Intel 8051 and 8031 are members of Intel MCS-51 family of 8-bit microcontrollers. In addition to on-chip clock oscillator, 2 16-bit timers, 4 I/O ports and a serial port, all 8051 and 80C51 chips have 128 bytes of RAM and 4 KB masked ROM. If necessary, 8051 memory can be expanded up to 128 KB by adding up to 64 KB of external program and data memory.
The 8031 requires external memory as it has none of its own.
This is the High-speed CMOS version w/ Burn-in screening.