Centralize and improve boot sector disk access

bootsect.asm

@@ -97,25 +97,15 @@ calc_constants:
 fat:
 	; FAT1 (the main one) follows right after bootsector(s)
 	mov ax, [reservedsectors]
-	call chs
-
-	; TODO: handle possible crossing of track boundary
-	mov ax, [sectorsperfat]
-	mov ah, 2
-	mov dl, [drivenumber]
 	mov bx, 0x8000
-	int 0x13
+	mov cx, [sectorsperfat]
+	call loadsectors
 
 root_dir:
 	mov ax, [rootdir]
-	call chs
-
-	; TODO: handle possible crossing of track boundary
-	mov ax, [rootdirsectors]
-	mov ah, 2
-	mov dl, [drivenumber]
 	mov bx, 0x500
-	int 0x13
+	mov cx, [rootdirsectors]
+	call loadsectors
 
 search_root:
 	mov bx, [rootdirentries]
@@ -198,27 +188,12 @@ found:
 		add ax, [rootdir]
 		add ax, [rootdirsectors]
 
-	; Load sectors
-	pop bx
-	xor cx, cx
-	mov cl, [sectorspercluster]
-	mov dl, [drivenumber]
-	.loop:
-		push ax
-		push cx
-
-		call chs
-
-		mov ah, 2
-		mov al, 1
-		int 0x13
-
-		pop cx
-		pop ax
-
-		add bx, 512
-		inc ax
-		loop .loop
+	.load:
+		; Load sectors
+		pop bx
+		xor cx, cx
+		mov cl, [sectorspercluster]
+		call loadsectors
 
 	.next:
 		pop ax
@@ -268,45 +243,77 @@ execute_kernel:
 	mov dl, [drivenumber]
 	jmp 0:0x500
 
-chs:
+; Note: bx will point to after the read data
+loadsectors:
 	push ax
-	push bx
+	push cx
+	push dx
 
-	; Save drive number
-	mov bl, dl
+	.loop:
+		push ax
+		push cx
 
-	xor dx, dx
-	; cylinder (track) - head - sector
-	; cylinder = LBA / sectorspertrack / heads
-	; head = LBA / sectorspertrack % heads
-	; sector = LBA % sectorspertrack + 1
-	div word [sectorspertrack]
-	; ax = LBA / sectorspertrack
-	; dx = LBA % sectorspertrack
+		.chs:
+			xor dx, dx
+			; cylinder (track) - head - sector
+			; cylinder = LBA / sectorspertrack / heads
+			; head = LBA / sectorspertrack % heads
+			; sector = LBA % sectorspertrack + 1
+			div word [sectorspertrack]
+			; ax = LBA / sectorspertrack
+			; dx = LBA % sectorspertrack
 
-	; sector
-	mov cl, dl
-	inc cl
+			; sector
+			mov cl, dl
+			inc cl
 
-	xor dx, dx
-	div word [heads]
-	; ax = LBA / sectorspertrack / heads
-	; dx = LBA / sectorspertrack % heads
+			xor dx, dx
+			div word [heads]
+			; ax = LBA / sectorspertrack / heads
+			; dx = LBA / sectorspertrack % heads
 
-	; head
-	mov dh, dl
+			; head
+			mov dh, dl
 
-	; cylinder (track)
-	mov ch, al
-	;shr ax, 1
-	;shr ax, 1
-	;and al, 0xC0
-	;or cl, al
+			; cylinder (track)
+			mov ch, al
+			shr ax, 1
+			shr ax, 1
+			and al, 0xC0
+			or cl, al
 
-	; Restore drive number
-	mov dl, bl
+		; Copy number of sectors to still read to ax and cap at 127
+		pop ax
+		push ax
+		cmp al, 127
+		jbe .read
+		mov al, 127
 
-	pop bx
+		.read:
+			mov ah, 2
+			mov dl, [drivenumber]
+			int 0x13
+
+		; Decrement cx by the number of sectors read
+		pop cx
+		xor ah, ah
+		sub cx, ax
+
+		; Increment bx by the number of sectors read times 512
+		xor dx, dx
+		mov dh, al
+		shl dx, 1
+		add bx, dx
+
+		; Increment ax by the number of sectors read
+		pop dx
+		add ax, dx
+
+		test cx, cx
+		jnz .loop
+
+	pop dx
+	pop cx
 	pop ax
 	ret