# How to launch a process with CPU affinity set

I was trying to get this internet explorer process launched so that it can use only 4 processors on a 8 core machine. It wasnt as easy as I thought initially.

Finally I figured out an easy way and I am sure it will save a lot of time for others who require to do this.

There are few different methods we can do this.

1. 1st Method
1. System wide we can set the boot.ini setting to use /numproc=4 so that the machine uses only 4 cores irrespecitve of the number of cores available (provided the available cores is more than 4)
2. 2nd Method
1. Launch a command window and run the highligted commands from the next steps
2. Run cd c:\program files (x86)\internet explorer
3. Run start /affinity F iexplore.exe
4. Now you can check the affinity from the task manager OR process explorer.

If you are using start /affinity, you will need to specify the value for affinity which is a hexadecimal value. Here is a simple way of calculating this affinity value.

Here is how the processors will be numbered. This may be different based on the number of nodes. However the calculation remains the same. Please use terms cores/processors interchangeably.

 CPU ID Associated value (n) Formula (2n-1) Affinity in Hex (h) CPU0 1 1 1 CPU1 2 3 3 CPU2 4 7 7 CPU3 8 15 F CPU4 16 31 1F CPU5 32 63 3F CPU6 64 127 7F CPU7 128 255 FF

Based on the above formula, you can run the following command. Replace h with the value in the Affinity column. This will result in using all the CPU’s listed above the specified value including the current.

Example:

If you specify a value of 1F  for affinity, it will use CPU4, CPU3, CPU2, CPU2 and CPU0

If you want to use specific CPU’s, you will need to SUM the associated values and use the corresponding HEX value.

Example:

If you want to run a process on CPU0 & CPU4, you can sum the values

Start /affinity h iexplore.exe

Enjoy launching processes with CPU affinity ðŸ™‚

1. karan Sharma says:

Thanks for sharing this.Is there a way to Start a Application with first set of 4 CPU and Another application with Another set (Remaining 4 cpu's)/

2. khalid says:

this commnad not work on Window xp i have tried this  "Invalid Swithc /affinity Error "

3. Andreas says:

Thanks for this great information. But how do i run only on CPU 6? I need to direct some parallel batch skripts to a specific CPU. For example Batch 1 to CPU 0, Batch 2 to CPU 2, Batch 3 to CPU 4, Batch 4 to CPU 6. I figured out the first ones, but do not get the clue for CPU 6.

4. SanthoshOnline says:

HEX(64) should run on the process on PROC 6. Use HEX(Associated Value) for specific process

5. max says:

how do i run a specific on a specific cpu?

6. Tom Watson says:

Another article with plenty of information on CPU affinity is :-

waynes-world-it.blogspot.co.uk/…/processor-affinity-on-windows-server.html

7. pars says:

is the formula right? should it be 2*n-1

8. Common Developer says:

The formula is right. Two possibilities in a bit (0 or 1) powed to the bit offset. Is a geometric progression and you need an AND operation to sum the numbers. This is called boolean algebra.

9. Balint says:

Hi!

Is there a way to tell the OS that i want this process to run on 2 cores of 4 but i do not want to specify the cores. I don't want to specify the cores because it should be changed based on the core's temperature. Ideally it would rotate between core0&2 and core1&3. I have seen this in action using linux and i am wondering if it can be done in Windows.