1. A data processing system for coupling execution of channel programs which control data transfer between input/output devices, the data processing system comprising: a buffer; means for generating a pair of channel programs having a plurality of channel command words with operands; channel subsystem means connected to the input/output devices and to the main storage for executing the channel programs to transfer data between input/output devices and the buffer; the channel programs having corresponding segments referencing the buffer; means for issuing start subchannel instructions to initiate execution of the pair of channel programs; means responsive to a write channel command from a channel program becoming current for determining if the buffer has been filled by a read operation from another channel program; means responsive to determination that the buffer has not been filled for suspending execution of the channel program having the current write channel command; means responsive to filling of the buffer for issuing a resume instruction to a subchannel; and wherein the means for suspending execution includes: suspend channel command words in a channel program; and means for tracking the number of resume commands issued from one channel program less the number of suspend commands encountered for another channel program for indicating if the buffer area is filled.