The SST89E5xxRD2 and SST89V5xxRD2 are FlashFlex51 family 8-bit microcontroller devices designed and manufactured using SST's unique and proprietary Super Flash CMOS semiconductor manufacturing technology. SST's clients gain significantly from the split-gate device structure and thick-oxide tunnelling injector in terms of cost and reliability. The devices are pin-for-pin compatible with ordinary 8051 microcontroller devices and use the 8051 instruction set. The devices include 16/24/40/72 K Byte of on-chip flash EEPROM programme memory that is divided into two separate programme memory blocks. The primary Block 0 has 8/16/32/64K Bytes of internal programme memory space, whereas the secondary Block 1 has 8K Bytes of internal programme memory space. The 8-KByte secondary block can be identified to the minimum address point in the 8/16/32/64 K Byte address space, as well as concealed from the programme counter and utilised as an independent EEPROM-like data store. In addition to the on-chip EEPROM programme memory of 16/24/40/72 K Bytes, the devices may address up to 64 K Bytes of external programme memory. In addition to 1024 x 8 bits of on-chip RAM, external RAM of up to 64 K Bytes may be addressed.