Kenbak-1

The Kenbak-1 is considered by the Computer History Museum, the Computer Museum of America and the American Computer Museum to be the world's first "personal computer", invented by John Blankenbaker (born 1929) of Kenbak Corporation in 1970, and first sold in early 1971. Less than 50 machines were ever built using Bud Industries enclosures as its housing. The system first sold for US$750. Today, only 14 machines are known to exist worldwide, in the hands of various collectors and museums. Production of the Kenbak-1 stopped in 1973 as Kenbak failed, and was taken over by CTI Education Products, Inc. CTI rebranded the inventory and renamed it the 5050, though sales remained elusive. Since the Kenbak-1 was invented before the first microprocessor, the machine didn't have a one-chip CPU but instead was based purely on small-scale integration TTL chips. The 8-bit machine offered 256 bytes of memory, implemented on Intel's type 1404A silicon gate MOS shift registers. The clock signal period was 1 microsecond (equivalent to a clock speed of 1 MHz), but the program speed averaged below 1000 instructions per second due many clock cycles needed for each operation and slow access to serial memory. The machine was programmed in pure machine code using an array of buttons and switches. Output consisted of a row of lights. Internally, the Kenbak-1 has a serial computer architecture, processing one bit at a time. The Kenbak-1 has a total of nine registers. All are memory mapped. It has three general-purpose registers: A, B and X. Register A is the implicit destination of some operations. Register X is also known as the index register and turns the direct and indirect modes into indexed direct and indexed indirect modes. It also has program counter, called Register P, three "overflow and carry" registers for A, B and X respectively as well as an Input Register and an Output Register.