History of Computers: The first single chip CPU was the Intel 4004, a 4-bit processor meant for a calculator. It processed data in 4 bits, but its instructions were 8 bits long. Program and data memory were separate, 1K of data memory and a 4K of program memory (in the form of a 4 level stack, used for CALL and RET instructions). There were also sixteen 4-bit general purpose registers

The 4004 consists of 2,300 transistors and has a 10mm*mm area.  The microprocessor had 46 instructions. The 4040 was an enhanced version of the 4004, adding 14 instructions, larger stack (8 levels) and 8K program space.