DRIVER_IRQL_NOT_LESS_OR_EQUAL is a famous Blue Screen of Death error,
often caused by a bad driver somewhere.
The Internet is awash with reports of variants of this error.
This particular variant, L1C63x64.sys,
was an occasional occurrence for me on Windows 7,
but seemingly only while torrenting.
Upgrading to Windows 10 turned the error from occasional to frequent within the first few days, prompting me to finally make a serious effort to track down the cause.
When Windows experiences a BSOD
it leaves a diagnostic file in C:\Windows\Minidump.
The first six characters of the file name are the numerical,
zero-padded month, day, and year,
which is both very close to and very far from properly sortable file names.
Minidumps are binary files and so require special tools to inspect. The Internet is not too helpful in finding out how to read those files. There appears to be an official Microsoft tool, WinDbg, but my CS degree was insufficient to figure out how to successfully install it.
I finally came across BlueScreenView, which immediately gave me all the information I needed, including presenting the suspect in a red line:
L1C63x64.sys L1C63x64.sys+6cb3 fffff801`a4080000 fffff801`a40a3000 0x00023000 0x51e4ee9f 16-07-2013 08:56:31 Qualcomm Atheros Ar81xx series PCI-E Gigabit Ethernet Controller Qualcomm Atheros Ar81xx series PCI-E Gigabit Ethernet Controller 2.1.0.17 Qualcomm Atheros Co., Ltd. C:\WINDOWS\system32\drivers\L1C63x64.sys
Note: the driver version appears to be the currently installed version, not the one implicated in the crash.
Okay, so, a bad ethernet driver.
Trying to update the driver via Windows, it simply reports that the driver is already the latest version.
According to Google we can find Qualcomm Atheros drivers at https://www.atheros.cz/, the "unofficial Qualcomm Atheros drivers website". Uh…
The page for network drivers showed that there were no Windows 10 drivers, but there were 64-bit Windows 8 drivers for my model and that was bound to be better than the Windows 7 version. Clicking the relevant link took me to another page that listed all the driver versions, which also told me that a more recent version number was available. Unfortunately, clicking the download link, for any of the drivers, consistently led to a 404. The unofficial Qualcomm Atheros drivers website was no more use past this point.
I ultimately managed to find the above driver on Driver Scape and Windows installed it without complaint. For the entire next day, not a single crash happened and I thought my troubles solved.
The day after that, exactly the same error manifested and I was a sad panda.
The problem is not solved, however, the driver update has helped a lot.