An Operating Systems Vade Mecum

156 Transput Chapter 52 THE DEVICE INTERFACEThe speed and complexity of a device determine how it is connected to the central processingunit (cpu). Slow devices, like terminals, usually have a simple, computationintensiveconnection, whereas faster devices, like magnetic tape drives, could not transfereffectively if computation must be involved. Simple devices can be given commands andreturn results in just a few bytes. Complex devices may need more information beforethey are ready to execute a command. We will examine three styles of connection, rangingfrom the simplest to the most complex.2.1 Device registersDevices may be connected to the computer’s cpu by device registers, which are accessibleeither directly as part of physical store or indirectly via transput instructions providedby the hardware. Figure 5.4 shows a simple view of a device connected by registers tothe computer. Device registers are used for four purposes.To transfer status information from the device to the cpuTo transfer instructions from the cpu to a deviceTo transfer data from the cpu to a deviceTo transfer data from a device to the cpu.processeskerneldevice driverstatusinstructionsdatainterruptsdevice interfacedeviceFigure 5.4 Device connected by registers