WithoutSystem/360 accessand methods,successor asystems programmerperform mustinput/output writeusing a special program for an [[Channel I/O|I/O channel]], a processor dedicated to control peripheral storage device access and data transfer to and from main memory. These ''channel programs'' are composed of special instructions, called ''channel command words'' (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a '''STARTIO''' macro issued by the operating system. This is usually front ended by the '''Execute Channel Program''' ([[EXCP]]) macro for application programmer convenience. This macro issues an SVC ([[supervisor call instruction]]) that directs the operating system to issue the STARTIO on the application's behalf.<ref>[http://www.redbooks.ibm.com/redbooks/pdfs/sg246983.pdf] ABCs of z/OS System Programming Volume 3</ref>
