1. COMP1321 Digital Infrastructures
Richard Henson
November 2017

2. Week 6: Hard & Solid State Disk Storage
Objectives:
explain what a file is and why it is such a useful way to store instructions and data
name the commonly used file systems used to store digital data off the motherboard
explain how data is organised into files by the CPU
explain how files are saved to secondary storage for fast retrieval

3. Storing Data
Overview: 9RV7twk

4. Files “file” ~ conventional name for a package of bytes of data
Created and Used in Primary Storage :
set of stored data usually in consecutive memory locations
controlled directly by CPU instructions

5. Secondary Storage Devices
Connected to the Motherboard
directly (SATA connection)
may need to be configured with operating system on ROM (CMOS settings)
allocates a drive letter (e.g. D, E, F)
indirectly (USB connection)
drive letter allocated through “plug and play”

6. Finding data on Secondary Storage (1)
Secondary storage: controlled on CPU via disk controller programs & file system manager
storage locations given addresses when the media is “formatted”
works just like memory but has to be loaded into memory to be CPU accessible
need an EOF (end of file) marker

7. File Attributes
When a file is saved, and data is transferred from RAM to disk, a number of “attributes” are also saved:
File name & suffix:
File starting address on disk:
File size:
Date/time saved:
Whether: read-write/read-only; hidden/visible

8. Overview of File Organisation
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Secondary storage devices need to organise files for quick access via starting address…
tapes fine for writing, but too slow for reading!

9. Primary/Secondary Storage of data as files
If Windows = operating system, each drive/partition allocated a letter (e.g. C:, D:, etc.

10. File Organisation on Disk(1)
Partition: area earmarked for formatting to meet the needs of a particular file system
Could be several partitions on a single disk..
Each has its own drive letter
Each could be formatted for a different file system

11. File Organisation on Disk (2)
Disk surface physically divided up into sectors
size of sectors depends on formatting type
512 locations (i.e. bytes)
2048 locations
files laid down in locations within sectors

12. Finding data on Secondary Storage
Each partition creates a table/index/catalogue for files that are written to it
includes file attributes otherwise file very difficult to retrieve…
Method depends on filing system chosen when partition formatted…
as well as formatting the surface to receive data, each filing system formatting type structures the media to receive data in its own unique way…

13. “Boot Sector” Important part of PC boot up involves secondary storage
Starts from “boot sector” (first sector)…
provides configuration information for effective communication with CPU
if damaged, boot up halted!
should have a backup… needs to be copied to boot sector to overwrite corrupted data

14. Partitions and Boot Up Whilst a disk can have multiple partitions…
Only one of these contains the boot sector
Boot sector configuration options…
Selectable via screen menu
Can point to different operating systems

15. Booting up: loading an Operating System…
Needs to be loaded into RAM
some operating systems load everything from ROM (e.g. smartphone)
others (e.g. PC) use a combination…
some loaded first from ROM
rest from hard disk or other source
disk needs a bootable partition or can’t load rest of operating system into RAM

16. Data Storage on Disk Partition
Sectors numbered Files stored in specified sector address ranges

17. Disk Index/Catalogue
Stores location for start of file, and size of file
Also, organises files into directories/folders
top folder (C:) = root
rest of folders link hierarchically from the root
Index/Catalogue logically allocates each file to a folder for ease of retrieval

18. “Fragmentation” (Data in Secondary Storage)
General problem with hierarchical data storage…
deleted data items leave holes in the structure
New data items saved try to fill the gaps
large files can be broken into fragments
fragments linked by address pointers
slows down retrieval

19. Removing Fragmentation
If disk only partly fragmented…
defragmented files copied into memory
remaining files moved around to close up holes
Previously defragmented files copied back to disk as complete files
If disk >75% fragmented
most effective solution is to copy all files to another partition
can copy back later once original partition has all data deleted

20. Is it true that deleted files aren’t really deleted?
Absolutely!
Two things happen when a file is deleted:
the first data item stored in the file (first character of filename) is changed to “?”
the catalogue entry ceases to recognise & display the filename
starting address shows “?” Character
file system is programmed to ignore ? at such a location
Rest of the data is untouched…
easily demonstrated through use of a Hex editor program: can show file contents “before” and “after”

21. “Normal” Loading of a File from Secondary Media
File catalogue essential for data retrieval
application reads file catalogue
displays folders and files
user chooses file, application uses disk addresses to load into memory
File catalogue corruption?
can’t find files!
backup copy on disk, in separate sector

22. Direct access by address on Secondary Media
If both file catalogues are damaged…
file (and its data) cannot be located
ideally, needs to be a backup catalogue elsewhere
“Hex editors” available to do the equivalent of debug –d (peek) and –e (poke)
enables full search of all addresses for particular ASCII string(s)
essential for recovery of data…

23. “Undelete” Tools also available for restoring recently deleted files
“?” character poked, and restored to a real character
then picked up & shown on catalogue display

24. WinHex Probably the most popular tool to examine hard disks
readout quite similar to debug –e
data presented byte-by-byte according to catalogue address
range of options for extracting, overwriting data, and (like debug) writing consecutive raw data items to a file

25. WinHex, Disk Investigation Tools, and Forensics
Essential for
police: collecting evidence
data recovery:
to rebuild a damaged or accidentally deleted catalogue
to repair a boot sector so that computer can reboot