=> R0 = 2 (&02)
R1 = stream selection
<= R0 preserved
R1 = value before being overwritten
R2 corrupted
This call select the device from which all subsequent input is taken by OS_ReadC. Note that on exit only 0 or 1 can be returned, so 0 may also mean 2. The state of this variable can be read with OS_Byte 177. If Exec is enabled (see OS_Byte 198) then the Exec file handle is used as the current input stream.
See PRM 1, page 854
Input streams
Value Meaning
0 Keyboard input with serial input buffer disabled
1 Serial input
2 Keyboard input with serial input buffer enabled
The difference between 0 and 2 is that the latter allows characters to be received into the serial buffer under interrupts as the same time as the keyboard is being used as the primary input. If the input streams are subsequently switched to the serial device, then those characters can then be read.
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