The LayGO system uses a pool of buffers allocated during initialization for data transmission and reception and for internal communication. The buffer pool may have up to 4 subpools, each subpool containing buffers of the same size. The buffers from all pools are shared by all lines.
Buffer size is specified in terms of the number of bytes of user data the buffer can hold. Added to this is the number of prefix bytes to reserve for the use of protocols. For most applications, the smallest useful buffer is 8 bytes with prefix bytes of 8. The largest useful buffer depends on the protocol. Since small buffers are required for both internal communication and protocol control messages, it is a good idea to allocate a large supply of small buffers. The largest buffers are always used when receiving data because it is impossible to predict the actual size of a frame until it has been completely received. So as many buffers as possible should be allocated of the largest size. Intermediate sizes are useful only for transmission of data of less than maximum size. Typically, only 2 subpools should be configured.
|Number of subpools||A subpool is a pool of buffers all of which are the same size. There may be 1-4 subpools.|
|Critical level for largest buffer subpool||When the number of largest buffers available falls below the critical level, protocols which implement flow-control, such as HDLC LAPB, will issue and XOFF request.|
|Safe level for largest buffer subpool||When the number of largest buffers available falls below the critical level, the XOFF condition will remain in effect until the safe level is reached.|
|Ignore 64K page boundaries for DMA?||Buffers used by host DMA
must not cross 64K page boundaries. If the allocated
buffers are to be used for DMA, this must be set to
|Number of extra prefix bytes||Protocol layers often place header bytes on data packets sent. This is the number of prefix bytes to reserve for this purpose.|
|Number of reference buffers||An error-free protocol, such as HDLC LAPB, must retain transmitted buffers until they are acknowledged by the receiver. The use of reference buffers allows the layer to keep a copy without making a copy, saving both time and space. This parameter should be set to the number of error-free devices being used times the send window size of the protocol.|
The following parameters are required for each subpool:
|Number of buffers||The number of buffers to allocate in the subpool.|
|Size of buffers||The data capacity of the buffers in the subpool. The minimum is 8.|