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AH = 16h DL = drive number (00h-7Fh) SI = 0000h (to avoid crash on AT&T 6300)
Return:
CF clear if change line inactive AH = 00h (disk not changed) CF set if change line active AH = status 01h invalid command (SyQuest) 06h change line active or not supported 80h drive not ready or not present
Notes: Call AH=15h first to determine whether the drive supports a change line. This call also clears the media-change status, so that a disk change is only reported once
BUGS: Some versions of Award 386 Modular BIOS and AMI BIOS fail to clear the media-change status. AT&T 6300 WGS systems crash if SI <> 0 on entry.. Some pre 1986/08/04 Compaq ROM BIOS have a serious bug where this function may re-configure a hard disk depending on what is located at ES:[BX] and data indexed to by it. MS-DOS/PC DOS IO.SYS/IBMBIO.COM install a special filter when they detect Compaq ROM BIOSes with earlier dates.. Some Compaq 286 systems have a bug in all INT 13h functions >= 16h, which causes the byte at DS:0074h to be destroyed when called for hard disks (DL >= 80h). MS-DOS/PC DOS IO.SYS/IBMBIO.COM performs a test on this bug using this sub-function, and if found installs a special filter which points DS into ROM, so that it cannot cause any harm.. Some drives (or controllers???) forget the change line status if another drive is accessed afterwards. The DOS BIOS takes care of this by not relying on the reported change line status when the change line is not active and a different drive is accessed, instead it reports "don't know" to the DOS kernel.
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