Which architecture use which instruction format

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Suppliers Arm supply base is a source of excellence, quality standards and innovation for third-party products, goods and services. Trust Center End-to-end security offerings and our ongoing commitment to keeping our customers secure. Policies Website terms and policies. However, you have program sequencing and control instructions that help you change the flow of the program. This is best explained with an example. Consider the task of adding a list of n numbers.

A possible sequence is given below. After all the numbers have been added, the result is placed in memory location SUM. Instead of using a long list of Add instructions, it is possible to place a single Add instruction in a program loop, as shown below:.

The loop is a straight-line sequence of instructions executed as many times as needed. During each pass through this loop, the address of the next list entry is determined, and that entry is fetched and added to R0. The address of an operand can be specified in various ways, as will be described in the next section. For now, you need to know how to create and control a program loop. Assume that the number of entries in the list, n, is stored in memory location N.

Register R1 is used as a counter to determine the number of times the loop is executed. Hence, the contents of location N are loaded into register R1 at the beginning of the program. Then, within the body of the loop, the instruction, Decrement R1 reduces the contents of R1 by 1 each time through the loop.

The execution of the loop is repeated as long as the result of the decrement operation is greater than zero. You should now be able to understand branch instructions. This type of instruction loads a new value into the program counter.

As a result, the processor fetches and executes the instruction at this new address, called the branch target, instead of the instruction at the location that follows the branch instruction in sequential address order.

The branch instruction can be conditional or unconditional. An unconditional branch instruction does a branch to the specified address irrespective of any condition.

A conditional branch instruction causes a branch only if a specified condition is satisfied. If the condition is not satisfied, the PC is incremented in the normal way, and the next instruction in sequential address order is fetched and executed.

This means that the loop is repeated as long as there are entries in the list that are yet to be added to R0. At the end of the n th pass through the loop, the Decrement instruction produces a value of zero, and, hence, branching does not occur. Instead, the Move instruction is fetched and executed.

It moves the final result from R0 into memory location SUM. Some ISAs refer to such instructions as Jumps. The processor keeps track of information about the results of various operations for use by subsequent conditional branch instructions. This is accomplished by recording the required information in individual bits, often called condition code flags. These flags are usually grouped together in a special processor register called the condition code register or status register.

Individual condition code flags are set to 1 or cleared to 0, depending on the outcome of the operation performed. Some of the commonly used flags are: Sign, Zero, Overflow and Carry. The call and return instructions are used in conjunction with subroutines. A subroutine is a self-contained sequence of instructions that performs a given computational task. During the execution of a program, a subroutine may be called to perform its function many times at various points in the main program.

Each time a subroutine is called, a branch is executed to the beginning of the subroutine to start executing its set of instructions. After the subroutine has been executed, a branch is made back to the main program, through the return instruction. Interrupts can also change the flow of a program. A program interrupt refers to the transfer of program control from a currently running program to another service program as a result of an external or internally generated request.

Control returns to the original program after the service program is executed. The interrupt procedure is, in principle, quite similar to a subroutine call except for three variations: 1 The interrupt is usually initiated by an internal or external signal apart from the execution of an instruction 2 the address of the interrupt service program is determined by the hardware or from some information from the interrupt signal or the instruction causing the interrupt; and 3 an interrupt procedure usually stores all the information necessary to define the state of the CPU rather than storing only the program counter.

Therefore, when the processor is interrupted, it saves the current status of the processor, including the return address, the register contents and the status information called the Processor Status Word PSW , and then jumps to the interrupt handler or the interrupt service routine.

Upon completing this, it returns to the main program. Suppose you are designing an embedded processor which is meant to be performing a particular application, then definitely you will have to bring instructions which are specific to that particular application.

Examples of specialized instructions may be media and signal processing related instructions, say vector type of instructions which try to exploit the data level parallelism, where the same operation of addition or subtraction is going to be done on different data and then you may have to look at saturating arithmetic operations, multiply and accumulator instructions.

The data types and sizes indicate the various data types supported by the processor and their lengths. The operation field of an instruction specifies the operation to be performed. This operation must be executed on some data that is given straight away or stored in computer registers or memory words. The way the operands are chosen during program execution is dependent on the addressing mode of the instruction.

The addressing mode specifies a rule for interpreting or modifying the address field of the instruction before the operand is actually referenced. In this section, you will learn the most important addressing modes found in modern processors. Computers use addressing mode techniques for the purpose of accommodating one or both of the following:. Virtualizing the Processor System-level Communication Parallel Processing Future of Computing Authored by Steve Ward.

Instruction Set Architectures. Steps toward a General Purpose Computer. Datapath vs. Control Circuitry. Generalizing Datapath Design. The Stored-Program Computer. Anatomy of a von Neumann Computer. CPU Datapath. CPU Control. Instruction Set Architecture as an Abstraction.

In order to execute the instruction, the CPU needs to know three things. The instruction also contains the details of direct or indirect referencing. In instruction format , the bits are grouped together in three parts. The first part of the instruction format is called addressing mode. The addressing mode is defined as the manner in which data is represented into the instruction format.

The addressing mode is the third field in the instruction format represented by a single bit. For example , in 16 bit instruction format the addressing mode is represented by the 15 th bit. During the program execution , the instruction is fetched into the instruction register. The instruction register IR is a 16 bit register and a part of the IR is used to store the operation code.

The second part of the instruction format is called operation code. CPU Registers. The control unit is one of the three important component of the microprocessor architecture. In computing , the OPERAND part of the instruction format specifies either data or the address of the data that needs to be operated by the processor.

After decoding the instruction, the CPU next needs to know the data on which the operation is to be performed as per the operation code. Depending upon the type of the format supported by the processor , the instruction might contain zero to three operands. The zero address type of instruction does not contain the operand part. The operand can either contain the data or the address of the data. The addressing mode value directs the CPU to fetch the data.

If the value of the addressing mode is 0 zero then it is referred as direct referencing. For direct referencing the operand contains the data. Whereas, If the value of the addressing mode is 1 one then it is referred as indirect referencing. For indirect referencing the operand contains the memory address that points to the data. In computer architecture , the instruction formats can be classified into two main groups.

The first type is based on the number of operands present into the instruction format. And second types of instruction group is based on the memory organization of the CPU. As the name suggest , the zero address instruction format does not have any operand field. This is a instruction format without any operand. The zero address instruction format is supported by the stack organized computer system. In order to evaluate the arithmetic expressions in stack organized computer, it is necessary to convert the expression into reverse polish notation.

The one address instruction format makes use of an implied accumulator for all data manipulation operations. An accumulator AC is a type of register used by the CPU to perform the mathematical and logical operations. In this type of instruction format , the accumulator is implied and does not require explicit reference.