TOPIC 2—MAN/MACHINE INTERFACES

indirect word addressed by the instruction. In computers with an 8-bit word, no special instruction is needed because each character has its own address. Double-Length Instructions Double-length instructions consist of two adjacent words stored in memory. Multiple-Word Instructions Multiple-word instructions can be used to process two or more sequential words from memory. This concept is commonly used in microcomputers where the instruction word is 16 bits and the memory word size is 8 bits (a byte). In this case two or more sequential bytes from memory are transferred into two or more 16-bit registers for processing; or multiple word store instructions are used to process 16-bit registers into sequential bytes in memory (two bytes for each register). Refer back to figure 8-4 for an example of a multiple-word instruction format. TOPIC 2—MAN/MACHINE INTERFACES To use or maintain a computer, you must be able to control the computer’s operation through some form of a man/machine interface. The man/machine interface is accomplished by the CPU and will vary with the type of computer. However, there are no major differences in the functions performed by the interfaces. You studied the controlling devices in chapter 3. The controlling devices allow you to interface with the computer. The methods are discussed in this topic. The controlling devices used by operator and maintenance personnel vary with different types and generations of computers. In some cases the particular devices used are the same for both general system operation and the more specific maintenance functions. In many cases the man/machine interfaces have evolved from large panels containing many pushbutton/ indicators, and pushbutton/toggle switches, and switches (fig. 8-16) on a maintenance panel to more Figure 8-16.—Pushbutton/indicators, pushbutton/toggle switches, and switches on a maintenance panel. 8-13