The Scatter-Gather List

134 contains a count of the number of pieces in the scatter-gather chain. If 135 is zero, then 136 points to the data buffer for the SCSI command, and 137 is the length of this buffer in bytes. Otherwise, 138 points to an array of 139 structures, and 140 will indicate how many such structures are in the array. The use of 141 is non-intuitive and confusing. Each element of the 142 array contains an 143 and a 144 component. If the 145 flag in the 146 structure is set to 1 (see section~#secdma#1423> for more information on DMA transfers), the address is guaranteed to be within the first 16~MB of physical memory. Large amounts of data will be processed by a single SCSI command. The length of these data will be equal to the sum of the lengths of all the buffers pointed to by the 147 array.