Buffer Underrun and Overrun Scenarios

By Stephen Bucaro

Buffer underrun and buffer overrun are occurrences that can result in some very frustrating errors. This is not a "how-to" article about fixing buffer underrun and buffer overrun errors, but a basic description of what a buffer is, why we need buffers, and what causes buffer underrun and buffer overrun.

Buffer Underrun

The most common occurrence of buffer underrun is CD recorders. Let's imagine an example of a CD recording session. The computer has an ATA hard drive capable of transferring data at a rate of 8 MBps (Mega Bytes per second). The CD recorder has a recording rate of 8 MBps. Everything should work fine, right?

Note: The data transfer rates mentioned in this article do not apply to any specific device. They're just for purposes of discussion.

The 8 MBps specification for hard drive is for "burst" mode. In other words, it can transfer data at a rate of 8 MBps for only a few seconds. Then transfer rate drops much lower, and if hard drive hasn't been maintained, for example it has not been defragmented recently, transfer rate can drop even lower.

Whereas a hard drive can skip from cluster to cluster while reading and writing, a CD recorder must burn data track in a continuous stream without stopping. The design of a CD recorder requires a "sustained" transfer rate.

When two devices that operate at different transfer rates must communicate, we can make them work together by placing a buffer between them. A buffer is a block of memory, like a bucket for bytes. When you start CD recording session, hard drive begins filling buffer. When buffer is almost full, CD recorder begins drawing bytes out of buffer.

If everything goes smoothly, hard drive will be able to keep enough bytes in buffer so that speedy CD recorder won't empty buffer. If buffer runs dry, CD recorder has no data to burn into CD, so it stops. Buffer underrun error.

We can reduce chances of buffer underrun by configuring a larger buffer. Then hard drive will be able to put more bytes in bucket before CD recorder starts drawing them out. However, sometimes you can't increase size of buffer because computer doesn't have a large amount of RAM installed. When computer needs more RAM, it uses "virtual" RAM. That is, it allocates part of hard disk and pretends like that's RAM. Now, even though you've increased size of buffer, you have caused hard drive to work even slower.

Buffer Underrun and Overrun Scenarios

Buffer Overrun