2.1 Module 2: top-level view of computer organization  (Page 6/7)

Figure 9: Sequential interrupt processing

The second approach is to define priorities for interrupts and to allow an in­terrupt of higher priority to cause a lower-priority interrupt handler to be itself interrupted (Figure 10)

Figure 10: Transfer of Control with Multiple Interrupts

3. interconnection structures

A computer consists of a set of components or modules of three basic types (proces­sor, memory, I/O) that communicate with each other. In effect, a computer is a net­work of basic modules. Thus, there must be paths for connecting the modules.

The collection of paths connecting the various modules is called the intercon­nection structure. The design of this structure will depend on the exchanges that must be made between modules.

Figure 11 suggests the types of exchanges that are needed by indicating the major forms of input and output for each module type:

• Memory
• Input/Output
• CPU

The interconnection struc­ture must support the following types of transfers:

• Memory to processor: The processor reads an instruction or a unit of data from memory.
• Processor to memory: The processor writes a unit of data to memory.
• I/O to processor: The processor reads data from an I/O device via an I/O module.
• Processor to I/O: The processor sends data to the I/O device.
• I/O to or from memory: For these two cases, an I/O module is allowed to ex­change data directly with memory, without going through the processor, using direct memory access (DMA).

Over the years, a number of interconnection structures have been tried. By far the most common is the bus and various multiple-bus structures.

Figure 11 Computer Modules

4. bus interconnection

A bus is a communication pathway connecting two or more devices. A key charac­teristic of a bus is that it is a shared transmission medium. Multiple devices connect to the bus, and a signal transmitted by any one device is available for reception by all other devices attached to the bus (broadcast). If two devices transmit during the same time period, their signals will overlap and become garbled. Thus, only one device at a time can successfully transmit.

Typically, a bus consists of multiple communication pathways, or lines. Each line is capable of transmitting signals representing binary 1 and binary 0. Overtime, a sequence of binary digits can be transmitted across a single line. Taken together, several lines of a bus can be used to transmit binary digits simultaneously (in par­allel). For example, an 8-bil unit of data can be transmitted over eight bus lines.

Computer systems contain a number of different buses that provide pathways between components at various levels of the computer system hierarchy. A bus that connects major computer components (processor, memory, I/O) is called a system bus. The most common computer interconnection structures are based on the use of one or more system buses.

Bus structure

A system bus consists, typically, of from about 50 to hundreds of separate lines. Each line is assigned a particular meaning or function. Although there are many dif­ferent bus designs, on any bus the lines can be classified into three functional groups (Figure 12): data, address, and control lines. In addition, there may he power dis­tribution lines that supply power to the attached modules.