ahcifix.386

VxD to allow Windows 3.1 Enhanced Mode to be used with Intel AHCI (SATA) controllers

This is a small and simple Virtual Device Driver ("VxD") for Windows 3.1 in 386 Enhanced Mode (and probably also Windows 95/98, though I haven't tested it with those!). Its purpose is to work around two bugs in the firmware for Intel AHCI controllers, which normally cause Windows 3.1 Enhanced Mode to crash on disk access.

The Bugs

The AHCI firmware contains code needed to service int 13h calls from Real/Virtual 8086 Mode, which works pretty well for the most part. In Virtual 8086 Mode, it uses the Virtual DMA Specification to cooperate with the running Virtual Machine Manager (e.g. EMM386, or Windows 3.1) and ensure that data gets read into / written from the correct physical addresses. Unfortunately, there are two mistakes in its use of the Spec:

1. When it locks a contiguous or scatter / gather region, it sets the ES register to the Extended BIOS Data Area (EBDA), but when doing the corresponding unlock(s), it instead uses DS! This is wrong, and causes Windows 3.1 to crash because it's trying to read a DMA Descriptor Structure from arbitrary memory that wasn't used to set one up in the first place (but EMM386 is more resilient…).
2. This one's a bit more subtle, and it may be particular to my motherboard / system: the firmware sets up an Extended DMA Descriptor Structure at the beginning of the EBDA, instead of somewhere deep inside it. This means that, depending on how large the scatter / gather regions are, it can overwrite varying amounts of data that are supposed to be at the beginning of the EBDA. For example, I found that when I worked around Bug 1, Windows would boot, but the mouse didn't work anymore because the "pointing device" info was getting overwritten in the EBDA! The offset within the EBDA is supposed to be specified at a certain location in the firmware ROM, but for some reason it's set to zero (again, this may just be on my motherboard…).

Building

To build the VxD, you need the Win16 Driver Development Kit. Once you install it, you can place the sources in a subfolder of the 386 directory (e.g. 386\AHCIFIX) and run nmake. This should create the file AHCIFIX.386 which can be loaded by Windows.

By the way, here's a tip for using the DDK on a modern system: to make the debugger WDEB386 work, you need to change some bytes:

• At position 63D8, you need to change 0F 24 F0 to 66 33 C0
• At position 63DF, you need to change 0F 24 F8 to 66 33 C0

This removes references to the TR6 and TR7 registers, which crash the system since they only existed on the 386, 486 and a few other less-well-known chips!

Usage

Simply add device=ahcifix.386 to the [386Enh] section of C:\WINDOWS\SYSTEM.INI. You should specify the full path, or else copy the file AHCIFIX.386 to C:\WINDOWS\SYSTEM. You can get the file by building it as described above, or by downloading a release.

Alternatively (or additionally), if using a DOS Memory Manager, such as EMM386, you can add ahcifixd.sys to your CONFIG.SYS file, including the full path to ahcifix.386 after it on the same line. For example:

DEVICE=C:\AHCIFIXD\AHCIFIXD.SYS C:\WIN16DDK\386\AHCIFIX\AHCIFIX.386

NOTE: This driver works for me, on the one motherboard I have tested it on (this motherboard)! Please proceed with caution if trying it on a different motherboard!

Bootstrapping Windows 9x

This driver may be required to bootstrap Windows 9x, but once you get it up and running, you can set up the PCI bus and install rloew's AHCI driver, which enables 32-bit disk access and hence improved performance.