So, the MemRead and MemWrite control signals are set to 0. These operations along with the required control signals are indicated in Figure 9. We can think of a load instruction as operating in five steps:. A store instruction is similar to the load for the address calculation. It finishes in four steps The control signals that are different from load are:. The MemtoReg field is irrelevant when the RegWrite signal is 0. Since the register is not being written, the value of the data on the register data write port is not used.
The Branch control signal is set to 1. The ALU performs a subtract on the data values read from the register file. The control signals and the data flow for the Branch instruction is shown in Figure 9. Now, to the subset of instructions already discussed, we shall add a jump instruction.
The jump instruction looks somewhat similar to a branch instruction but computes the target PC differently and is not conditional. Like a branch, the low order 2 bits of a jump address are always The next lower 26 bits of this bit address come from the bit immediate field in the instruction, as shown in Figure 9. The upper 4 bits of the address that should replace the PC come from the PC of the jump instruction plus 4. One additional control signal is needed for the additional multiplexor.
This control signal, called Jump , is asserted only when the instruction is a jump—that is, when the opcode is 2. Since we have assumed that all the instructions get executed in one clock cycle, the longest instruction determines the clock period. The single cycle implementation may be acceptable for this simple instruction set, but it is not feasible to vary the period for different instructions, for eg.
Floating point operations. Also, since the clock cycle is equal to the worst case delay, there is no point in improving the common case, which violates the design principle of making the common case fast.
In addition, in this single-cycle implementation, each functional unit can be used only once per clock. Therefore, some functional units must be duplicated, raising the cost of the implementation.
A single-cycle design is inefficient both in its performance and in its hardware cost. These shortcomings can be avoided by using implementation techniques that have a shorter clock cycle—derived from the basic functional unit delays—and that require multiple clock cycles for each instruction.
In the next module, we will look at another implementation technique, called pipelining, that uses a datapath very similar to the single-cycle datapath, but is much more efficient.
Next, we shall briefly discuss another type of control, viz. In the case of hardwired control, we saw how all the control signals required inside the CPU can be generated using hardware.
There is an alternative approach by which the control signals required inside the CPU can be generated. This alternative approach is known as microprogrammed control unit. In microprogrammed control unit, the logic of the control unit is specified by a microprogram. A microprogram consists of a sequence of instructions in a microprogramming language.
For a CPU to execute these instructions, each one must first be translated into machine code — simple binary codes that activate parts of the CPU. The CPU only performs a few basic functions:.
A piece of software, such as a game or web browser , combines these functions to perform more complex tasks. The ALU carries out calculations and makes decisions on the data sent to the processor. So let us start. Before studying more about the instruction let us understand the basic format of instruction.
Below is the figure showing the general format of the instruction The Opcode shown above determines the nature of the operation to be performed. The operands , Operand 1 and operand 2 identify the data on which operation has to be performed. The operand can be a memory location, a processor register, immediate value or logical data.
Now, as we have studied above, the instructions could be of many types. The operation specified in the instruction decides the nature of that instruction. Generalizing all types of instructions, they can be categorized into four types as follows:.
The source location is from where the value is to be read and the destination location is where the value is to be stored. These locations are given a symbolic name for the convenience. The instruction 1 indicates the value in the memory location having symbolic name VAR3 is transferred to the processor register R4. Here, VAR 3 is placed in square bracket [ ], as the square bracket around a location denotes the content or value stored at that location.
Similarly, in instruction 2 the value inside the processor register R3 and R2 is added and then stored in the processor register R4. Here, also the old content of R4 will get overwritten by the sum of R3 and R2. Always remember the value at the right-hand side of the RTN expression is value to be operated or considered for the transfer and the left-hand side is the name of the location where that value has to be stored.
Assembly Language notations are used to represent the machine instructions. C Programming. Control System. Data Mining. Data Warehouse. Javatpoint Services JavaTpoint offers too many high quality services. In a basic computer, each instruction cycle consists of the following phases: Fetch instruction from memory.
Decode the instruction. Read the effective address from memory. Execute the instruction. Input-Output Configuration In computer architecture, input-output devices act as an interface between the machine and the user.
The following block diagram shows the input-output configuration for a basic computer. The input-output terminals send and receive information. The amount of information transferred will always have eight bits of an alphanumeric code.
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