AGP ISA Aliasing

Common Options : Enabled, Disabled

Quick Review

The AGP ISA Aliasing BIOS feature allows you to determine if the system controller will perform ISA aliasing to prevent conflicts between ISA devices.

The default setting of Enabled forces the system controller to alias ISA addresses using address bits [15:10]. This restricts all 16-bit addressing devices to a maximum contiguous I/O space of 256 bytes.

When disabled, the system controller will not perform any ISA aliasing and all 16 address lines can be used for I/O address space decoding. This gives 16-bit addressing devices access to the full 64KB I/O space.

It is recommended that you disable AGP ISA Aliasing for optimal AGP (and PCI) performance. It will also prevent your AGP or PCI cards from conflicting with your ISA cards. Enableit only if you have ISA devices that are conflicting with each other.

Details

The origin of the AGP ISA Aliasing feature can be traced back all the way to the original IBM PC. When the IBM PC was designed, it only had ten address lines (10-bits) for I/O space allocation. Therefore, the I/O space back in those days was only 1KB or 1024 bytes in size. Out of those 1024 available addresses, the first 256 addresses were reserved exclusively for the motherboard’s use, leaving the last 768 addresses for use by add-in devices. This would become a critical factor later on.

Later, motherboards began to utilize 16 address lines for I/O space allocation. This was supposed to create a contiguous I/O space of 64KB in size. Unfortunately, many ISA devices by then were only capable of doing 10-bit decodes. This was because they were designed for computers based on the original IBM design which only supported 10 address lines.

To circumvent this problem, they fragmented the 64KB I/O space into 1KB chunks. Unfortunately, because the first 256 addresses must be reserved exclusively for the motherboard, this means that only the first (or lower) 256 bytes of each 1KB chunk would be decoded in full 16-bits. All 10-bits-decoding ISA devices are, therefore, restricted to the last (or top) 768 bytes of the 1KB chunk of I/O space.

As a result, such ISA devices only have 768 I/O locations to use. Because there were so many ISA devices back then, this limitation created a lot of compatibility problems because the chances of two ISA cards using the same I/O space were high. When that happened, one or both of the cards would not work. Although they tried to reduce the chance of such conflicts by standardizing the I/O locations used by different classes of ISA devices, it was still not good enough.

Eventually, they came up with a workaround. Instead of giving each ISA device all the I/O space it wants in the 10-bit range, they gave each a much ISA device smaller number of I/O locations and made up for the difference by “borrowing” them from the 16-bit I/O space! Here’s how they did it.

The ISA device would first take up a small number of I/O locations in the 10-bit range. It then extends its I/O space by using 16-bit aliases of the few 10-bit I/O locations taken up earlier. Because each I/O location in the 10-bit decode area has sixty-three16-bit aliases, the total number of I/O locations expands from just 768 locations to a maximum of 49,152 locations!

More importantly, each ISA card will now require very few I/O locations in the 10-bit range. This drastically reduced the chances of two ISA cards conflicting each other in the limited 10-bit I/O space. This workaround naturally became known as ISA Aliasing.

Now, that’s all well and good for ISA devices. Unfortunately, the 10-bit limitation of ISA devices becomes a liability to devices that require 16-bit addressing. AGP and PCI devices come to mind. As noted earlier, only the first 256 addresses of the 1KB chunks support 16-bit addressing. What that really means is all 16-bit addressing devices are thus limited to only 256 bytes of contiguous I/O space!

When a 16-bit addressing device requires a larger contiguous I/O space, it will have to encroach on the 10-bit ISA I/O space. For example, if an AGP card requires 8KB of contiguous I/O space, it will take up eight of the 1KB I/O chunks (which will comprise of eight 16-bit areas and eight 10-bit areas!). Because ISA devices are using ISA Aliasing to extend their I/O space, there’s now a high chance of I/O space conflicts between ISA devices and the AGP card. When that happens, the affected cards will most probably fail to work.

There are two ways out of this mess. Obviously, you can limit the AGP card to a maximum of 256 bytes of contiguous I/O space. Of course, this is not an acceptable solution.

The second, and the preferred method, would be to throw away the restriction and provide the AGP card with all the contiguous I/O space it wants.

Here’s where the AGP ISA Aliasing BIOS feature comes in.

The default setting of Enabled forces the system controller to alias ISA addresses using address bits [15:10] – the last 6-bits. Only the first 10-bits (address bits 0 to 9) are used for decoding. This restricts all 16-bit addressing devices to a maximum contiguous I/O space of 256 bytes.

When disabled, the system controller will not perform any ISA aliasing and all 16 address lines can be used for I/O address space decoding. This gives 16-bit addressing devices access to the full 64KB I/O space.

It is recommended that you disable AGP ISA Aliasing for optimal AGP (and PCI) performance. It will also prevent your AGP or PCI cards from conflicting with your ISA cards. Enableit only if you have ISA devices that are conflicting with each other.

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