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Enabling PC to access more memory

Question : I tried to install Windows 98 on a 266-megahertz central processing unit to access some old applications. During the installation, the following error message appeared: "Standard mode: bad fault in ms-dos extender Fault: 000D stack dump: 0000 0000 0070 raw fault frame: 00000 IP..." Before installing Windows 98, I had formatted the hard disk to use FAT32. I have another computer of the same specifications which the vendor managed to install Windows 98 successfully. So what seems to be the problem?

Answer : The Web page http://support.microsoft.com/kb/q87239/ contains the answer to this question. According to the Microsoft support, there are several reasons why this would happen. One likely possibility (in this case) is that HIMEM.SYS was unable to control the A20 gate. HIMEM.SYS was the "high memory" driver for Windows 9x systems.

When the first "IBM-compatible" PCs were made, the maximum amount of memory they could address was one megabyte (MB). This was further divided into roughly two sections. The first 640 kilobytes (KB) was called conventional or base memory and this was all that most applications could use at that time. The area above 640KB was used mainly for optimising the system, thus was unavailable for application use.

Eventually, as operating systems and applications outgrew these boundaries, a solution was needed that would enable the PC to access more random access memory (RAM) and at the same time remain compatible to the earliest PCs manufactured.

Several methods were used to try to break the 640KB barrier. These software became known as DOS extenders. Two of the most popular DOS "extension" methods were known as Expanded Memory Specification and Extended Memory Specification. These methods are supported in the Windows 9x series via the HIMEM.SYS and EMM386.EXE drivers.

Now that we've seen the software part of the equation, let's look at the hardware part. As mentioned earlier, the earliest PCs could only address (access) a total of 1 MB RAM. This is because they only had 20 address lines, each capable of carrying one bit. The maximum amount of locations thus becomes 220, or 1,048,576, which is exactly 1 MB. What was more interesting was that the last location in RAM was "connected" to the first location, so anyone going past the last location would end up at the first location. This got to be such a standard that people were writing software that actually took advantage of the "wraparound" effect.

When more advanced processors were introduced, they could address more RAM. This essentially got rid of the wraparound effect at the 1 MB barrier. This also made software that relied on the wraparound effect incompatible with the new processors.

In an effort to achieve full compatibility, system designers put in a "gate". This gate protected the area of RAM above I MB. Closing the gate produced a wraparound effect at the 1 MB barrier. Opening it allowed program access to any available RAM above it. The old method (wraparound effect, memory limited to 1 MB) is usually called real mode. The new mode was called protected mode. This gate is called Gate A20.

Now that there's a way to switch between modes, there needs to be a way to control the switch. The standard way was to use a spare pin in the keyboard controller. This was later found to be too slow, so some chipset manufacturers put in an option for the gate to be controlled via the mainboard chipset. This option can be controlled via the basic input/output system (BIOS) firmware -- it's usually called Fast Gate A20, Gate A20 Option or something similar.

Locate this option in the BIOS and select another setting. This should fix any problems the operating system has controlling the Gate A20.

Update the BIOS if possible, and check to see that all settings in the BIOS firmware are accurate, especially RAM "speeds" and sizes if they need to be set manually.

If the other PC has the exact same specifications, try using the exact same BIOS settings on the problem PC that the good PC uses. If this doesn't work, try replacing the RAM on the problem PC with the ones from the good PC.
If it works then, the problem is likely to be that the RAM on the problem PC is failing. By replacing the RAM one module at a time, the problem module can be identified and replaced.

For best results, it's recommended that the hard disk be erased before partitioning and formatting are attempted. This can be done with a "zero-fill" utility usually available for download at the hard disk manufacturer's Web site.


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