Due to limitations with the 6502 architecture at the time, opening an I/O channel requires three system calls. The first typically sets the logical filename through the codice_3 system call. The second call, codice_4, establishes the GPIB/IEEE-488 "device" address to communicate with. Finally codice_5 is called to perform the actual transaction. The application then used codice_6 and codice_7 system calls to set the application's current input and output channels, respectively. Applications may have any number of concurrently open files (up to some system-dependent limit; e.g., the C64 allows for ten files to be opened at once). Thereafter, codice_8 and codice_1 prove useful for actually conducting input and output, respectively. codice_10 then closes a channel.