Real Time Systems

1

Troisième Partie Chapitre 7 Lorganisation
des mémoires périphériques et la gestion des
fichiers
2
Contents

• Physical disk organization
• Logical to physical mapping
• DOS Windows File Access Table (FAT16)
• UNIX LINUX inode
• Windows NT Master File Table
• Disk directories
• System calls

3
Disk Organization
Track
Sector
Cylinder
4
Clusters vs. Sectors

• What is the optimal size of a disk sector ?
• Small sectors good for small disks
• Reduce slack
• Large sectors good for large disks
• Increase disk throughput
• Reduce number of bits in sector number
• It is desirable to have a single sector size for
  all disks !
• Solution define clusters as groups of sectors
  with size optimized for each specific disk.
• Transfer entire clusters instead of sectors
• Each cluster on disk has a Physical Cluster
  Number (PCN)

5
Format of 3.5 diskettes for PCs.
Double Density
High density
2
Tracks/cylinder
2
Cylinders
80
80
Sectors/track
9
18
Bytes/sector
512
512
Total Capacity(in bytes)
737 280
1 474 560
Sectors/Cluster
2
1
6
Format of 428 MBytes Hard Disk.
15
899
62
512
428 067 840
Sectors/Cluster
16
7
File/disc mapping
Logical file structure
Physical Disc Structure
8
MS/DOS the FAT 16
Stream a
0
1
2
9
Stream b
3
4
Stream c
8
...
Stream d
16
15
17
18
19
...
PCN
...
Free
6
7
9
File Access Table(MS/DOS)
Cluster x
FAT
One entry per Physical Cluster Number
Cluster 0
DISK
Two copies of the FAT are stored on track 0.
10
Minimum Cluster Size (with a 16 bit FAT)
When cluster numbers have only 16 bit, no more
than 65536 clusters can be defined on any disk,
this implies very large clusters on large disks
Disk Size 128 MB 256 MB 512 MB 1 GB 2 GB 4 GB 8
GB
Cluster Size 2 kB 4 kB 8 kB 16 kB 32 kB 64
kB 128 KB
Sectors/Cluster 4 8 16 32 64 128 256
11
UNIX/LINUX inode
10 Logical clusters of data on disk
256
256
256
...
...
...
Logical clusters of data on disk (up to 256)
256
256
...
...
256
256
...
...
Logical clusters of data on disk (up to 256256)
256
256
256
256
...
...
...
...
Logical clusters of data on disk (up to
256256256)
12
LCN to PCN mapping in MS/NT
starting LCN
starting PCN
no.of clusters
4
5
6
7
8
LCN
0
1
2
3
1588
1589
1590
1591
1592
PCN
1355
1356
1357
1358
13
Directory Tree
\
info
ex
1b
2b
acs
3E
\info\ex\3E
14
MS/DOS Directory Entry
Base Name
8 bytes
Extension
3 bytes
Time of creation
2 bytes
Date of creation
2 bytes
Starting FAT entry
2 bytes
File size
4 bytes
15
Unix Directories
\
info
txt
ex
new
3E
\info\ex\3E
\txt\new
16
MS/NT Master File Table
17
MS/NT Data Files
Small file
File Name
Security Descriptor
Standard Information
Actual Data
Large file
File Name
Security Descriptor
Standard Information
LCN to PCN mappings
0
1
2
3
4
5
6
7
8
LCN
1355
1356
1357
1358
1588
1589
1590
1591
1592
PCN
18
MS/NT Directories
Small directory
File Name
Security Descriptor
Standard Information
Index of files in MFT f1,f2,f3
Large directory (organized as a Btree)
File Name
Security Descriptor
Standard Information
LCN to PCN mappings bitmap
f3
f7
0
1
2
3
4
5
6
7
8
9
10
11
f6
f1
f2
f4
f5
f8
f9
1355
1356
1357
1358
1588
1589
1590
1591
1648
1649
1650
1651
19
System CallsFile Manipulation

• create file, delete file
• open, close
• read, write, reposition
• get file attributes, set file attributes