A formal description of SYSTEM/360

ciated with a MW of a matrix V. Control unit u, desiring servicefor one of its attached devices, sets Vi = 1 and at the same timeindicates the channel wanted by holding the channel number inVy,. If V;4 = 1 and Vt;, = c, this clearly represents a call forchannel c, and all such possibilities are monitored by the orover the conjunction. The numbers available to control unit ufor Vy, must, of course, be restricted to those of the channelson whose interfaces the control unit is indeed connected.'In response to request-in the channel attempts to establisha working connection with a device by setting select-out (PG)to 1 with the tags a"/ U set to zeros (line 39). Control units areconnected serially on the interface with respect to select-out.Only the first control unit will sense Pi, and it will pass the signalon (setting PE = 1 and Pg = 0) if it does not require service.Successive control units will pass the signal on in a similar wayuntil either a control unit responds with address-in or the lastcontrol unit, passing the signal on in turn, sets select-in (P&)to 1. A tag other than address-in, or wrong parity on an incomingaddress, will cause an interface control check (line 41).For a multiplexor channel, two major typesservice from a device are possible:of request for1. from a device still in the data transfer state (Sg was 1 lasttime it disconnected at line 61) ;2. a) from a device for which a termination status byte wasstacked,b) from a device for which device end is due, orc) from a device presenting an externally generated signal suchas attention or a change from not-ready to ready (whichwill be indicated as device-end) .For a selector channel, only types 2b and 2c are possible.A ndtiplexor channel always honors requests of the first type,but attempts to suppress others if the illterruption buffer (ax//"and os/ Bo) is loaded, as indicated by B: = 1. This is shown inis set to the value of Bg.lines 36 and 39 where suppress-out ( q)When e = I, a control unit must eithersuppress requests of'the second kind by not activating Vl and Vr2 or else pass alongselect-out when the channel tries to establish a connection underthese circumstances. On a selector channel, BE is zero during theidle phase and all requests are, therefore, honored.If a connection is established (Q becomes 0 and lJi becomes 1,lines 40, 42), a lnultiplexor channel will immediately set selectoutto zero (line 44), giving the devicc control over the connection.Operational-in (q) will have become 1 with U;, andnow the channel (always passing through lines 59 and 61 beforereturning to idle) will be constrained to hold the connection aslong as 17: = 1.Line 44 has no effect on a selector channel, which simply continuesby placing the device address in the working address228 A. D. FALKOFF, K. E. IVERSON, AND E. H. SUSSENGUTH