not modify data or program storage associated with the system state. Addressing, data, or operation exception errors made the machine enter the system state through a controlled routine so the operating system could try to correct or terminate the program in error. Similarly, it could recover certain processor hardware errors through the machine check routines. Channels Peripherals interfaced to the system via channels. A channel is a specialized processor with the instruction set optimized for transferring data between a peripheral and main memory. In modern terms, this could be compared to direct memory access (DMA). The S/360 connects channels to control units with bus and tag cables; IBM eventually replaced these with (Enterprise Systems Connection (ESCON) and Fibre Connection (FICON) channels. Byte-multiplexor and selector channels There were initially two types of channels; byte-multiplexer channels (known at the time simply as "multiplexor channels"), for connecting "slow speed" devices such as card readers and punches, line printers, and communications controllers, and selector channels for connecting high speed devices, such as disk drives, tape drives, data cells and drums. Every System/360 (except for the Model 20, which was not a standard 360) has a byte-multiplexer channel and 1 or more selector channels, though the model 25 has just one channel, which can be either a byte-multiplexor or selector channel. The smaller models (up to the model 50) have integrated channels, while for the larger models (model 65 and above) the channels are large separate units in separate cabinets: the IBM 2870 is the byte-multiplexor channel with up to four selector sub-channels, and the IBM 2860 is up to three selector channels. The byte-multiplexer channel is able to handle I/O to/from several devices simultaneously at the device's highest rated speeds, hence the name, as it multiplexed I/O from those devices onto a single data path to main memory. Devices connected to a byte-multiplexer channel are configured to operate in 1-byte, 2-byte, 4-byte, or "burst" mode. The larger "blocks" of data are used to handle progressively faster devices. For example, a 2501 card reader operating at 600 cards per minute would be in 1-byte mode, while a 1403-N1 printer would be in burst mode. Also, the byte-multiplexer channels on larger models have an optional selector subchannel section that would accommodate tape drives. The byte-multiplexor's channel address was typically "0" and the selector subchannel addresses were from "C0" to "FF." Thus, tape drives on System/360 were