Channel Programming

 Before IOS issue a SIO command, it firstly must set up the address of the channel program, a eries of channel command words in a special memory location called channel status word. If the channel responded positively to SIO, the channel start fetching and executing channel programs and frees the CPU to do other words. 

Channel command word contains the command codes such as read or write, the data address n memory from or to which the data is the be transferred and several flags that nodify the execution of the command word. 

Chain Data flag caused tue channel to continue execute the same command with the data address in the next word. This resulted in an effect of dispersed IO which read or write from few buffers. 

Chain command flag caused the channel to execute the next command on the same device. 

Skip flag caused the channel to read the data but not transmit to the memory. This is used to check the data just written (write check). 

SLI suppresssed length indicator is to ask channel to not abend if the IO byte counts differs from the one specified in the command word. This is to handle variable length record. 

TIC transfer in channel flag is the branch instruction in channel progra. CD and CC flag continue with TIC. If the next work is a TIC, the program will amend. If the next word is not CD or CD, it t indicated end of program.