8253 cheat sheet:

8253 programmable interval timer (PIT) registers:
I/O address	register
-----------	--------
0x40		read/write channel 0 MSB/LSB
0x41		read/write channel 1 MSB/LSB
0x42		read/write channel 2 MSB/LSB
0x43            write-only mode byte

bits
in
mode
byte    description
----    -----------
b7:b6   Select Channel
SC1:SC0         00=channel 0
                01=channel 1
		10=channel 2
                11=illegal
b5:b4   Read/Load
RL1:RL0         00=latch the count of selected channel for later read
                01=read/load one byte; LSB only
                10=read/load one byte; MSB only
                11=read/load two bytes; LSB then MSB
b3:b1   Mode
M2:M0           000=interrupt on terminal count
                001=programmable one-shot (pulse generator)
                010=rate generator (divide-by-N, pulse output)
                011=square wave generator (divide-by-N)
                100=software-triggered strobe
                101=hardware-triggered strobe
                110=illegal ?
                111=illegal ?
b0      Binary Coded Decimal
BCD            0=count 0-255 or 0-65535
               1=count 0-99 or 0-9999
