fat32 bootloader: implement directory recursing and kernel load

this is actually working now.

boot/fat32.asm

@@ -50,13 +50,15 @@ mloop:	lodsb
 mend:	push si
 	ret
 
+dskerr: call msg
+	db "DISK", 0
 err:	call msg
 	db "ERR", 0x0A, 0x0D, 0
 hlt:	hlt
 	jmp hlt
 
 	; offsets relative to FS
-offd:	; add offset of cluster data area to DX:AX
+readd:	; add offset of cluster data area to DX:AX
 	push cx
 	mov di, [rde]
 	mov cl, 4
@@ -64,7 +66,7 @@ offd:	; add offset of cluster data area to DX:AX
 	add ax, di
 	adc dx, 0
 	pop cx
-offr:	; add offset to rootdir to DX:AX (FAT12/FAT16 only)
+readr:	; add offset to rootdir to DX:AX (FAT12/FAT16 only)
 	push cx
 	xor ch, ch
 	mov cl, byte [fn]
@@ -72,17 +74,14 @@ fatlp:	add ax, [sf]
 	adc dx, [sf+2]
 	loop fatlp
 	pop cx
-offf:	; add offset to FAT table to DX:AX
+readf:	; add offset to FAT table to DX:AX
 	add ax, word [rsc]
 	adc dx, 0
-	ret
-
 readp:	; read sector DX:AX from partition
 	add ax, word [po]
 	adc dx, word [po+2]
 	jc err
-
-readd:	; read sector DX:AX from disk
+read_:	; read sector DX:AX from disk
 	; qword sector number DX:AX
 	push cs
 	push cs
@@ -105,16 +104,15 @@ readd:	; read sector DX:AX from disk
 	mov dl, 0x80
 	stc
 	int 0x13
-	jc err
+	jc dskerr
 
 	add sp, bp
 
 	ret
 
-next:	; Advances [clus] to next FAT entry
-	; get current cluster number
-	mov ax, [clus]
-	mov dx, [clus+2]
+next:	; Advances DX:AX to next FAT entry
+	push ax
+	push bx
 
 	; shift 2 left for dword-sized FAT entries
 	; shift 9 right for sector size
@@ -124,31 +122,30 @@ shftl:	clc
 	rcr ax, 1
 	loop shftl
 
-	call offf
-
 	mov bx, 0xA0
 	mov cx, 1
-	call readp
+	call readf
 
-        ; get lower part of cluster number
-        mov si, [clus]
+	pop bx
+
+	; get lower part of cluster number
+	pop si
 	; 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
+	; load dword from FAT
+	lodsw
+	mov dx, [si]
 
 	ret
 
 	; reads current cluster into [dest]
 readc:	; load cluster number
-	mov ax, [clus]
-	mov dx, [clus+2]
+	push ax
+	push dx
 
 	; subtract the two dummy entries from FAT start
 	sub ax, 2
@@ -156,47 +153,82 @@ readc:	; load cluster number
 
 	; convert cluster number to sector number
 	; this is some cheapo multiplication with 2^n
-	mov ch, [sc]
+	mov cl, [sc]
 l02:	shr cl, 1
 	jz l02e
 	clc
-        rcl dx, 1
         rcl ax, 1
+        rcl dx, 1
 	jmp l02
 l02e:
 	; dx:ax is now sector num in data area
-	call offd
-	mov bx, [dest]
 	mov cx, [sc]
 	xor ch, ch
 
-	call readp
+	call readd
 
-	xchg bx, cx
+	xchg bp, cx
 	mov cl, 5
-	sal bx, cl
-	add [dest], bx
+	sal bp, cl
+	add bx, bp
+
+	pop dx
+	pop ax
 
 	ret
 
-loadf:	call readc
+	; load cluster chain DX:AX to [dest]:0
+load:	mov bx, 0x07C0
+lloop:	call readc
 	call next
-	cmp word [clus+2], 0x0FFF
-	jne loadf
+	cmp word dx, 0x0FFF
+	jne lloop
 	ret
 
-main:	call loadf
-	mov ax, [0x7C00]
-	mov cx, [0x7C02]
-	int 3
+	; Search for first entry with matching fsattr
+search:	mov si, (0x7C00-0x20)
+loop:	add si, 0x20
+	; check if entry is valid
+	mov al, [si]
+	; unallocated direntry
+	test al, al
+	jz end
+	; deleted files
+	cmp al, 0xE2
+	je loop
+	; check attr
+	mov al, [si+11]
+	and al, 0x5C
+	cmp al, ah
+	jne loop
+	ret
 
-	xor ah,ah
-	int 0x16
-	int 0x19
-	jmp hlt
+main:	mov ax, 2
+	xor dx, dx
+	call load
 
-	; current cluster number
-clus:	dd 2
+	; search for an system directory
+	mov ah, 0x14
+	call search
+	jne end
+
+	mov ax, [si+0x1A]
+	mov dx, [si+0x14]
+
+	call load
+	mov ah, 0x04
+	call search
+	jne end
+
+	mov ax, [si+0x1A]
+	mov dx, [si+0x14]
+
+	call load
+	jmp 0:0x7C00
+
+end:	call msg
+	db "DIR", 0
+	jmp err
 
 	; segment register for data
 dest:	dw 0x07c0