Fix rootdir.asm re-using potentially wrong DS

kernel/fcb.asm

@@ -33,6 +33,8 @@ fcbdrv	push ds
 	inc dl
 	mov [si], dl ; 1-based
 .log	dec dl
+	mov ax, cs
+	mov ds, ax
 	call logdrv ; 0-based
 	pop dx
 	pop ds

kernel/rootdir.asm

@@ -46,7 +46,7 @@ fndnxt	mov bp, sp
 	mov ax, [es:di+FCBSIZ]
 	inc word [es:di+FCBSIZ]
 	; bail out if we are at end of dir
-	cmp ax, [bpb+BPBRDE]
+	cmp ax, [cs:bpb+BPBRDE]
 	jnc .err
 	; load entry and first byte
 	push ds
@@ -58,7 +58,7 @@ fndnxt	mov bp, sp
 	test byte [cs:di+0x0B], 0xDA
 	jnz .loop
 	; bail out if end of dir
-	mov al, [di]
+	mov al, [cs:di]
 	cmp al, 0
 	je .err
 	; next if deleted entry
@@ -85,11 +85,13 @@ fndnxt	mov bp, sp
 	movsb
 	; copy filename
 	push ds
+	push si
 	mov ax, cs
 	mov ds, ax
 	mov si, bx
 	mov cx, 11
 	rep movsb
+	pop si
 	pop ds
 	; done!
 	xor ax, ax
@@ -103,7 +105,8 @@ open	call fndfst
 	test al, al
 	jnz .ret
 	; read start cluster
-	mov ax, [cs:bx+26]
+	mov ax, [cs:bx+0x1A]
+	mov bx, dx
 	mov [ds:bx+FCBCLUS], ax
 	; reset record number
 	mov byte [ds:bx+FCBRN], 0