Input-Output and Interrupt
A computer can serve no useful purpose unless it communicates with the external environment. Instructions and data stored in memory must come from some input device. Computational results must be transmitted to the user through some output device. Commercial computers include many types of input and output devices. To demonstrate the most basic reqttirements for input and output communication, we will use as an illustration a terminal unit with a keyboard and printer. Input-output organization is dicsussed further in Chap. 11.
The terminal sends and receives serial information. Each quantity of information has eight bits of an alphanumeric code. The serial information from the keyboard is shifted into the input register INPR. The serial information for the printer is stored in the output register OUTR. These two registers communicate with a communication interface serially and with the AC in parallel. The input-output configuration is shown in Fig. 5-12. The transmitter interface receives serial information from the keyboard and transmits it to INPR. The receiver interface receives information from OUTR and sends it to the printer serially. The operation of the serial communication interface is explained in Sec. 11-3.
The input register INPR consists of eight bits and holds an alphanumeric input information. The 1-bit input flag FGI is a control flip-flop. The flag bit is
set to 1 when new information is available in the input device and is cleared to 0 when the information is accepted by the computer. The flag is needed to synchronize the timing rate difference between the input device and the computer. The process of information transfer is as follows. Initially, the input flag FGI is cleared to 0. When a key is struck in the keyboard, an 8-bit alphanumeric code is shifted into INPR and the input flag FGI is set to 1. As long as the flag is set, the information in INPR cannot be changed by striking another key. The computer checks the flag bit; if it is 1, the information from INPR is transferred in parallel into AC and FGI is cleared to 0. Once the flag is cleared, new information can be shifted into INPR by striking another key.