Technical information

Serial Solutions NewBIOS Reference
Service 1H, Send Character._________________________
Compatibility._____________
This Service is fully compatible with the PC ROM BIOS.
Parameters.___________
AH = 1H Denotes service 1H
AL = byte Character to send
DX = 0...31 Port number, 0=COM1...31=COM32
Return Values._____________
AH is 16450 Line status or time-out, where
bit 7 = Time-out. If true then remaining bits
are undefined
bit 6 = Transmitter shift register empty
bit 5 = Transmitter holding register empty
bit 4 = Break interrupt
bit 3 = Framing error
bit 2 = Parity error
bit 1 = Overrun error
bit 0 = Data ready
AL is preserved
Meaning._________
Service 1H, send char, sends the character specified in AL
to the port specified in DX. The service performs a handshake
by asserting the RS232 output lines RTS and DTR, then waits
for the CTS and DSR lines to go true. If they do not the routine
fails with a time-out error. If they do go true then the service
waits again for the transmitter buffer empty (TBE) bit in the
16450 to go true, and if it does not fails with a time-out error. If
TBE goes, or is already, true the character to send is written to
the 16450’s transmitter buffer and the service returns.
Page 78 Chapter 5