Technical information

NewBIOS Reference Serial Solutions
The same timing loop is used as for service 1H.
RTS and DTR are left asserted, and as a side effect loop,
out1 and out2 (the other bits in the modem control register) are
left reset. This is important because IBM-style serial cards (ie
almost all) use out2 to gate interrupts off and on. Resetting out2
prevents interrupts from the 16450 reaching the host processor
bus. This is fine for the BIOS routines which do not use
interrupts, but NewCOM, BASIC and any sensible serial
communications software, do. Use of this BIOS service
alongside these is therefore not recommended.
Chapter 5 Page 81