A computer system having a kernel for mapping virtual memory address space to physical memory address space. The computer system uses a method for performing an input/output operation. A physical memory buffer is registered with a subsystem, and the physical memory buffer is associated with a first virtual address, a size and a key. The physical memory buffer is dynamically associated with a second virtual address which is different from the first virtual address. As part of an application program an input/output operation is requested regarding the second virtual address. An application table is used to obtain the first virtual address, the key and the size. The first virtual address, the key and the size are supplied to the subsystem. The subsystem uses the first virtual address, the key and the size, to determine the physical memory buffer and performs an input/output operation using the physical memory buffer without intervention of the kernel.