Byte Merge
Common Options : Enabled, Disabled
Quick Review of Byte Merge
The Byte Merge BIOS feature is similar to the PCI Dynamic Bursting feature.
When enabled, the PCI write buffer accumulates and merges 8-bit and 16-bit writes into 32-bit writes. This increases the efficiency of the PCI bus and improves its bandwidth.
When disabled, the PCI write buffer will not accumulate or merge 8-bit or 16-bit writes. It will just write them to the PCI bus as soon as the bus is free. As such, there may be a loss of PCI bus efficiency when 8-bit or 16-bit data is written to the PCI bus.
Therefore, it is recommended that you enable Byte Merge for better performance.
However, please note that Byte Merge may be incompatible with certain PCI network interface cards (also known as NICs). So, if your NIC won’t work properly, try disabling this feature.
Details of Byte Merge
The Byte Merge BIOS feature is similar to the PCI Dynamic Bursting feature.
If you have already read about the CPU to PCI Write Buffer feature, you should know that the chipset has an integrated PCI write buffer which allows the CPU to immediately write up to four words (or 64-bits) of PCI writes to it. This frees up the CPU to work on other tasks while the PCI write buffer writes them to the PCI bus.
Now, the CPU doesn’t always write 32-bit data to the PCI bus. 8-bit and 16-bit writes can also take place. But while the CPU may only write 8-bits of data to the PCI bus, it is still considered as a single PCI transaction. This makes it equivalent to a 16-bit or 32-bit write in terms of PCI bandwidth! This reduces the effective PCI bandwidth, especially if there are many 8-bit or 16-bit CPU-to-PCI writes.
To solve this problem, the write buffer can be programmed to accumulate and merge 8-bit and 16-bit writes into 32-bit writes. The buffer then writes the merged data to the PCI bus. As you can see, merging the smaller 8-bit or 16-bit writes into a few large 32-bit writes reduces the number of PCI transactions required. This increases the efficiency of the PCI bus and improves its bandwidth.
[adrotate group=”1″]This is where the Byte Merge BIOS feature comes in. It controls the byte merging capability of the PCI write buffer.
If it is enabled, every write transaction will go straight to the write buffer. They are accumulated until there is enough to be written to the PCI bus in a single burst. This improves the PCI bus’ performance.
If you disable byte merging, all writes will still go to the PCI write buffer (if the CPU to PCI Write Buffer feature has been enabled). But the buffer won’t accumulate and merge the data. The data is written to the PCI bus as soon as the bus becomes free. This reduces PCI bus efficiency, particularly when 8-bit or 16-bit data is written to the PCI bus.
Therefore, it is recommended that you enable Byte Merge for better performance.
However, please note that Byte Merge may be incompatible with certain PCI network interface cards (also known as NICs). Boar-Ral explains :-
I noticed that some PCI cards really despise Byte Merge, in particular the 3Com 3C905 series of NICs. While this may only apply to certain motherboards, in my case, the P3V4X; I feel that this is probably not the case and that it is a rather widespread problem.
Issues I have encountered with Byte Merge enabled, range from Windows 98 SE freezing at the boot screen to my NIC not functioning at all. This issue has been confirmed with others using the same NIC and is what alerted me to the issue in the first place.
Prozactive concurs :-
I wanted to confirm the observation posted by Boar-Ral concerning the “Byte Merge” BIOS setting. After enabling “Byte Merge” and making other recommended BIOS setting changes, I suddenly lost all network I/O from my system. And yes, I happen to be using a 3Com 3C905B-TX NIC (with an Asus A7V motherboard). After a great deal of trial and error troubleshooting, I found that disabling “Byte Merge” lets everything work again.
On the other hand, Cprall discovered that he was able to use the NIC in Windows 98 SE but not in Windows 2000. Check out what he has to say :-
I’ll even third this to say I was recently bitten by the same (A7V motherboard at BIOS 1009 and 3C905B-TX network card). I do have one slight addition to what was seen here. With Byte Merge enabled, I was able to access the network under Windows 98 SE, but not Windows 2000. With Byte Merge disabled, the network card works under both.
So, if your NIC (Network Interface Card) won’t work properly, try disabling Byte Merge. Otherwise, you should enable Byte Merge for better performance.
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