An FPGA-based LGP-30 Replica

Summary

Vintage Computing – LittleGP-30 An FPGA-based LGP-30 Replica The LGP-30 was a commercial computer, released in 1956. Due to its simple design and relatively low cost, it may be seen as the first “personal computer” – to be used by a single user as their “desk computer”. (It could sit by your desk, and was the size of a desk too.) Designed in the age of vacuum tubes, it needed only 113 tubes in total, of which only 24 were used in the CPU itself! This simplicity was achieved by a bit-serial CPU design, which was tightly integrated with the magnetic drum storage unit. The magnetic drum contained not only the main memory, but also the CPU’s three 32-bit registers, and several tracks with timing signals to control the instruction decoding and execution. This page describes a replica that is true to the bit-serial implementation and its timing, but uses modern components. The CPU and the magnetic drum storage are recreated in an FPGA – by implementing the complete logic equations published by the LGP-30’s inventor (in a scientific paper and in the computer’s service manual). The magnetic drum, while implemented in on-board memory inside the FPGA, is made tangible via an optional video display of its contents. This way, one can play with all the quirks of the LGP-30, including the timing behavior of programs, which depends critically on the position of instructions and data on the magnetic drum. For those who want to really dive into the details of the bit-serial design, the clock rate can be slowed down (all the way to single step, bit-by-bit clocking), and the contents of the CPU’s tube-based flip-flops can be inspected on an LCD (all 15 bits of them!). The original LGP-30: 'Smaller than a desk'! The FPGA-based replica: Much smaller than a desk... This replica is based on a low-cost FPGA development board, the Numato Mimas (Xilinx Spartan 6 FPGA). I have added a front-end board to provide all necessary I/O, and programmed the FPGA to implement the LGP-30 logic unit, tra...