Custom TTL design by Marvin Minsky. There are two displays: one
raster scan for bitmapped characters, and another random scan for
vector graphics. There is also a keyboard, and a UART for talking
to a host computer.
The computer is normally booted off a ROM which reads and starts a
secondary loader from the UART. The loader is responsible for reading
the payload, which comes in checksummed blocks. The LOAD command
accepts files in same format.
