PC-DOS 4.0

by Lloyd Borrett
Technical Cornucopia, January–February 1989

This latest version of DOS boasts support for hard disk volumes that hold an entire gigabyte (1 billion bytes) of data. It also recognises the Expanded Memory Standard (EMS) and can use it for its own buffers and RAM disk.

In addition, a simplified user interface or shell lets you move through all disks, directories, and files, and perform most actions with pull down menus and cursor keys or a mouse. A minor, but nonetheless interesting, feature in this version is support for 43 or 50 line text with an EGA or VGA card.

In September 1988, IBM quietly started shipping PC-DOS 4.0 with disks labeled 4.01. This maintenance release fixed some of the bugs that had caused user complaints but left many other problems unanswered.

DOS 4.0 and EMS memory

PC-DOS 4.0 is the first product from IBM that recognises EMS memory. Unfortunately IBM have not fully conformed to the EMS 3.2, EEMS, or EMS 4.0 standards. They have implemented their own form of EMS memory that only works with the IBM XMA memory card.

IBM PC-DOS 4.0's use of EMS memory makes assumptions that are entirely contrary to the standard. Thus existing users with EMS 3.2, EEMS, or EMS 4.0 conforming memory cards, can not use the EMS features in IBM PC-DOS 4.0. Thus the only safe way to run IBM PC-DOS 4.0 is to leave it all in standard memory and ignore the expanded memory options.

Microsoft have announced that their MS-DOS 4.0 will work with any EMS 4.0 compatible memory card.

Larger hard disk volumes

Previous PC-DOS versions restricted hard disk size to 32 MBytes, viewing volumes larger than that as multiple hard drives. PC-DOS 4.0 is the first version of PC-DOS that supports hard disk volumes larger than 32 MBytes.

DOS 4.0 makes no changes in sector size or in the file allocation table (FAT); it continues to use 512-byte sectors, a 12-bit FAT for smaller disks, and a 16-bit FAT for everything else. For disk volumes up to 128 MBytes, DOS 4.0 uses allocation units of 2 KBytes. For larger disks, it simply increases cluster sizes. This means that no DOS 4.0 disk volume may ever hold more than 65,520 files.

The waiting game

Many popular utility programs, especially disk optimizers and other programs that access disks directly, are incompatible with DOS 4.0. Some are completely incompatible with DOS 4.0, while others only have difficulties if you have a disk volume that is larger than 32 MBytes. So if you switch to DOS 4.0, you should plan to upgrade many of the utility programs you use.

There are some conflicts between DOS 4.0 and some memory resident programs. Some will not pop up at all, while others have screen display conflicts. Many of the problems seem to be caused by specific hardware configurations as they will work okay on some systems but not on others.

We can assume that all of the incompatibilities and bugs in PC-DOS 4.0 will eventually be ironed out by updates from software manufacturers and bug fixes from IBM. It's to be hoped that by the time Microsoft ships MS-DOS 4.0 it will already have many of the problems resolved.

If you need hard disk volumes larger than 32 MBytes, DOS 4.0 may be the best choice for you. And once MS-DOS 4.0 is available, or IBM solves their 4.0 EMS incompatibility problems, moving DOS storage areas to expanded memory will help keep large programs from running out of space.

However, I'll stick to using DOS 3.30 until the DOS 4.0 versions of the utility programs I use are available. Unless you really need the expanded capabilities, you may be better off sticking with your present operating system until DOS 4.1 appears.