Scheme's input and output is based on the "port" datatype. (R5RS sec 6.6) R5RS defines two default ports, accessible with the procedures codice_93 and codice_94, which correspond to the Unix notions of standard input and standard output. Most implementations also provide codice_95. Redirection of input and standard output is supported in the standard, by standard procedures such as codice_96 and codice_97. Most implementations provide string ports with similar redirection capabilities, enabling many normal input-output operations to be performed on string buffers instead of files, using procedures described in SRFI 6. The R6RS standard specifies much more sophisticated and capable port procedures and many new types of port.