The microprocessor performs two basic tasks during its normal sequence of operation: (1) it fetches instructions from the memory and (2) it executes the instructions that it fetches. This sequence of events is performed endlessly as the microprocessor executes a program.
An instruction is fetched from the memory when the microprocessor sends out the address of the instruction, on its address bus, and issues a memory read signal (R/#W = 1). INstructions in the 680XX microprocessor are always stored in multiples of 2, 3, 6, 8, or 10 bytes in length.
To keep track of the program and the location of the next instruction in the program, the microprocessor uses a program counter (PC). The program counter is a register that counts up through the memory so that the next sequential instruction can be located at the next higher memory location.
In the 680XX microprocessor, the program counter is 32 bits in width. Not all versions of the 680XX microprocessor use all 32 bits of the program counter. The 68008 uses 20 or 22 bits, and the 68000 and 68010 use 24 bits; only the 68020, 68030, and 68040 use the full 32 bits of the program counter. Once the instruction has been removed from the memory and placed into an internal instruction register, the microprocessor decodes the instruction and enters the execution phase of its operation.
The microprocessor performs three main tasks as it executes instructions: (1) data transfer, (2) arithmetic and logic, and (3) decisions. Data transfers, which include fetching instructions, encompass most of the microprocessor's operation. Data transfers occur between the microprocessor and the memory, between the microprocessor and I/O, and between internal registers of the microprocessor.
Srithmetic and logic operations are performed on data in the memory or in the internal registers. The 680XX series of microprocessors can accomplish arithmetic and logic operations on 8-, 16-, and 32-bit data. Some of the major arithmetic and logic instructions are listed in Table 1-3 (not shown in this excerpt). Notice that the 680XX is capable of simple arithmetic and logic operations, If more complicated operations (sine, cosine, etc.) are required, a program must be written to accomplish them. In a system that intensly relies on complicated arithmetic, a floating-point coprocessor is often added to accomplish arithmetic rapidly, without the need to write software procedures. The floating-point numeric coprocessor performs these tasks at up to 100 times faster than software.
Decisions, the last type of operation described here, are probably one of the most inportant operations performed by the microprocessor. Two things make the computer system imporant and powerful in today's society: (1) its ability to execute a program stored in the memory at a high rate of speed and (2) its ability to make decisions. Without either of these two elements, the computer would be no better than a simple four-function calculator, which relies on the operator's memory to store the program and also to make decisions.