What is meant by GB data?

Answer 1

GB is acronym for gigabyte and is used to indicate storage capacity on a computer. A gigabyte is 2^30 bytes or 1,073,741,824 bytes.

Note that disk manufacturers and memory chip manufacturers use different notations. Memory chips are precisely specified such that a 1 GB memory chip is guaranteed to provide storage for exactly 1,073,741,824 bytes. However, hard drives only give approximate storage capacities such that 1 GB only guarantees storage capacity for at least 10^9 bytes, or 1 billion bytes, which is the equivalent of a 0.93 GB memory chip.

Technically, hard-drive manufacturers use the correct notation since the prefixes kilo-, mega- and giga- literally equate to some power-of-ten (10^3, 10^6 and 10^9 respectively). However, it is not possible to create a memory chip with exactly 10^3 bytes, it has to be an exact power-of-two (2^10, 2^20 and 2^30, respectively).

The reason memory has to be an exact power-of-two stems from the way in which memory is addressed on a binary computer. In order to address 1000 bytes we'd need at least 10 bits in the range 0000000000 through 1111100111 (0 to 999 decimal) which means there are 24 invalid addresses in the range 1111101000 through 1111111111 (1000 to 1023 decimal). This over-complicates circuit design; it's much easier to make all bit patterns valid by adding on those missing 24 addresses thus giving us the full kilo-binary-byte capacity of 1024 bytes. Hard-drive manufacturers are less constrained in the way storage is addressed, because the addressing is subject to the operating system and the way in which the hard-drive is formatted. Typically, a hard-drive is divided up into addressable clusters which are themselves some power-of-two bytes in length, such as clusters of 512, 1024, 2048 or 4096 bytes.

In an effort to avoid confusion, the terms kilo-binary-byte (KiB), mega-binary-byte (MiB) and giga-binary-byte (GiB) are used to indicate precise memory capacities while KB, MB and GB are used to indicate the more generalised hard disk capacities. However, these terms is not widely adopted. For instance, Windows operating systems use the term KB to mean 1024 bytes whether reporting memory capacity or hard drive capacity, hence a 300 GB hard-drive only has capacity for 279.4 GB of storage on Windows. But 279.4 GB is 300 billion bytes so, strictly-speaking, it is quite correct to call it a 300 gigabyte drive.