Public mainframe system

On 18/04/2013 21:28, Cory Smelosky wrote: I think that was more "Marketing" than anything real. All S/360 i/o is via what is called a "Channel" which is physically implemented as a BUS. However its not directly connected to memory like an ISA or MCA card. Instead the channel has a dedicated programmable interface usually just refered to as a "Channel" but its a really an intelligent controller/cpu which runs something called "channel programs". Its a fairly simple program but it will do scatter/gather i/o, loop, test device status etc. so offloading much of the work in doing i/o away from the main CPU. All a program had to do to perform i/o was construct a channel program that defined what i/o it wanted doing and issue a Sart IO (SIO) command and the channel program does the rest. So for a communications line you can leave a channel program running in an endless loop and it will only interupt when a byte arrives... So whilst MCA had DMA it only had one DMA path to memory and there was no equivalent of a "Channel Program" even though it was called "Micro Channel Architecure".... If I remember properly he S/360 architecture allowed for 8 channels running at 1.5 MBytes/Sec (so 12Mbits), S/370 upped this to 4Mbytes/sec or 32Mbits/sec and allowed more channels. In XA there can be 256 channels... but they are no longer real channels... Each channel has 256 addresses, 00 thru FF. Typically we add the channel number to the start so a S/360 could address up to 2048 (8x256) 000 thru 7FF devices. As logic was expensive typically devices were connected by a controller. Only the controller actually connects directly to the channel via a standard interface called Bus+Tag. The devices connect to the controller, but via some interconnect appropriate to the device. So Tapes and Disks had similar cables to the Bus+Tag, but screens connected via the 93ohm co-ax which is where we started. The original plan was that these were set up as 16 controllers each with up to 16 devices but later on devices such as printers appeared with built controllers and controllers that could support more than 16 devices became common so tweaks were made. There are pictures of the Bus+Tag connectors that connected the cables used to connect a channels controllers here:- http://en.wikipedia.org/wiki/IBM_System/360