=> R0 = 156 (&9C)
R1 = EOR value
R2 = AND value
<= R0 = preserved
R1 = state before being overwritten
R2 = keyboard disable flag (see OS_Byte 201)
This call reads or writes the serial port state. It is provided for backwards compatibility - use OS_SerialOp 0 and 1 instead, and OS_Byte 2 to change the receive interrupt/control bit.
When writing the effects depend on R1:
Bits Value Effect
0-1 0-2 No effect
3 Reset transmit, receive, control registers
2-4 0 7 bit, even parity, 2 stop bits (7E2)
1 7 bit, odd parity, 2 stop bits (7O2)
2 7 bit, even parity, 1 stop bits (7E1)
3 7 bit, odd parity, 1 stop bit (7O1)
4 8 bit, no parity, 2 stop bits (8N2)
5 8 bit, no parity, 1 stop bits (8N1)
6 8 bit, even parity, 1 stop bits (8E1)
7 8 bit, odd parity, 1 stop bit (8O1)
5-6 0 RTS low, transmit interrupt disabled
1 RTS low, transmit interrupt enabled
2 RTS high, transmit interrupt disabled
3 RTS low, transmit interrupt disabled, break level on transmit data
7 0 Receive interrupt disabled
1 Receive interrupt enabled
You should only write bit 7 with this call (and really should be using OS_Byte 2). The value can also be read with OS_Byte 192.
The default value for bits 2-4 is stored in CMOS byte 16.
See PRM 2, page 446
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