The IDA Pro Book, 2nd Edition

Alternative Calling Conventions

In Chapter 6, we discussed the most common calling conventions utilized in C and C++ code. While adherence to a published calling convention is crucial when attempting to interface one compiled module to another, nothing prohibits the use of custom calling conventions by functions within a single module. This is commonly seen in highly optimized functions that are not designed to be called from outside the module in which they reside.

The following code represents the first four lines of a function that uses a nonstandard calling convention:

.text:000158AC sub_158AC       proc near
  .text:000158AC
 .text:000158AC arg_0 = dword ...