external status words and data words returned (input) from the channel. The starting address of the chain (and other  data)  is  provided  by  the  load  control  memory command. The chain’s starting address is stored in the channel’s chain  address  pointer  portion of I/O control memory. The contents of the I/O control memory are used by the IOC to control all channel operations including execution of chaining instructions. TOPIC  3—INPUT/OUTPUT OPERATIONS Input/output operations are initiated by the CPU. Computers with an IOC will begin I/O control functions only after an initiate I/O or equivalent instruction is executed by the CPU. I/O operations under the control of   the   computer   program   control   the   external equipment. Computer instructions inform the external equipment which type of operations to perform with function codes. Computer instructions also specify memory  areas  for  input  and  output  information. Input/output  operations  do  not  accept  data  from external  equipments  or  send  information  to  them  unless memory areas for the data have been specified by the computer  programs.  Whenever  an  external  equipment is ready to send or receive data, a request signal is sent to  the  computer.  How  the  I/O  section  notifies  or interrupts  the  control  section  that  an  external  equipment is  ready  to  send  or  receive  information/data  depends  on the type of computer. Some constants in all I/O operations include the following: When the transfer will begin, How many words or bytes will be transferred, Word or byte size, When each individual word or byte is actually transferred, and, When the transfer will terminate. I/O operations require circuitry that must take action in a specific sequence of events to communicate with the external equipment. In I/O operations, we examine  operating  modes,  I/O  circuits,  and  I/O functions. OPERATING MODES Similar to the CPU, some computers have the capability to select operating modes. These options are usually found with computers that have an IOC. They can  be  found  on  the  computer’s  controlling  device, usually a maintenance panel or some equivalent. You can  use  this  option  for  troubleshooting  purposes. Consult the operator’s section of your computer’s technical  manual.  As  far  as  the  operating  modes  for  I/O operations, these options are usually established at the factory. Again, they usually apply to computers that have an IOC. Some of the operating modes for I/O operations include the following: Single-channel   —The  single-channel  operating mode  allows  external  equipments  to  communicate  with the computer via one input/output channel. Dual-channel  —The dual-channel operating mode is used by computers with smaller word sires, say 16 bits, to communicate with external devices using a larger word size (30 or 32 bits). In a dual-channel mode, the data lines for two channels are combined under control   of   the   lower   order   channel.   A   pair   of sequentially numbered channels (0 and 1, 2 and 3, and so forth) is used for dual-channel operations. The even numbered  channel  provides  the  control  signals  and lower half or lower order data bits. The odd numbered channel provides the upper half or upper order data bits only.  The  exchange  of  information  over  the  dual channel is controlled by the even numbered channel’s interface signals. Dual channels may use the computer peripheral or intercomputer channel signals. Externally  specified  address  (ESA)  —The externally  specified  address  mode  provides  the  external devices with a means of specifying an absolute memory location  for  storage  (write)  or  retrieval  (read)  of information on a word-by-word basis. Externally   specified   index   (ESI)  —The externally specified index mode is identical to regular transfers (input, output, external interrupt, and external function) except that the IOC requires the external device to specify an index address in main memory. Intercomputer channel (IC) —The intercomputer channel  mode  permits  communication  between  two CPUs. In this mode, each computer appears as an external  device  to  the  other.  During  operations,  the computer that is outputting the data is defined as the sending computer. The computer that is receiving the data from the sending computer is defined as the receiving   computer. I/O CIRCUITS In chapter 4, we discussed the circuits used by computers. We also discussed some of the same circuit types in the CPU and memory sections. I/O is no 7-11