Advance on bootloader: load cluster data

Makefile

@@ -1,7 +1,7 @@
 FLOPPY = 360
 
 PROGRAMS = hello.com
-DISTFILES = $(PROGRAMS) rdos86.bs
+DISTFILES = $(PROGRAMS) kernel.bs
 ROMS =
 
 QEMU_ARCH = $(shell uname -m)

fat32.asm

@@ -1,9 +1,12 @@
 	; Bootsector for FAT32 filesystems
 	org 0x0800
-	jmp near init
+	jmp short init
+	nop
 
 	; WORD reserved sector count
 rsc:    equ ( $$ + 0x00E )
+	; BYTE number of sectors per cluster
+sc:	equ ( $$ + 0x00D )
 	; BYTE number of FATs
 fn:	equ ( $$ + 0x010 )
 	; DWORD hidden sector count (partition offset)
@@ -53,6 +56,7 @@ read:	pop bp
 	mov dl, 0x80
 	mov si, sp
 
+	mov di, [si+4]
 	int 0x13
 	jc err
 
@@ -61,31 +65,33 @@ read:	pop bp
 	push bp
 	ret
 
-	; Advances cluster number to next entry
-next:	; Upper two words for sector num
+	; Advances [clus] to next FAT entry
+next:	; push upper two words for sector num
 	xor ax, ax
 	push ax
 	push ax
 
-	; Lower two words for sector num
+	; get current cluster number
 	mov ax, [clus]
 	mov dx, [clus+2]
 
-	; 32-bit >> 7
+	; shift 2 left for dword-sized FAT entries
+	; shift 9 right for sector size
 	mov cl, 7
 shftl:	clc
 	rcr dx, 1
 	rcr ax, 1
 	loop shftl
 
-	; Add reserved sector count
+	; add reserved sector count
 	add ax, word [rsc]
 	adc dx, 0
 
+	; push lower dword of sector number
 	push dx
 	push ax
 
-	; target buffer
+	; push ptr to target buffer
 	push cs
 	mov ax, buf
 	push ax
@@ -93,13 +99,16 @@ shftl:	clc
 	mov cx, 1
 	call read
 
-        ; Stow off offset in FAT sector
+        ; get lower part of cluster number
         mov si, [clus]
+	; multiply with 4
         sal si, 1
         sal si, 1
+	; make sure its within sector range
         and si, 0x1FF
 	add si, buf
 	mov di, clus
+	; copy dword from FAT to [clus]
 	movsw
 	movsw
 
@@ -107,9 +116,67 @@ shftl:	clc
 	add sp, 12
 	ret
 
-main:	call next
+	; reads current cluster into [dest]
+readc:	; load cluster number
 	mov ax, [clus]
 	mov dx, [clus+2]
+
+	; subtract the two dummy entries from FAT start
+	sub ax, 2
+	sbb dx, 0
+
+	; convert cluster number to sector number
+	; this is some cheapo multiplication with 2^n
+	mov ch, [sc]
+l02:	shr cl, 1
+	jz l02e
+	clc
+        rcl dx, 1
+        rcl ax, 1
+	jmp l02
+l02e:
+	; dx:ax is now sector num in data area
+
+	; get number of FATs
+	xor ch, ch
+	mov cl, byte [fn]
+	; add their sizes up
+floop:  add ax, [sf]
+	adc dx, [sf+2]
+	loop floop
+
+	; add reserved sector count
+	add ax, [rsc]
+	add dx, 0
+
+	; push sector num QWORD
+	xor cx, cx
+	push cx
+	push cx
+	push dx
+	push ax
+
+	; push target buffer DWORD
+	; dest gives the segment register
+	mov ax, [dest]
+	push ax
+	push cx
+
+	; push sector count
+	; always an full cluster
+	mov cx, [sc]
+	xor ch, ch
+
+	call read
+
+	cmp cl, [sc]
+	jne err
+
+	add sp, 12
+	ret
+
+main:	call readc
+
 	int 3
 
 	xor ah,ah
@@ -117,7 +184,17 @@ main:	call next
 	int 0x19
 	jmp hlt
 
+	; the packing is so that it looks nice in a hexdump
+	times (0x1E0 - ($-$$)) db 0
+	; directory the kernel is in
+tdir:	db "RDOS       "
+	; current cluster number
 clus:	dd 2
+	times (0x1F0 - ($-$$)) db 0
+	; filename for hte kernel
+tfile:	db "KERNEL  BS "
+	; segment register for data
+dest:	dw 0x07c0
 
 	; Padding and signature
 	times (0x1FE - ($-$$)) db 0