Tandy 1000 HX Technical Reference Manual page 405

Exit Conditions:
AH = RS-232 status, as follows (set = true):
bit 0 = data ready
bit 1 = overrun error
bit 2 = parity error
bit 3 = framing error
bit 4 = break detect
bit 5 = transmitter holding register empty
bit 6 = transmitter shift register empty
bit 7 = timeout occurred
AL = modem status, as follows (set = true):
bit 0 = delta clear to send
bit 1 = delta data set ready
bit 2 = trailing edge ring detector
bit 3 = delta receive line signal detect
bit 4 = clear to send
bit 5 = data set ready
bit 6 = ring indicator
bit 7 = receive line signal detect
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