=> R0 = 3 (&03)
R1 = bit mask for output streams
<= R0 preserved
R1 = value before being overwritten
R2 corrupted
This call select the device(s) to which all subsequent output from OS_WriteC etc. is sent. The state of this variable can be read and written with OS_Byte 236.
Enabling the serial device and printer with this OS_Byte and setting the printer to the serial device with OS_Byte 5 causes characters to be placed in both serial and printer buffers, which both are then fed to the serial port.
See PRM 1, page 506
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