I think you are confusing the IOFilterInterruptEventSource FIlter and Action routines. Specifically the Action context is just the work loop. In general while you are on the work loop context you can do anything you want, except block for 'too' long. The Filter is called on the primary hardware interrupt context. Very little of the system is available at this time. The IOLog doc says that you can't run at interrupt time, the sub text is that you can't call it at hardware interrupt time, or in IOKit terms at Filter time. Does this help? Godfrey At 15:34 -0500 01-11-12, Chris Edgington wrote: The docs for IOLog say "IOLog should not be called from interrupt context." However, it appears that Apple breaks this rule - in AppleUSBOHCI_Interrupts.cpp they definitely call IOLog from PollInterrupts - which is called from InterruptHandler. Am I missing something or is the doc for IOLog not right? I'm porting some OS9 code to OSX. We've used DCON for debug output in the past, I'm trying to determine of IOLog will meet our needs as a replacement for DCON on OSX.