Logical Block Addressing

(L-B-A)

A translation method that allows drives larger than 504MB to be used with an enhanced BIOS.

Note: The limitation was built into the IBM BIOS and ATA specification which, in combination, limited hard drive sizes to 16 heads over 1024 cylinders with 63 sectors per track and 512 bytes per sector. Multiply these together and you get 528,482,304 bytes. Divide by 1024 twice to convert bytes to megabytes and you get 504.

More modern BIOS limits increase the head cound to 256 for a maximum disk size of 8GB. However, operating systems like Windows bypass the BIOS hard disk services and so support ever-increasing sized drives.

More Information

• PC Guide Article
• Storage Interface Chapter
• Hard Drive Terms