Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Digital systems, linear algebra, introduction to formal proof / 2 / e

Each address uses $32/8=4$ bytes, so an address block holds $q=2048/4=512$ pointers. Use zero-based file offsets. The requested file offset hasThus select direct pointer index 1, read its file data block, and take the byte at offset 952 within it. No address block is needed:Fetching an uncached inode adds one read. If “byte 3000” instead uses one-based numbering, the within-block offset is 951; it is still the second file data block, with the same read count. Pathname lookup is separate from accessing a known inode.

The logical file data block index and within-block file offset areDirect pointers cover logical indices 0 through 9. The singly indirect region covers 10 through $10+512-1=521$. The requested file data block therefore lies in the doubly indirect region, at relative indexFollow the doubly indirect block pointer in the inode, read its top address block and select entry 14, read that second address block and select entry 511, then read the resulting file data block and take byte offset 1568. These entry indices are zero-based.This assumes a resident inode and no useful buffer cache entries for the three blocks; an uncached inode adds one read. Under one-based byte numbering the last offset is 1567, without changing either address-block index or the read count.