Processor's basic blocks
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| basic blocks of processor |
Processor
A processor is the logic circuitry that responds to and processes the basic instructions that drives a computer. The term processor has generally replaced the term central processing unit (CPU). The processor in a personal computer or embedded in small devices is often called a microprocessor.
A central processing unit (CPU) is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logical, control and input/output (I/O) operations specified by the instructions. The term has been used in the computer industry at least since the early 1960s. Traditionally, the term CPU refers to a processor, more specifically to its processing unit and control unit (CU), distinguishing these core elements of a computer from external components such as main memoryand I/O circuitry. The block diagram of basic processor is as shown in the figure1.
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Figure 1 Basic processor
Basic blocks of processor :
ALU
An arithmetic logic unit (ALU) is a digital electronic circuit that performs arithmetic and bitwise logical operations on integer binary numbers. This is in contrast to a floating-point unit(FPU), which operates on floating point numbers. An ALU is a fundamental building block of many types of computing circuits, including the central processing unit (CPU) of computers, FPUs, and graphics processing units (GPUs). A single CPU, FPU or GPU may contain multiple ALUs.
The inputs to an ALU are the data to be operated on, called operands, and a code indicating the operation to be performed; the ALU's output is the result of the performed operation. In many designs, the ALU also exchanges additional information with a status register, which relates to the result of the current or previous operations. The diagram of ALU is as shown in the figure2.
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Figure2 Arithmetic Logic Unit
Control Unit
The control unit (CU) is a component of a computer's central processing unit (CPU) that directs operation of the processor. It tells the computer's memory, arithmetic/logic unit and input and output devices how to respond to a program's instructions.
The Control Unit (CU) is digital circuitry contained within the processor that coordinates the sequence of data movements into, out of, and between a processor's many sub-units. The result of these routed data movements through various digital circuits (sub-units) within the processor produces the manipulated data expected by a software instruction (loaded earlier, likely from memory). The diagram of control unit is as show in the figure3.
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figure3 : control unit
Registers
A register is a quickly accessible location available to a digital processor's central processing unit (CPU). Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-only. Registers are typically addressed by mechanisms other than main memory, but may in some cases be memory mapped.
Almost all computers, whether load-store architecture or not, load data from a larger memory into registers where it is used for arithmetic operations and is manipulated or tested by machine instructions. Manipulated data is then often stored back to main memory, either by the same instruction or a subsequent one. Modern processors use either static or dynamic RAM as main memory, with the latter usually accessed via one or more cache levels. Processor registers are normally at the top of the memory hierarchy, and provide the fastest way to access data. The term normally refers only to the group of registers that are directly encoded as part of an instruction, as defined by the instruction set. The diagram of register unit is as shown in the figure4.
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Figure4. Register Unit
Program Counter
The Program Counter is a register which is also known as PC. PC contains the memory address of the next instruction to be executed. In the beginning PC contains the address of the memory location where the program begins. Here, the program counter is a 32 bit register which needs a 32 bit input and a clock. The output obtained is a 32 bit register. Instructions are usually fetched sequentially from memory, but control transfer instructions change the sequence by placing a new value in the PC. These include branches (sometimes called jumps), subroutine calls, and returns. A transfer that is conditional on the truth of some assertion lets the computer follow a different sequence under different conditions. The diagram of program counter is as shown in the figure5.
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Figure5 Program Counter
Data Memory
The Data Memory Register is also known as Memory Data Register. The Memory Data Register is the register of a computer's control unit that contains the data to be stored in the computer storage (e.g. RAM), or the data after a fetch from the computer storage. It acts like a buffer and holds anything that is copied from the memory ready for the processor to use it.
The MDR is a two-way register. When data is fetched from memory and placed into the MDR, it is written to go in one direction. When there is a write instruction, the data to be written is placed into the MDR from another CPU register, which then puts the data into memory.
The Data Memory Register is half of a minimal interface between a microprogram and computer storage, the other half is a memory address register .
The Memory Data Register (MDR) contains the data value being fetched or stored. It is a common mistake to say that the MDR should be W bits wide, where W is the cell size. However, on most computers the cell size is only 8-bits, and most data values occupy multiple cells. Thus, the size of the MDR is usually a multiple of 8 bits. Typical values of MDR width are 32 and 64 bits, which would allow us to fetch, in a single step, either an integer or a float value.
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