TimeStamp in Cache-time attacks on AES
The opcode 15 49 (0F 31 - Intel opcodes are more typically listed in hexidecimal) is the RDTSC instruction; this loads the number of cycles since the last rest into EDX:EAX.

By running this function, running some code, and then running this function again, and subtracting the results, you get precisely how long the code takes (in number of CPU cycles).