QMCS 360
Class Notes # 19
Dr. Rick Smith
Quantitative Methods
and
Computer Science
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last update:
Tuesday, November 22, 2005
Scheduling software
Deadline for spring CIG grants - Tue, Nov 29
Outline
CP/M File System
File system for 8-bit computers
Based on DEC OSes, especially RT-11 (device letters)
Heavily influenced MS-DOS
Never made the jump to 16-bits
Flat file system - no hierarchy
Directory entry format
User ID (1 byte)
File name (8 bytes)
File type/extension (3 bytes)
Extent # (1 byte) - which part of the file this describes (incremental)
Block pointers used (1 byte)
List of 16 block pointers (2 bytes each)
Internal fragmentation problem - no end of data marker
FAT File System
Classic MSDOS file structure, has tried to evolve to keep up with disk sizes
FAT = File Access Table
Centerpiece of the disk structure
All data accessed in chunks of blocks/sectors called "Clusters"
Files consist of sets of clusters linked together in order
Links appear in the FAT - one entry per cluster
If the link = 0, the cluster is free to allocate
Usually have 2 or more copies of FAT for reliability/recovery
Different FATs:
FAT12 = 12-bit cluster addresses
FAT16 = 16-bit
FAT32 = guess
Elements of the disk layout in FAT
Reserved sectors (boot data, configuration data)
FAT #1
FAT #2 (backup of #1)
Root Directory (Cluster #1)
rest of disk consists of Clusters #2 through N)
FAT directory entry contents
File name (8 bytes) - first byte flags free or 'special' (LFN)
Extension (3 bytes)
Attributes (1 byte)
Time/Date (2 bytes each)
First block number (2-4 bytes, depending on 12/16/34)
Long file names (LFN)
Special format entries in the directory
NTFS
Introduced for Windows NT
File system for "real players"
Implements state of the art features: User IDs, access control lists, huge devices, etc
"Everything is in a file" unlike FAT
Uses clusters, like FAT
Hierarchical
Inheritance of ownership permissions