Intel 4004
The world's first microprocessor, released in 1971. The 4004 contained 2300 transistors (compared with 5.5 million in the 1996 Pentium Pro) and was intended for use in a calculator. It processed data in 4 bits, but its instructions were 8 bits long. Program and Data memory were separate, it had 1 kilobyte of data memory and a 12-bit PC for 4K of program memory (in the form of a 4 level stack, used for CALL and RET instructions). There were also sixteen 4-bit (or eight 8-bit) general purpose registers. The 4004 had 46 instructions.