- A microprocessor unit is a complex logic element that performs arithmetic, logic, and control operations.

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1 Microprocessors By Salim Reference: Microprocessors, Heathkit Educational Systems - A microprocessor unit is a complex logic element that performs arithmetic, logic, and control operations. - A computer contains a microprocessor and other circuits such as memory devices, interface adapters to connect with the outside world, and a clock to act as a master timer for the system. Clock MPU Bus Memory Memory Bus Interface Adapter Microcomputer I/O Port I/O Devices

2 Computer Words In computer terminology, a word is a group of binary digits that occupy a storage location. Even though the word may be made up of several binary digits, the computer handles each word as if it were a single unit. word is the fundamental unit of information used in the computer. One of the most important characteristics of any microprocessor is the word length it can handle. A word may be binary numbers to be treated as data, an instruction that tells the computer what to do, or an address that tells the computer where a piece of information is located. In an 8-bit word, we can specify positive numbers between (00 16 ) and (FF 16 ) which is 0 and Of course, to represent larger numbers, we have to use two or more words. If the 8 bit word is an instruction, we can specify any of possible operations. More on the word size as it comes up appropriately. 1 byte word is an 8-bit word 16 bit word is two bytes Higher order byte Lower order byte

3 Microprocessor Unit A simple microprocessor unit is shown in greater details below. Only the major registers and circuits are shown. In this simple unit, the counters, registers, and busses are 8 bits wide. Fig.1 Elementary MPU

4 ALU: It performs logic or arithmetic operations. Has two inputs: one comes from the accumulator and one from the data register. It can add/subtract the two inputs or perform a logical operation. The control lines C determine the operation to be performed. The two inputs are called the operands. Figure 2 illustrates the process. Fig. 2 ALU

5 Some definitions: - accumulator: It is the most useful register in the microprocessor. During the arithmetic and logic operations, it performs dual functions. Prior to the operation, it holds one of the operands. It then holds the result after the operation. The accumulator receives several instructions. For example, the load accumulator instruction causes the contents of some memory location to be loaded into the accumulator. Also, the store accumulator instruction stores the contents of the accumulator to some memory location. - Data Register: It is a temporary storage location for data going to or coming from the data bus. For example: it holds instructions while the instruction is being decoded. It also holds a data byte while the data is being stored in memory. - Address register: Temporarily holds the address of the memory location or I/O device that is used in the operation currently being performed. - Program Counter: It controls the sequence in which the instructions in a program are performed. It normally does this by counting the sequence 0,1,2. At any given instance, the count indicates the location in memory from which the next byte of information is to be taken. - Instruction Decoder: After an instruction is pulled from memory and placed in the data register, the instruction is decoded by the circuit. The decoder examines the code and decides what operation is to be performed. - Controller Sequencer: It produces different control signals to carry out the instructions. Different combinations of control signals are produces for different instructions. It also determines the sequence of evens the instructions are carried out.

6 Fetch and Execute Sequence A microprocessor goes through a fundamental sequence of steps when executing a program. A program consists of instructions that tell the microprocessor exactly what to do. The instructions must be stored in an orderly manner in memory. The instructions must be FETCHED one at a time from memory and then the instruction is executed. Start Here Fetch and instruction Execute the instruction THUS: The operation of the microcomputer can be broken into two phases: When a microprocessor is initially started, it enters the fetch phase. During this phase, an instruction is taken from memory and decoded by the MPU. Once the instruction is decoded, the MPU switches to the execute phase. During this phase, the MPU carries out the operation specified by the instruction. Note: the fetch phase always consists of the same series of operations. Thus it always takes the same amount of time. Whereas the execute phase consists of different sequences of events depending on the type of instruction and thus the time for this phase may vary from one instruction to the next.

7 Let us now play with a simple program and see what happens. Fist, a program is placed in memory We have a five byte program The main registers are shown The program counter is set to the memory address of the first instruction. Fig 3. Simple program execution.

8 Fetch Phase Fetch the instruction first. The event is controlled by the controllersequencer. 1- the contents of the program counter are transferred to the address register (this is the address of the first instruction.) Fig. 4. Fetch phase -1

9 2- Once the address is in the address register, the program counter is incremented by one. Fig. 5. Fetch phase -2

10 3- The contents of the address register are now placed on the address bus. The memory circuits decode the address and select the memory location. Fig. 6. Fetch phase -3

11 4- Now, the contents of the memory location are placed on the data bus and are transferred to the data register in the MPU. After this operation, the operational code (opcode) for LDA instruction will be in the data register. Fig.7. Fetch phase 4.

12 5- The next step is to decode the instruction. The opcode is transferred to the instruction decoder. This circuit (instruction decoder) recognizes that the opcode is that of an LDA instruction. It informs the controller sequencer of this fact and the sequencer produces the necessary control signals to carry out the instruction. Fig. 8. Fetch phase 5. Completed for the first instruction

13 Execute Phase Now that the first instruction is fetched and decoded, the MPU knows that this is an LDA instruction. During this phase, the instruction must be carried out by reading the next byte in memory and placing it in the accumulator. 1- The first step is to transfer the address of the next byte from the program counter to the address register. Fig. 9. Execute phase -1

14 2- the program counter is incremented in anticipation of the next step. 3- the contents of the address register are placed on the address bus. Fig. 10. Execute phases 2, 3

15 4- The address is decoded and the memory contents of the specific location are loaded into the data register (this is the number 7) An instant later, the number is transferred to the accumulator. This ends the first execute phase with number 7 loaded into the accumulator Fig. 11. Execute phase -4 The first execute phase just ended.

16 Fetching the add instruction: The next instruction in the program is the ADD instruction. This instruction is fetched from memory using the same steps as earlier. 1- the contents of the program counter are transferred to the address register. 2- the program counter is incremented 3- the address is placed in the address bus 4- the contents of the selected memory location are transferred to the data register 5- the contents of the data register are decoded by the instruction decoder. Fig. 12. Fetching the ADD instruction.

17 The fetched data word from memory is the opcode for the ADD instruction. Thus, the controller-sequencer produces the necessary control signals to execute the instruction. Executing the ADD instruction: 1- the contents of the program counter are transferred to the address register. 2- the program counter is incremented by 1 3- the address of the operand is placed on the address bus 4- the operand (10 10 ) is transferred to the data register 5- the operand is transferred onto one input of the ALU 6- Simultaneously, the other operand (7) is transferred from the accumulator to the other input of the ALU. 7- The ALU adds the two operands. Their sum (`17 10 ) is loaded into the accumulator (destroying the number 7 that was init.)

18 Fig. 13. Executing the ADD instruction The program is not finished until it tells the computer to stop executing instructions.

19 Fetching and Executing the HLT instruction