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The 8254 programmable interval timer uses four registers at addresses
BASE+12, BASE+13, BASE+14 and BASE+15. Register functions
are listed below:
Register
BASE+12
BASE+13
BASE+14
BASE+15
Since the 8254 counter uses a 16-bit structure, each section of read/
write data is split into a least significant byte (LSB) and most
significant byte (MSB). To avoid errors it is important that you make
read/write operations in pairs and keep track of the byte order.
The data format for the control register appears below:
BASE+15 - 8254 control, standard mode
Bit
D7
D6
Value SC1
SC0
SC1 & SC0
Select counter
Counter
0
1
2
Read-back command
Function
Counter 0 read/write
Counter 1 read/write
Counter 2 read/write
Counter control word
D5
D4
RW1
RW0
SC1
0
0
1
1
D3
D2
M2
M1
SC0
0
1
0
1
Chapter 7 Programmable Pacer
D1
D0
M0
BCD
45
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