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--- my_mbr [2017/03/03 13:58]
admin
+++ my_mbr [2017/03/27 10:36] (current)
admin
@@ Line 1: / Line 1: @@
 =====My MBR=====
-I'm using NASM to write my [[real mode]] code, and I use file extension "nasm" to separate them from other assembler files. This makes the build process able to automatically select the correct assembler.
+I'm using NASM to write my [[real mode]] code, and I use file extension "nasm" to separate them from other assembler files. This makes the build process able to automatically select the correct assembler.\\
-<code asm>
+\\
-cli
+**Current state:**\\
-blahblahblah
+. Copy 512 bytes to new location.\\
-jmp far far away
+. Load 64-bit set-up code.\\
-</code>
+. Load kernel loader.\\
+. Execute 64-bit set-up code.\\
+Function ''seek'': Convert absolute sector number into CHS values directly acceptable for INT13h (AH=02).
+<file asm>
+[bits 16]
+struc	PT_ENT		;Partition table entry
+	BootFlag		resb	1
+	BeginHead		resb	1
+	BeginSector		resb	1
+	BeginCyl		resb	1
+	SystemID		resb	1
+	EndHead			resb	1
+	EndSector		resb	1
+	EndCyl			resb	1
+	RelSectorLow	resw	1
+	RelSectorHigh	resw	1
+	NumSectorsLow	resw	1
+	NumSectorsHigh	resw	1
+endstruc
+%include "mbr/mem.inc"
+[org loc_MBR]
+SEGMENT START align=16			;At 0:7C00
+start:
+; register 'dl' contains boot disk number
+	cli
+	xor	cx, cx			; Set segment registers to zero
+						; cs - Code Segment (we're here, so it's obviously zero - or erroneously 0x07C0)
+	mov	es, cx			; Extra Segment
+	mov	ss, cx			; Stack Segment
+	mov	sp, loc_STACK	; Top of stack
+	mov	ds, cx			; Data Segment
+	mov	di, loc_MBR		; Destination for stream
+	mov	si, bootpoint	; Source of stream
+	cld					; Clear direction flag (stream increment)
+	mov	ch, 1			; cx = 256
+	rep movsw			; word move = 512 bytes copy, one HD sector
+	jmp	0x0000:continue	; far JMP to copy of self
+SEGMENT MAIN align=16			;At 0:7A00 + START.length
+continue:
+	mov [drive.disk], dl
+	sti					; enable interrupts
+	; grab drive geometry for LBA/CHS conversion
+	mov ah, 8			; check drive parameters
+	xor di, di
+	int 13h
+	mov [drive.H], dh		; last head
+	mov al, cl
+	and al, 0x3F			; mask sectors
+	mov [drive.S], al		; sectors per track
+	xchg ch, cl				; swap ch/cl - 76543210.98xxxxxx -> 98xxxxxx.76543210
+	shr ch, 6				; shift ch   - xxxxxx98.76543210
+	mov [drive.C], cx		; last cylinder
+	mov al, [drive.H]		; grab last head
+	inc al					; make head-count
+	mul byte [drive.S]			; make sectors per cylinder
+	mov [drive.HSmpx], ax	; sectors per cylinder
+load64:
+	mov	si, 3			; three tries
+try64:
+	mov ax, 1			; sector 1
+	call seek			; grab CHS -> dx 0080, cx 0002, ax 0000
+	mov	ah, 02h			; "read CHS"
+	mov	al, 20			; 20 = space between loader point and stack (20 x 512 = 10240)
+	mov	bx, loc_LOADER	; mem
+	int	13h				; 'dl' contains boot disk from MBR entry
+	jnc	loadloader		; success
+	dec	si				; retry count
+	jnz	try64
+	jmp fail
+loadloader:
+	mov	si, 3			; three tries
+try:
+	mov ax, 21			; sector 21
+	call seek			; grab CHS
+	mov	ah, 02h			; "read CHS"
+	mov	al, 40			; 10 sectors
+	mov	bx, loc_ELF		; mem
+	int	13h				; 'dl' contains boot disk from MBR entry
+	jnc	ok				; success
+	dec	si				; retry count
+	jnz	try
+	jmp fail
+fail:
+	mov	bx, 0x15		;   bold attribute
+	mov ah, 0x0E
+	mov al, 'F'
+	int 10h
+crap:
+	cli
+	hlt
+	jmp crap
+ok:
+	cli
+	jmp	loc_LOADER		; set up for 64-bit, long mode, and bootloader
+; input:
+;	ax: zero-based sector to find
+; output: (int13-02 compatible)
+;	dl: drive
+;	dh: head
+;	cx: cyl.sec - C 76543210.98xxxxxx - S xxxxxxxx.xx543210
+; example
+; dh: 02    H:0-2			3 heads per cylinder
+; cx: 0802	C:0-8 S:1-2		9 cylinders, 2 sectors per track
+; makes sector 21: C/H/S = 3/1/2 = 3x6+2x1+(2-1)
+;	mov	cx, 0x0302			; C=3 / S=2
+;	mov dh, 1				; H=1
+seek:
+	push bx
+	xor dx, dx				; DIV input DX:AX
+	div word [drive.HSmpx]	; operation DX:AX / HSmpx = AX.DX
+	xchg ah, al				; xxxxxx98.76543210 -> 76543210.xxxxxx98
+	shl al, 6				; 76543210.98xxxxxx
+	mov cx, ax				; set cylinder
+	mov ax, dx				; set remainder - ((LBA/mpx)*mpx) -> AX
+	xor dx, dx				; zero DX (DX:AX)
+	xor bh, bh				; zero
+	mov bl, byte [drive.S]	; grab sectors per track
+	div bx					; remainder / S
+	mov dh, al				; set head
+	inc dl					; remainder is sector, sector is 1-based
+	or cl, dl				; insert sector
+	mov dl, [drive.disk]	; set disk
+	pop bx
+	ret
+drive:
+	.disk	db 0		; BIOS-supplied drive number
+	.C		dw 0		; last cylinder, 0->
+	.H		db 0		; last head, 0->
+	.S		db 0		; sector per track, 1->
+	.HSmpx	dw 0		; HxS per cylinder
+; put segment FINAL at the end of the sector
+SEGMENT FINAL start=loc_MBR+0x200-2-4*PT_ENT_size
+PartitionTable	db	0x80		;BootFlag		resb	1
+				db	1			;BeginHead       resb	1	8 bit
+				db	1			;BeginSector     resb	1	6 bit
+				db	0			;BeginCyl        resb	1	10 bit
+				db	0xED		;SystemID        resb	1	ED - Evil Duck
+				db	2			;EndHead         resb	1	8 bit
+				db	2			;EndSector       resb	1	6 bit
+				db	0			;EndCyl          resb	1	10 bit
+				dw	2			;RelSectorLow    resw	1
+				dw	0			;RelSectorHigh   resw	1
+				dw	1000		;NumSectorsLow   resw	1
+				dw	0			;NumSectorsHigh  resw	1
+PartitionTable2  resb PT_ENT_size
+PartitionTable3  resb PT_ENT_size
+PartitionTable4  resb PT_ENT_size
+	dw	0xAA55
+</file>

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