checks parity, and examines a code to determine the correct sequence. After the sequence is established, the REI broadcasts the requests and the address to all DMIs connected  to  it.  The  signals  on  the  REI  external interface are expanded to guarantee capture at the DMI operating synchronously to the REI, which can be located up to 500 cable-feet away. Once the REI makes a request, it can send write data if it is performing a write operation or wait for a response and pass it to the requestor. The REI responds to the requestor just as memory does, including faults and aborts (terminates a process before it is completed). DIRECT   MEMORY   INTERFACE   (DMI) BUS.— The Direct memory interface is a responder or slave on the REI bus. The DMI bus is used in some computers that use an I, O, and IOC bus. The DMI bus is  used  to  send  requests  from  other  enclosures (computers) to the module (CPU or IOC) requested. It acts as the requestor and makes requests to the CPU. When it requests an IOC, it uses IOC read and write requests.  When  it  requests  memory,  it  uses  operand read or write, instruction read, or replace. Time Multiplexed Bus Another variation of the address and data bus is the time multiplexed bus. This single bus transmits both addresses and data using a four cycle clock (t1, t2, t3, and t4). The address is transmitted during the t1 clock cycle, the direction of data movement is selected during t2, and the data is transmitted during t3 and t4. BUS OPERATIONS The bus control function is performed by a bus interface unit or logic circuitry similar to it. Control of a bus line and the proper protocol of requesting a bus depends on the design of the computer. In computers with no IOC, the CPU has control of the bus lines. In computers with an IOC, the CPU will control the instruction and operand buses and the IOC will control the memory buses. Bus control is necessary to handle the large number of bus transactions that take place in a very short period of time in the computer. There are basically   two   factors   that   must   be   taken   into consideration  in  bus  communications:  transfer priority and source/destination of the data being transferred. Bus transfers are done on a priority basis. The priorities of bus transfers are determined by the design of the computer’s firmware. What part makes the request  is  also  determined  by  the  design  of  the computer’s  firmware;  requests  may  be  made  by  a  CPU, an IOC, and/or a DMI. Examples of priorities that a computer must deal with include the following (these examples are not in any type of priority and do not cover the full range of priorities you may encounter): Transfers  from  memory  to  the  CPU,  these transfers move instructions and operands to the CPU for execution and modification Transfers from the CPU to memory Transfers by the I/O in and out of memory The specific request will identify the source and the destination of the data. The computer’s controlling bus continually and repeatedly checks the bus signal lines for requests. When it receives a request, it provides the control  signals  needed  to  initiate  the  transfer.  Since most  transfers  deal  with  memory,  each  transfer  consists of an address exchange and a separate data exchange. The data will either parallel the address as in a write operation or move in the opposite direction after the data has been read from the memory word identified by the  address. In some computers, the bus systems use holding registers  in both the source and destination sections to prevent data loss and to help coordinate the data exchange. In the source logic, the data is placed in a holding register until it is accepted by the destination logic. The outputs of the holding register feed the bus circuitry. In the destination logic, the bus inputs to a holding  register. After  accepting  the  data,  the destination logic can then move the data from the holding  register  to  other  parts  of  the  logic  for processing. A variety of command signal names are used to coordinate the exchange of data on the buses by both the source and the destination logic. The source logic generates a ready or signal equivalent when the data is in the holding register and on the bus. The destination logic sends an accept or equivalent signal when it has sensed the ready signal and captured the data on the bus in its holding register or other logic circuits. MICROCOMPUTER  ARCHITECTURE AND BUSES The microcomputer has uses four main types of buses. These are the l  Processor  bus l  Address  bus 5-26