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Unformatted text preview: Topic 3 The Central Processing Unit Design 3-1 The Functional Requirements 3-1-1 An Overview of the CPU Functions As everybody knows, the basic computer system includes a central processing unit (CPU), a memory system, and an input/output system. The memory includes a read only memory (ROM) and a random access memory (RAM). The startup programs reside in the ROM, whereas other executable programs and their relevant data reside in the RAM. After the completion of the startup process and loading the operating system to the RAM the CPU executes the programs from the RAM. The program, as defined, is a limited sequence of instructions which performs a certain task or a group of tasks. So, executing the program means executing its instructions. To execute the instruction, the CPU needs to fetch it from the RAM, i.e. reads it from the RAM. We call this the (instruction Fetch) operation. If the instruction needs to operate on data residing in the RAM (Memory Operands), the CPU has to fetch (read) it from the RAM. We call this the (instruction Fetch) operation. The instruction may instruct the CPU to store the result, or other datum in the RAM. In this case the CPU has to writ data to the RAM. Apart from fetching and storing data, and in order to execute the instruction, the CPU must include functional units to understand (decode) and execute the fetched instruction. So, The CPU must include an Instruction Decoder (ID) and an execution unit, basically to execute the arithmetic and logic operations; an Arithmetic Logic Unit (ALU). To keep the instruction for the instruction decoder, the instruction must be latched to an instruction register (IR). The fetched data or the produced results may need to stay in the processor pending other instructions to operate on them. So they need a temporary storage. This means the processor must include a register file to store the data temporarily, whether for operations or for address generation. So, we can conclude by saying that the following functional units must be included in the CPU design (Refer to Figure 3-1 to see the block diagram of the CPU): 1- The Instruction Fetch (IF) Unit: This unit basically provides an access and read facility to the memory location/s that include/s the instruction. This includes providing at least, the address of the memory location that includes the instruction and a read control signal. The address is provided by a unit called the Address Generator (AG) , whereas the read signal is provided by the external control unit which that is associated with the instruction decoder. 2- The Operand Fetch Unit: The function of this unit is to access the data (the operands) in the memory. The access includes reading or writing data. The 1 address generator and the external control unit cooperate to provide the access by providing the memory address of the data plus a read or write signal....
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- Fall '10