The 0 bit is always reserved. Bits 1 through 3 contain the nfrags field. Bits 4 through 31 contain the addr field. The Disk Address Format figure illustrates the mapping of a file's data to the fragments contained on the disk. The JFS uses the indirect blocks to address the disk space allocated to larger files.
Indirect blocks allow the greatest flexibility for file sizes and the fastest retrieval time.
Because files larger than 32KB are allocated fragments of 4096 bytes, the nfrags field for addresses using the single indirect or double indirect method has a value of 0. When the file requires more than 32KB, an indirect block is used to address the file's disk space. These addresses point to the disk fragments for each allocation. The double indirect block contains 512 addresses that point to indirect blocks, which contain pointers to the fragment allocations. When memory map interface is used, 2GB can be addresed.