DESQview and QEMM on DOS was also popular. It wasn't fully multi-taking, but it worked quite well.
* https://en.wikipedia.org/wiki/DESQview
* https://en.wikipedia.org/wiki/QEMM
You also had to have a COM port that was connected to a 16550A UART, often done via a serial add-in card, because it had a whopping 16 byte buffer:
* https://en.wikipedia.org/wiki/16550_UART#The_16550_FIFO
DESQview and QEMM on DOS was also popular. It wasn't fully multi-taking, but it worked quite well.
* https://en.wikipedia.org/wiki/DESQview
* https://en.wikipedia.org/wiki/QEMM
You also had to have a COM port that was connected to a 16550A UART, often done via a serial add-in card, because it had a whopping 16 byte buffer:
* https://en.wikipedia.org/wiki/16550_UART#The_16550_FIFO