The account of USB Drive's capacity

Key Word: FLASH Memory, BLOCK, FAT

Flash memory is a solid-state, non-volatile, rewritable memory (NAND FLASH) that works like RAM and a hard-disk drive combined. Flash memory stores bits of electronic data in memory cells, just like DRAM and SRAM, but flash also works like a hard-disk drive because when the power is turned off, the data stays in memory. Flash memory is becoming increasingly popular for portable computing and communications applications.

By the original manufacturer design, The Flash memory make up of block. For example,128MB can be divided with 1024 blocks, as the follow:

Vided with 1024 blocks, as the follow:

128M byte divided 1024 blocks, and 1 block divided with 64 page
1 Page divided with 2K+64 bytes
But manufacturers use the account is that 1M=1000kb, 1K=1000Byte.
By this account, The capacity of 128MB can be accounted as follow:
128MB = 1024(block)*64(page)*2000(byte)=131072000 bytes
As we know the computer use bytes as a unit to account disks capacity.
The computer use the account is that 1MB=1024K 1K=1024Byte
By this account, 128M=1024*1024=1311768576 bytes. that we see the actual capacity is
131072000/(1024*1024)=125MB

But in fact, there are bad block in product process. A few bad block can be allowed. But the bad block can't be used, Bad blocks will be deducted from the 1024 good blocks. As hard disk, USB drive also uses FAT or FAT32 file system. FAT file system include MBR main boot partition, FAT file config table and ROOT directory. There will use a few capacity, several Kilo Bytes as general. (The capacity is more larger the lost capacity ), while USB controller will use 10 blocks as spare space for bad block of future using. In order to sure the stability of products, most big manufacturers (such as Toliba, SONY, ect.) always keep above 10 blocks to ensure the stability. The capacity of 125MB will be occupied 1~5M because of above reason. The capacity of 128M Flash memory is impossible to show on computer as 128MB(1311768576 byte).

The hard disk manufactures (as MAXTOR, WESTWOOD, HATACHI) all use the same way to account the capacity. A 80GB hard disk, the actual capacity of 76GB only.

Below is the capacity standard:

64M—60M
128M-120M
256M-240M
512M-475M-485M
1GB-960M-975M
2GB-1800M-1900M, show in computer is 1.75G-1.85G
4GB-3800M-3842M, show in computer is 3.7-3.75G
8GB-7600M-7900M, show in computer is 7.4-7.75G
16GB-15155M-16076M, show in computer is 14.7G-15.7G
32GB- 28672mb-29696MB, show in computer is 28G-29G