branches of a a 1 142 i a 2 142 j and a 3 142 k which lead to multiple branches of b b 1 144 i b 2 144 j and b 3 144 k each ai leads to three potential bj and thus nine combinations are possible one typical default way of allocating these ssif files is shown in fig1 b as allocation 140 however as is clear a jump from the end of ssif file 142 i to the beginning of ssif file 144 k which in this case is the maximum long jump could often be over 640 000 sectors thus interleaving is required the bottom diagram in fig1 b shows just such an interleaving and as noted above the points where ssif files are interleaved should occur at appropriate jump capable points as shown the longest jumps required are below the maximum of 640 000 sectors this maximum being termed u201c rule a u201d as is seen in this figure and as is commonly encountered the last extss of the group a files and the first extss of the group b files require smaller size extss files in order to keep the longest jump a 1 to b 3 within the required 640 000 sectors this can be achieved by estimating the size of these extss files to be roughly half of the prior extss files giving rise to a u201c rule b u201d where the jump distance is less than 320 000 sectors of course it will be understood that variations of these rules may be employed in a given circumstance fig1.