IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

Transmitter Holding Register
The Transmitter Holding Register contains the character to be serially
transmitted and is defined below:
Transmitter Holding Register (THR)
r'"'\
3F8 DLAB=O WRITE ONLY
BIT 7 6 5 4 3 2 1 0
I I • DATA BIT 0
L----==; DATA BIT 1
'---------1. DATA BIT 2
'---------.. DATABIT3
'-------------1. DATA BIT 4
L--------------a DATA BIT 5
'---------------____ DATA BIT 6
L-_______________----I~ DATA BIT 7
Bit 0 is the least significant bit and is the first bit serially transmitted.
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