FD 0 is the standard input , and is open for reading. FD 1 and FD 2 are the standard output and standard error output files, respectively, and are open for writing. Normally all three files refer to the controlling terminal, but the redirection syntax of the shell can be used to associate these descriptors with arbitrary files. Parameters to these calls are a file descriptor, the address of a buffer area in the program's memory, and the number of bytes to be transferred. Both calls return the number of bytes actually transferred, which is never greater than the number requested.
End of file is indicated by a transfer count of zero.
This is done through a parameter block . The block for read and write consists of four longwords. This section briefly describes those system calls supported under the vsim simulator. Section 2 of the UNIX Programmer's Manual should be consulted for details on these calls. Terminates execution of the program.