boot odds and ends. unfinished cd-detection work.

boot/boothead.s

@@ -1204,6 +1204,44 @@ bcd:	movb	ah, al
 	.data1 0xD5,10 ! aad	! ax = (al >> 4) * 10 + (al & 0x0F)
 	ret			! (BUG: assembler messes up aad & aam!)
 
+
+! void bootcdinfo(u32_t bufaddr, int *ret, int drive)
+! If booted from CD, do BIOS int 0x13 call to obtain boot CD device.
+.define	_bootcdinfo
+_bootcdinfo:
+	push bp
+	mov bp, sp
+	push ax
+	push bx
+	push cx
+	push dx
+	push si
+	push ds
+	mov	bx, 10(bp)	! drive number
+	mov	cx,  8(bp)
+	mov	ax,  4(bp)	! buffer address from stack
+	mov	dx,  6(bp)
+	call	abs2seg
+	mov	si, ax		! bios will put data in ds:si
+	mov	ds, dx
+!	movb	dl, #0x00
+	movb	dh, #0x00
+	movb	dl, bl
+!	mov	ax, #0x4b01	! command 0x4b, subcommand 0x01
+	movb	ah, #0x4b
+	movb	al, bh
+	int	0x13
+	mov	bp, cx
+	mov	(bp), ax
+	pop ds
+	pop si
+	pop dx
+	pop cx
+	pop bx
+	pop ax
+	pop bp
+	ret
+
 ! Support function for Minix-386 to make a BIOS int 13 call (disk I/O).
 bios13:
 	mov	bp, sp