This part is a kind of semiconductor called M81C55.

Function of this product has MSM81C55.

Manufacturers : OKI

Image and pinout :

Some of the text within the PDF file :

E2O0014-27-X2 ¡ Semiconductor MSM81C55-5RS/GS/JS ¡ Semiconductor 2048-Bit CMOS STATIC RAM WITH I/O PORTS AND TIMER This version: Jan. 1998 MSM81C55-5RS/GS/JS Previous version: Aug. 1996 GENERAL DESCRIPTION The MSM81C55-5 has a 2k-bit static RAM (256 bytes) with parallel I/O ports and a timer. It uses silicon gate CMOS technology and consumes a standby current of 100 micro ampere, maximum, while the chip is not selected. Featureing a maximum access time of 400 ns, the MSM81C55-5 can be used in an MSM80C85AH system without using wait states. The parallel I/O consists of two 8-bit ports and one 6-bit port (both general purpose). The MSM81C55-5 also contains a 14-bit programmable counter/timer which may be used for sequence-wave generation or termin [ ... ]

M81C55 PDF Datasheet Download