kernel: fall back to defaults if int 13h/ah=8 spews bogus data

kernel/drive.asm

@@ -18,7 +18,8 @@ drive_table:	resb (drives * drive_size)
 
 section .text
 
-drives_init:	int 0x11
+drives_init:	; CX = number of floppy drives in the system
+		int 0x11
 		mov cl, 6
 		shr ax, cl
 		inc ax
@@ -31,16 +32,20 @@ drives_init:	int 0x11
 .loop:		push cx
 		push dx
 		mov [bx+drive.biosnum], dl
-		; defaults if 13h,AH=8 fails (XT 360kb)
-		les di, [0x1e * 4]
-		mov cx, 0x2709 ; 40 cylinders, 9 sectors
-		mov dh, 1 ; 2 heads
-		; ask bios
-		push bx
+		; query bios for floppy format
 		mov ah, 8
+		push bx
 		int 0x13
 		pop bx
-		; copy dpt
+		; set up defaults if invalid data or fail
+		jc .defs
+		test cx, cx
+		jnz .load
+.defs:		; use defaults: 360k, 40 cyl, 9 sects
+		les di, [0x1e * 4]
+		mov cx, 0x270
+		mov dh, 1
+.load:		; copy dpt
 		push es
 		push ds
 		pop es