Technical information

NewBIOS Reference Serial Solutions

Service 5H, Extended Control.___________________________

Compatibility._____________

This service was first used in the ROM BIOS of IBM PS/2

machines and is supported by most modern PCs. The NewBIOS

version is fully compatible with the version documented in the

PS/2 BIOS technical reference.

Parameters.___________

AH = 05H Denotes service 5H

DX = 0...31 Port number, 0=COM1...31=COM32

AL = 00H Denotes Read modem control

AL = 01H Denotes Write modem control reg:-

BL is Modem control register

bits 7,6,5 Reserved

bit 4 = Loop

bit 3 = Out2

bit 2 = Out1

bit 1 = Request To Send (RTS)

bit 0 = Data Terminal Ready(DTR)

Return Values._____________

AH is 16450 Line status, where

bit 7 = 0, always

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 16450 Modem status, where

bit 7 = DCD, Data Carrier Detect

Chapter 5 Page 87