While working on the AltairZ80 documentation, I found some problems
with the SOL20 registers and fixed some formatting issues.
Added static and "vdm1_" prefix to "charset" variable in s100_vdm1.c
This PR adds support for the Processor Technology VDM-1 display adapter and Sol-20 computer system. The VDM-1 was integrated into the Sol-20 but has been separated out into its own device.
This PR adds the following devices to the AltairZ80 simulator:
VDM1 - Processor Technology VDM-1 display adapter
SOL20 - Processor Technology SOL20 with SOLOS ROMs
SOL20K - SOL20 Keyboard (callback from VDM1)
SOL20T - SOL20 Tape (reads/writes cassette file images)
SOL20S - SOL20 Serial Port (TMXR capable)
SOL20P - SOL20 Printer Port (TMXR capable)
