In the past few days I installed vDos on my new computer (the May 5, 2019 version) and after adjusting the config.txt files to handle some new and changed parameters I began running my dBase 4 programs. Unfortunately, the new version of vDos no longer supports the mouse even though I have MOUSE = ON specified. I found an installation file on my old PC for the 2015 version of vDos, which always worked well with my dBase programs, reinstalled it, readjusted the config.txt files, and now the mouse works fine. Apparently, something has changed between the 2015 and 2019 versions of vDos that makes the mouse fail for dBase 4.
dBase 4 frequently calls Int 16h, the BIOS keyboard interface, sometimes while the interrupt enable flag is cleared. A dBase4 mishap: Interrupt handling by the CPU should only be disabled in specific situations and for a short time. vDos 2019.05.01 traps some interrupts, bypassing the halt/exit of the emulated CPU to handle the interrupt. But it didn’t at first consider interrupts could be disabled, while an actual interrupt enables interrupts once completed.