We have already seen the components of CPU as well as the connections. i.e. communication pathways which are categorized as different buses. We have already learnt about the instruction and execution cycles. The instruction cycle is composed of two distinct steps called fetch and decode. Let us now see the cycle of operations executed by CPU, which are as follows:
1. CPU points to the first instruction in the memory.
2. Fetches the current instruction from the memory.
3. Determining the type of the fetched instruction.
4. IF any data is required for the instruction is checked for and if so, from which memory location.
5. Fetching the data if required from the memory.
6. Executing the current instruction that is fetched, into the memory.
7. Evaluate results and give the output.
8. Check for following instructions, if found then the repeat above steps from 2nd step.
9. If the instructions to be executed are completed then stop.
For all these above steps if hardly takes fraction of seconds which one cannot imagine, which is as less as few milliseconds, to an extent of about 100 milliseconds.
Define Computer Registers
These are temporary storage locations within CPU. These are very special as they are pretty fast in accepting, storing and transferring data and instructions, which are to be used currently. Because of this capacity of registers, the control unit of CPU retrieves the instruction and data from main memory and it is transferred to the registers. The registers play a vital role in increasing the processing speeds of the CPU, as the size and capacity as well as the number of registers in a CPU can affect the processing power. The bigger the size of the registers the more would be speed of processing. It usually comes in capacity like 8 bits, 16 bits, 32 bits, 64 bits, 128 bits, 256 bits etc. The 256 bit register is faster than the rest due to its bigger size.
From the point of view of “stored program computers”, the concept developed by J.V. Neumann as we have already seen. CPU structure can be seen as composed following components:
A) Instruction Interpreter
i) Memory Address Register (MAR)
ii) Memory Buffer Register and
iii) Memory Controller, to interact with the memory.
a) Instruction Interpreter: This is a combination of many electric circuits which perform the interpretation of the instructions that are fetched from the memory by the fetched step of instruction cycle.
b) Instruction Register (IR): It is that location where the copy of the instruction which is being executed is stored. This instruction is the current instruction which is being interpreted by the instruction interpreter.
c) Instruction Counter (IC): This is also known as Program Counter (PC) or Location Counter (LC). This is the memory device which keeps track of the location of the current instruction which is being processed.
d) Working Register (WR): These are these registers which as used as temporary storage buffers which are used for various calculations and other processing activities, by the instruction interpreter while interpreting an instruction.
e) General Register (GR): These are the extra storage locations used by the user or programmer to store instructions as well as special functions which are required by the program being executed.
i) Memory Address Register (MAR): This is that hardware which is used to store the addresses of the memory locations of the instructions which are to be read as well as the addresses of t he memory locations where the instructions or data is to be stored into.
ii) Memory Buffer Register (MBR): This is a hardware storage device where a copy of the memory location specified by the memory address register (MAR), after a read operation or the contents of the memory location prior to write operation.
iii) Memory Controller: This is that hardware, which is used for transferring data between the Memory Buffer Register (MBR) and the memory location, whose address is stored and specified in the Memory Address Register (MAR).
Simplified Instructional Computers (SIC)
As most of the system Point of sale of software depends on the computer hardware, unlike application programs. The system programs are basically intended to make the computer work whereas the application programs are concerned with tasks and they use computer to get the task done, therefore they do not depend on Hardware.
As the real computers come from different organizations, we need to know about each hardware, if we want to understand the system software and its working. It is quite a difficult task unsderstanding the configuration of each distinct is quite a difficult task for understanding the configuration of each distnict hardware. For this reason we build a hypothetical computer with the combination of general hardware features, so that we can unders
tand the system software. Simplified Instructional Computer (SIC) is such a hypothetical computer which we use to discuss the software. SIC includes the common hardware featurs which are found on most of the real computers, so that one can really eleimniate the differences in hardware so that we can understand the working of various system software.
This computer has been designed for illustrating various hardware features and concepts, which aer found in most of the computers, SIC is quite similar to the Typical Micro Computers. SIC usually comes in two versions, as we need to be compatible for extended versions. The following are the two versions of SIC: –
a) SIC standard version
b) SIC-XE, the extra equipment
These two different versions are used so that they are closely related to the real time computers. Let in: look into structure of SIC machines. We look into following aspects of the machine structure; –
a) Memory: Memory of SIC machines consist of 8-bit, I.e. byte. For representation a word it takes three addresses. Each word is addresses by the location of the lost numbered byte. There are about 32, 768 bytes in the memory of the computer.
b) Registers: The registers used in this machine are 24 bits. The registers are of five different categories, which are used for different specific purposes. The registers are addressed by numbering schemes and names i.e., Mnemonics. The numbers are chosen so that they are compatible with the XE version of the SIC.