1. The RealTek timer-count
A look at how we could utilize the timekeeping capabilities of our RealTek 8139 network controller

2. The Timer Count Register
32-bits
TCTR
0x48-0x4B
How it works:
This register may be read at any time.
Its value steadily increments, at a fixed frequency.
If any value is written, the register is reset to zero.

3. The Timer Interrupt Register
32-bits
TimerInt
0x54-0x57
How it works:
This register may be read or written at any time.
Its value is initialized to zero by a reset command.
So long as it’s zero, no timer-interrupt is generated.
If any nonzero value is written, the ‘timeout’ flag in
the Interrupt Status register will be set as soon as
the TCTR becomes equal to this TimerInt amount.

4. Registers IMR and ISR 0x3C-0x3D 0x3E-0x3F Interrupt Mask Register
16-bits
Interrupt Mask Register
(read/write)
(Bit: 1=enabled, 0=masked) T i m e O u t
0x3C-0x3D 14
16-bits
Interrupt Status Register
(read/only)*
(bit: 1=active, 0=inactive) T i m e O u t
0x3E-0x3F 14
Writing ‘1’ to an active bit in IntStatus clears the active condition

5. In-Class Exercise
Use our ‘nictimer.c’ device-driver, and also our ‘2seconds.cpp’ program (plus your knowledge of arithmetic), to determine how rapidly the nic’s Timer Count Register will be incrementing
Compute the expiration-value you should write to the TimerInt register in order that a task will be awakened by a timer-interrupt after enough time has elapsed for a maximum-size packet to be transmitted