In order to specify the rnicrooperations needed for the execution of each instruction, it is necessary that the function that they are intended to perform be defined precisely. Looking back to Table 5-2, where the instructions are listed, we find that some instructions have an ambiguous description. This is because the explanation of an instruction in words is usually lengthy, and not enough space is available in the table for such a lengthy explanation. We will now show that the function of the memory-reference instructions can be defined precisely by means of register transfer notation.
Table 5-4 lists the seven memory-reference instructions. The decoded output D; for i = 0, 1, 2, 3, 4, 5, and 6 from the operation decoder that belongs to each instruction is included in the table. The effective address of the instruction is in the address register AR and was placed there during timing signal T2 when I = 0, or during timing signal T3 when I = 1. The execution of the memory-reference instructions starts with timing signal T4• The symbolic description of each instruction is specified in the table in terms of register transfer notation. The actual execution of the instruction in the bus system will require a sequence of rnicrooperations. This is because data stored in memory cannot be processed directly. The data must be read from memory to a register where they can be operated on with logic circuits. We now explain the operation of each instruction and list the control functions and rnicrooperations needed for their execution. A flowchart that summarizes all the rnicrooperations is presented at the end of this section.