I can give you a few details.
The tachometers are read with an ASM interrupt on a 500uSec timer. Input ports are read and XORed against their last readings. If any bit has changed, a corresponding variable is incremented.
After 1 second (2000 interrupts), the variables are stored and the process repeats.

A main PBP loop reads the stored variables and calculates the fan RPMs. It also reads the A/Ds, calculates the required PWM values and writes that value directly to the hardware PWM registers, and writes to the LCD and LEDs. Another counter in the main timer interrupt routine paces everything.

Two PBP interrupts read the two USARTS and process input serial data and SNMP packets.

It does quite a few other things as well, but I have a conference call starting in a few minutes!