Secure Digital, officially abbreviated as SD, is a proprietary, non-volatile, flash memory card format the SD Association (SDA) developed for use in portable devices.
microSD card in a smartphone
This microSDHC card holds 8 billion bytes. Beneath it is a section of a magnetic-core memory (used until the 1970s) that holds eight bytes using 64 cores. The card covers approximately 20 bits (2+1⁄2 bytes).
Macro shot of a microSDXC memory card with eight gold plated electrical contacts
Back side of a Lexar UHS-II microSDHC card, showing the additional row of UHS-II connections
Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use the same cell design, consisting of floating gate MOSFETs. They differ at the circuit level depending on whether the state of the bit line or word lines is pulled high or low: in NAND flash, the relationship between the bit line and the word lines resembles a NAND gate; in NOR flash, it resembles a NOR gate.
A disassembled USB flash drive in 2005. The chip on the left is flash memory. The controller is on the right.
NOR flash by Intel
Serial Flash: Silicon Storage Tech SST25VF080B
An Intel mSATA SSD in 2020
