A subscriber identity module or SIM is a smart card that is designed to fit into mobile phone. It provides the identification of a user to a network, allowing him or her to access such services as telephony, email, internet and text messaging.
The SIM card contains a microcomputer as well as a certain amount of memory to process commands (Random Access Memory or RAM), and to store user files (Electronically Erasable Programmable Read Only Memory or EEPROM). The SIM also contains an amount of Read Only Memory (ROM) which stores the cards operating system.
When the SIM card is activated the microcomputer loads the operating system from ROM into the RAM of the card and processes commands as requested by the mobile equipment (ME) or card access device (CAD).
For more information visit SIM Structure, File Structure & Communication Protocols of Mobile Phones