Tales from the Trailing Edge

Standard, Enhanced, and Real Mode

Gregg Eshelman - 2001.11.06

I was going to write a bit about conventional, extended, and expanded memory, but while doing a bit of searching to make sure I remembered things correctly, I found a website that explains the fiddly technical bits better than I ever could.

If you run several older DOS applications and games that require expanded (EMS) memory, an expansion card that is configurable to make part or all of the RAM on it work as hardware EMS is a good thing to have. They may not be too easy to find today, though. Micron and Everex made them, along with Intel and some other companies. Make sure you get any setup software with the card, as I doubt that it will still be available from the manufacturer - if the company is even in business now.

Here's where I expand a bit on the things marked with a * in my previous article.

A "feature" (or bug) in the 80286 CPU was that, while it could switch from Real to Protected mode easily, it could not switch back without a reset of the CPU - and resetting the CPU resets the whole computer. A few programs for the 286 did use protected mode, but don't ask me to name any.

Microsoft came up with a way of using the enhanced features of the 286 so Windows could use extended memory and called it Standard Mode. This is not a "real" operating mode of x86 CPUs but a creation of Microsoft. Almost the instant that 386 computers were available, programmers gave up on the kludgy and inefficient Real and Standard modes for the more "roomy" 386 Enhanced Mode, which was just Microsoft's name for protected mode.

Real Mode is a 16-bit only mode. It's still there, even in the latest and greatest AMD Thunderbird, Cyrix III or Pentium 4. Real Mode is what you get when you boot from your emergency floppy disk. It's dark and cramped from a program's point of view. Various programs were created to extend the bounds of Real Mode so DOS could use all that extra memory. The most popular "DOS extender" is DOS4GW, used by many games of the late 80s and early 90s. Other games used expanded memory with either the DOS EMM386 driver on a 386 or 486, or used a hardware EMS expansion card on the 8088/86 or 286.

Protected mode on the 386 and up is 32-bits, even with a 16-bit operating system like DOS/Win3.x. Data is shoved around 32 bits at a time, greatly enhancing the overall speed of the system. The fly in the 32-bit ointment was the 16-bit nature of Windows and its DOS foundation. Every time the system had to read from or write to the hard drive, it had to "thunk"* down from 32-bit to the 16-bit Real Mode to transfer the data. Enabling 32-bit disk and file access in Windows For Workgroups 3.11 eliminated that constant mode switching for a healthy improvement in speed.

* Thunking is a computer term used to describe a change in the number of bits being processed at a time. For a 386 or higher CPU to access a 16-bit ISA card it must thunk down to 16 bits and wait more cycles for two 16-bit chunks of data to arrive for processing. Then it must thunk up to 32 bits to process the data.

Special Bonus Section

Got an old PC like a 486 and a small old hard drive you don't care about? FDISK and format the drive, but do not have format install the system files. Now try to install the Microsoft DOS 6.22 Upgrade. It will quit because there is no system already installed. Reboot with Disk 1, exit the installation, then enter SYS C: at the A:\> prompt. Remove the floppy and reboot. GOTCHA!

Unless you're the sort of person who can step in anything and have it turn to gold, you've just screwed up your hard drive. Wiping the drive with FDISK, then formatting and installing the system files, then installing the DOS 6.22 upgrade won't fix it. Modern disk utilities might be able to totally wipe the boot sector, but back then I just kept a 486 motherboard with an AMI BIOS that had the Media Analysis function. That BIOS would autodetect any IDE drive up to 8 gig, and Media Analysis would work on it even though DOS needed special overlay software to use any drive over 512 MB.

The lesson here is to always use FORMAT /S or sys the C: drive before installing the DOS 6.22 upgrade if you don't have the full version. Except for requiring a preexisting system, even if it is DOS 6.22, the upgrade is identical to the full version. LEPC

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