The classic CP/M approach was two serial port CON: and AUX:.
They were typically the same type of devices I used 8251As.
THe bis had both and also supported IObyte which was the
IO vector map.
To a program that used that structure was fairly simple:
It was also more portable.
The problem was early CP/M system often had only one serial port
or more often the second port was not supported by the BIOS and
it was required the modem progrram to support it directly (unportable).
Reason for that was IObyte and support fo it was built into the BDOS
and CCP but the BIOS it was often an unsupported as "optional".
The BIOS was an important part of the system most often early ones
were very minimal.? The Yabut it was possible to include into the bios
improved serial? and parallel support and even add system features
there like buffered? background printing.
By the early 80s that was often improved upon and dual serial ports were
common on systems like Kaypro, NS*, AmproLB+, SB180 and many more.
Single board system were usually two serial ports and one parallel
(centronics printer).
Allison
