Text 19. Types of Instructions

Communication between man and machine may be accomplished automatically or manually. In the automatic method a complete program of instruction words and a list of required data words are prepared beforehand. This information is then loaded into the ma­chine which then executes the instructions in proper sequence. The manual system of communication is accomplished by means of switches and controls on the front panel of the machine. Instruc­tion words and data words may be fed individually into the ma­chine; and instruction sequences may be modified. The machine keeps the operator up to date on events within the computer by means of display lights which indicate the informational content of various registers and whether any arithmetic errors or compo­nent malfunction have occured.

In keeping with the general organization of the computer, several different types of instructions are used by man to communicate with the machine.

Most of the actual arithmetic work of the machine is performed by the arithmetic unit. We inform the machine to do these opera­tions by means of arithmetic instructions which include: add, sub­tract, multiply, divide, square root, and shift right. Some of these instructions such as add and subtract refer to addresses in memo­ry where operands are located. Other instructions, such as shift right, do not refer to memory. The first type of instruction re­quires an address portion. The second type does not require an address.

Before operations can be performed by the arithmetic unit, oper­ands must be transferred to it. After the arithmetical unit has per­formed its work, results must be removed from it. Information must also be fed into and out of other registers in the computer. Man instructs the machine to perform these operations by means of trans­fer instructions. Transfer instructions may be of two types: read or write. In the first, a word is read out of memory into another regis­ter in the machine. In the second, a word is taken from another register and written into the memory. The memory location in both cases is given by the address portion of the instruction.

As long as the computer is performing simple arithmetic opera­tions, transfer and arithmetic instructions are sufficient for commu­nication. But the great advantage of the digital computer is that it can follow one sequence of instructions under one set of conditions and another sequence of instructions under another set of condi­tions. An instruction which asks the computer to do this is called a branch instruction. The branch instruction enables man to communi­cate with the computer’s control unit to modify the sequencing of computer instructions.

Notes

to accomplish [a'komplif] — виконувати

to load ['loud] — завантажувати

to modify ['madifai] — видозмінювати

the machine keeps the opera- — Ця машина попереджає onepamo-

tor up to date on events pa про me, що відбувається у ком-

within the comptuter... п’юторі

to indicate [‘indikcit] — вказувати

to perform [pa'foim] — виконувати

to remove [ri'muiv] — усувати

sufficient [sa'fi/ant] — достатній