14.1 A CLOSER LOOK AT CENTRAL PROCESSING UNIT

Transcription

1 FORM 5 COMPUTER STUDIES Chapter 14: The Central Processing Unit March, A CLOSER LOOK AT CENTRAL PROCESSING UNIT Still remember what CPU is? If not, take a look of the following: CPU is a device to accept and execute the instructions stored in RAM; CPU is built on a single integrated circuit chip called microprocessor; CPU contains a Control Unit (CU), Arithmetic and Logic unit (ALU) and registers; Information (data, address and control signals) is transmitted via various buses; A register ( 寄存器 )is a special memory cell used by the CPU for temporarily storing data during the execution of an instruction Control Unit (CU) controls the overall operations of the computer controls access to the main store controls the sequence in which instructions are executed regulates the timing of all operations carried out within the CPU sends and receives control signals to and from peripheral devices consists of the program counter (PC), instruction register (IR), instruction der and processor status register Program Counter (PC) Register (IR) Der Processor Status Register (SR) is a register that holes the address of the next instruction to be fetched from the main store hold the current instruction fetched from the main store is a logic circuit which interprets the instruction currently hold in the instruction register holds important information (processor status) about the operations done by the ALU Condition flag A flag is set to 1... Zero bit (Z) if the result is zero Negative bit (N) if the result is negative Overflow bit (O) if the result is an overflow Carry bit (C) if there is a carry over of the most significant 1

2 Arithmetic and Logic Unit (ALU) responsible for the manipulation (calculation) of data; consists of an accumulator (ACC) and a set of logic circuits( 邏輯電路 ) Accumulator (ACC) Set of logic circuits 邏輯電路 ) a register storing intermediate and final results of calculations. It is the main working area of the ALU. used for doing computations 14.2 MEMORY UNIT consists of the main store (memory chips), the memory data register (MDR), the memory address register (MAR) and the address der. Main Store (Memory Chips) Memory Data Register (MDR) often called random access memory (RAM) holds the data word when it is transferred to and from the main memory Memory Address Register (MAR) holds the address of the data word to be accessed Address Der interprets the address in the MAR and selects the cell in the appropriate memory chip to be accessed 2

3 14.3 THE MOD'S REGISTER LAYOUT Logic circuits Accumulator ALU Processor Status Register External Bus Prgram Counter (PC) register (IR) der Memory address register (MAR) Address Der Control Unit Main memory chips Memory Unit Memory data register (MDR) Input / Output Port 3

4 14.4 THE INSTRUCTION CYCLE CPU normally carries out the instructions in the main memory sequentially. The sequence of actions required to carry out one machine instruction is called instruction cycle. an instruction cycle consists of three stages : Fetch( 取 ), De( 解碼 )and Execute ( 執行 ). Address of instruction sent to memory received from memory and placed in IR Program counter incremented by one in IR de carried out Fetch De Execute Fetch instruction from memory to IR Increment PC Execute the instruction De the instruction Cycle 14.5 MACHINE CODES a computer can only understand one language - the binary machine ; a machine instruction is an instruction which does one single elementary operation; machine language is a program (instruction set) written in form of machine instructions Basic Format of a Machine consists of two parts : Operation (op ) 操作碼 specifies what operation is to be carried out by the computer; Operand 操作數 is the data or address of memory location that contains tha data to be used for the operation. Model computer (MOD) uses one byte (8 bits) to store each machine instruction 4

5 op operand MOD's Set operation of each instruction is given an abbreviation called an mnemonic instruction ( 助記碼 ): Op Description Operation 0000 CLR Clear the contents of the memory 0001 INP Input a data value into the memory 0010 OUT Output the contents of the memory 0011 LOA load the contents of the memory into the accumulator 0100 STO Store the contents of the accumulator in the location indicated by operand 0101 ADD Add the contents of the memory to the contents of the accumulator, and leave the answer in the accumulator 0110 SUB Subtract the contents of the memory from the contents of the accumulator, and leave the answer in the accumulator (operand) 0 (operand) input port Output port (operand) (ACC) (operand) (operand) (ACC) (ACC) (ACC) + (operand) (ACC) (ACC) - (operand) 0111 JMP Unconditional jump to the instruction 1000 JPN Jump to the instruction location indicated by the operand if the contents of the accumulator are less than zero 1001 JPZ Jump to the instruction location indicated by the operand if the contents of the accumulator are zero 1111 HAL Halt (stop) the execution of the program 5

6 Composing a Machine Language Program e.g. Below is a program to input two numbers and output them in ascending order Address (denary) Machine Operand (denary) Operation INP 14 Input the numbers and store INP 15 them in addresses 14 and LOA 14 Subtract the number in address SUB from that in address 14, and JPN 8 jump to instruction at address 8 if the result is negative OUT OUT HAL 0 Operand filled with 0s to complete the instruction OUT OUT HAL 0 Operand filled with 0s to complete the instruction 6

7 e.g. Consider the following program written in MOD's machine language : address Machine (a) During program execution, the denary numbers 4, 7, 10, 0 are inputted on at a time. Dry run the program and draw a table to show the contents (in denary) of addresss 1110 and 1111 each time after the instruction at address 1 is executed. Translate the program into a form which is easier to understand. Machine Meaning in address Operand symbol (in denary) Op Operand (in denary) (hexa) CLR 15 F INP 14 E input LOA 14 (ACC) E JPZ 7 Jump to address 7 if (ACC) = ADD 15 (ACC) (ACC) + F STO 15 F (ACC) JMP 1 Jump to address HAL 0 Stop the program Value of input data Contents just after instruction at address 1 is executed Address 1110 Address (b) Briefly describe the purpose of this program. The purpose of the program is to add all inputted numbers until zero is inputted. The sum is stored in address end of chapter 14 7