Processor Level and CPU Level

Processor Level and CPU Level

Processor Level

An image below presents the generic block diagram of a processor system. It represents the interface between the processor, memory, and I/O devices through the bus that we discussed in the section on memory interfacing in.

The internals of a processor are a microcosm of the processor system shown in Figure above. Figure below shows a central processing unit (CPU) acting as the brains of the processor connected to memory and I/O devices through an internal bus within a single chip. The internal bus is much simpler than the bus the processor uses to connect its external devices. There are a number of reasons for this.

First, there are fewer devices to interface with, so the addressing scheme does not need to be that complex. Second, the external bus needs to be able to adapt to many different configurations using components from many different manufacturers. The internal bus will never change for that particular model of processor. Third, the CPU accesses the internal components in a well-defined, synchronized manner allowing for more precise timing logic.

The following is a description of the components of the processor shown in Figure below.

• Central processing unit (CPU) – This is the brain of the processor. The execution of all instructions occurs inside the CPU along with the computation required to determine addressing.

• Internal memory – A small, but extremely quick memory. It is used for any internal computations that need to be done fast without the added overhead of writing to external memory. It is also used for storage by processes that are transparent to the applications, but necessary for the operation of the processor.

• Data buffer – This buffer is a bidirectional device that holds outgoing data until the memory bus is ready for it or incoming data until the CPU is ready for it. This circuitry also provides signal conditioning ensuring the output signals are strong enough and the fragile internal components of the CPU are protected.

• Address latch – This group of latches maintains the address that the processor wishes to exchange data with on the memory bus. It also provides signal conditioning and circuit protection for the CPU.

• I/O ports – These ports represent the device interfaces that have been incorporated into the processor’s hardware.

• Configuration registers – A number of features of the processor are configurable. These registers contain the flags that represent the current configuration of the processor. These registers might also contain addressing information such as which portions of memory are protected and which are not.

CPU Level

If we look at the organization inside the CPU, we see that it in turn is a microcosm of the processor block diagram of image shown above. Figure below presents the organization inside a typical CPU.

• Control unit – Ask anyone who has worked in a large business what middle management does and they might say something like, “Not a darn thing.” Ask them what expertise middle management has and you are likely to get a similar answer.

This of course is not true. Middle management has a very important task: they know what needs to be done, who best can do it, and when it needs to be done. This is the purpose of the control unit. It knows the big picture of what needs to be done, it knows which of the CPU’s components can do it, and it controls the timing to do it.

• Arithmetic logic unit (ALU) – The ALU is a collection of logic circuits designed to perform arithmetic (addition, subtraction, multiplication, and division) and logical operations (not, and, or, and exclusive-or). It’s basically the calculator of the CPU. When an arithmetic or logical operation is required, the values and command are sent to the ALU for processing.

• Instruction decoder – All instructions are stored as binary values. The instruction decoder receives the instruction from memory, interprets the value to see what instruction is to be performed, and tells the ALU and the registers which circuits to energize in order to perform the function.

• Registers – The registers are used to store the data, addresses, and flags that are in use by the CPU.