For what complier/C library? malloc/free is not provided by any OS I'm aware of. Instead your C library (which might come with the compiler, might be included) provides it. In some Unix systems (and I suspect Windows as well), the library asks the OS for more memory than you requested, and then holds that in a buffer. If you malloc more, it will give you memroy from that same buffer if there is room. When you free the memory it marks it unused, but doesn't return it to the OS, instead it will hold onto it in case you need more memory latter. On many systems the OS will only allow you to allocate 4Kb chunks (this varies depending on both the OS and the design of the hardware). The library is designed so that you can malloc a few bytes, without getting a full 4kb. Also, in many cases the library will write something to most of that memory when it is allocated. The OS however may just mark your application as allowed to use that much memory, but not actually give it to you, until you use it. In effect it is optimized for small accesses, and you are sending it a large acccess which it isn't designed for. If you don't need portability you might be able to speed things up by using OS specific functions to get the memory from the OS yourself. Note that none of this will help if you don't have enough physical memory. If you want to use 256M of memory you should have at least 400M of physical memory so the OS doesn't have page everything else out.