shr bx, 12
+	mov bx, [HEXABET+bx]
+	mov [HEX_TEMPLATE+2], bl
+	mov bx, dx
+	and bx, 0x0FFF
+	shr bx, 8
+	mov bx, [HEXABET+bx]
+	mov [HEX_TEMPLATE+3], bl
+	mov bx, dx
+	and bx, 0x00FF
+	shr bx, 4
+	mov bx, [HEXABET+bx]
+	mov [HEX_TEMPLATE+4], bl
+	mov bx, dx
+	and bx, 0x000F
+	mov bx, [HEXABET+bx]
+	mov [HEX_TEMPLATE+5], bl
+	call pm_print_string
+	pop si
+	pop bx
+	ret
+
 inc_cursor:
 	push eax
 	mov ax, [CURSOR_X]
@@ -106,3 +134,26 @@ update_vga_cursor:
 	pop ecx
 	pop ebx
 	ret
+
+check_magic:
+	push eax
+	push ebx
+	push ecx
+	push edx
+	push esi
+	mov eax, NOF_LOAD_SECTORS	; number of 512-byte sectors
+	shl eax, 9			; 512 bytes per sector
+	mov ecx, 0x7e00			; offset of stage 2
+	add ecx, eax
+	sub ecx, 11			; the length of the magic string
+	mov dx, cx
+	call pm_print_hex
+	mov dx, [ecx]
+	call pm_print_hex
+	call pm_print_newline
+	pop esi
+	pop edx
+	pop ecx
+	pop ebx
+	pop eax
+	ret