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CompTIA A+ Certification All-in-One Exam Guide

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Figure 9-16 32-bit FAT

NOTE Hexadecimal characters cover the decimal numbers 0–15,

numbering from 0–9 and then A–F; each character reflects the state of four

binary characters. You add them up to make the number. So, 0000 in binary

shows zero numbers and the hex number is 0. When you go up numerically in

binary to 0001, this represents the number 1 in decimal and also in hex. The

key to hex is when you reach the number 10. In binary, this looks like this:

1010. But because hex sticks with a single digit, it’s represented as A. B

translates as 11 in decimal or 1011 in binary, and so on.

We call this type of FAT a 32-bit FAT or FAT32. And it’s not just hard

drives and SSDs that have FATs. Many USB flash drives use FAT32.

The right column of the FAT contains information on the status of

clusters. All hard drives, even brand-new drives fresh from the factory,

contain faulty blocks that cannot store data because of imperfections in the

construction of the drives. The OS must locate these bad blocks, mark them

as unusable, and then prevent any files from being written to them. This

mapping of bad blocks is one of the functions of high-level formatting. After

the format program creates the FAT, it marches through every block of the

entire partition, writing and attempting to read from each block sequentially.

If it finds a bad block, it places a special status code (0000FFF7) in the

block’s FAT location, indicating that the cluster is unavailable for use.

Formatting also marks the good blocks with code 00000000 (see Figure 9-

17).

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