SRAM Timekeeper 64K, 48T18, DIP28 - 

The M48T18-100PC1 is a TIMEKEEPER® Non-volatile Static RAM and Real-time Clock which is pin and functional compatible with the DS1642. This is a non-volatile pin and function equivalent to any JEDEC standard 8K x 8 SRAM. It also easily fits into many ROM, EPROM and EEPROM sockets providing the non-volatility of PROMs without any requirement for special write timing or limitations on the number of write that can be performed.