effect on the shellcode execution. While this method greatly improves the chances that an attack will be successful, it is not without problems. Exploits using this technique still must rely on some amount of luck that they will guess offsets on the stack that are within the NOP-sled region. An incorrect guess will usually result in the target program crashing and could alert the system administrator to the attacker's activities. Another problem is that the NOP-sled requires a much larger amount of memory in which to hold a NOP-sled large enough to be of any use. This can be a problem when the allocated size of the affected buffer is too small and the current depth of the stack is shallow (i.e., there is not much space from the end of the current stack frame to the start of the stack). Despite its problems, the NOP-sled is often the only method that will work for a given platform, environment, or situation, and as such it is still an important technique. The entropy of a NOP slide is dependent upon the constraints placed on it. If it can be determined that certain registers are not in use (that is to say, they will be set to a known value before their next use), instructions which manipulate them arbitrarily may be used in the NOP slide. Additionally, if the alignment of both the NOP slide and the instruction pointer are deterministic, multi-byte instructions can be used in a NOP slide without regard to the results of unaligned execution. If the input providing the attack vector into which the NOP slide and payload are to be introduced are filtered (such as accepting only printable characters), the field of possible instructions for inclusion is limited. While instructions that are part of an architecture extension (such as SSE) may frequently be irrelevant to program state, they cannot be used in a NOP slide targeting a computer on which the extension is not supported. Detection Many techniques exist to detect the presence of NOP slides in memory. For example, in 2005, Greek researchers found that they can be easily detected by checking whether a memory image contains a lengthy sequence of bytes such that each starting offset within the sequence is valid and leads execution to the same place. See also Heap spraying, a technique which is complementary to the use of NOP slides Worm memory