Secure Digital (SD) memory cards provide flash memory storage for cameras, computers and a host of other electronic devices. The SD Association, a trade organization made up of SD card manufacturers, sets standards for how data is stored and retrieved on SD memory cards. These guidelines help to ensure that these handy, removable memory devices adhere to standardized protocols and design specifications, making them compatible with computers and other devices produced by many different manufacturers.
The standard for memory blocks on Secure Digital memory cards adopted by the SD Association is a fixed size of 512KB, although not all cards adhere to this standard. In a June 2007 article published on technology website ZD Net, columnist Robin Harris notes that each block on a SD card is further divided into “pages” of about 2KB each.
Although the standard block size on an SD card is 512KB, many different flash memory devices are advertised as SD cards, and some manufacturers use other block sizes. In the ZD Net article, Harris observes that many SD cards use block sizes of only 128KB. Block sizes typically remain consistent on a given SD card.
Smaller block sizes allow an SD card to write data more quickly. When an electronic device writes to flash memory, it must first erase the entire block and then rewrite the data in that block along with the new information. Writable data is stored in a buffer to speed the process, but erasing and recreating larger blocks can take slightly longer than it does for smaller blocks, leading to a delays when writing larger files to an SD card.
Electronic devices follow a specific sequence when writing data to blocks on an SD memory card. The write process begins with block zero, the first block, and proceeds in order through block 64. This helps speed up the write process but can cause a delay or wasted storage space when writing small amounts of data.
Because SD cards use a constant block size of 512KB or less, large capacity cards contain a considerable number of storage blocks. For example, a 64GB card formatted to 128KB blocks holds more than a million blocks. In addition, SD cards, like other flash storage media, rely on software drivers that allow the removable media to interact with electronic devices much like a traditional hard drive. This software occupies space on the SD card, reducing the card’s overall storage capacity.
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