But this is simply an implementation detail that allows the control unit to run faster. The essence of how the control unit executes a program is represented by the single instruction register model. Since instructions are simply bit patterns, they can be stored in memory.
The instruction pointer register always has the memory address of points to the next instruction to be executed. In order for the control unit to execute this instruction, it is copied into the instruction register. The memory address where the first instruction is located is copied to the program counter.
Memory responds by sending a copy of the state of the bits at that memory location on the data bus, which the CPU then copies into its instruction register. The instruction pointer is automatically incremented to contain the address of the next instruction in memory.
Steps 3 , 4 , and 5 are called an instruction fetch. Notice that steps 3 — 7 make up a cycle, the instruction execution cycle. It is shown graphically in Figure 8. We will give a very brief look at interrupts in Chapter 17 , but the details are beyond the scope of this book. For now, it is sufficient to understand that the wfi instruction stops the program execution cycle. Actually, the ARM fetches two instructions at a time.
But this is an implementation detail that helps speed up program execution. The logical flow of a program follows the instruction execution cycle shown in Figure 8. How do we get the instructions into memory? The instructions for a program are stored in a file on a storage device; for example, a disk, device memory, or an SD card.
Create a free Team What is Teams? Learn more. Asked 2 years, 8 months ago. Active 2 years ago. Viewed times. Improve this question. So if the program counter is always incrementing by 1, how it can jump to a new instruction? Add a comment. Active Oldest Votes. Improve this answer.
David Richerby David Richerby Ran G. Should it be changed other way, the special instruction is placed which tells processor address of the next instruction other ones just contain in each instruction address of the next instruction. This makes code larger, but you have advantage of having to execute less commands and therefore be a little faster. Bulat Bulat 1, 1 1 gold badge 9 9 silver badges 16 16 bronze badges. Sathvik Swaminathan Sathvik Swaminathan 2 2 bronze badges.
Sign up or log in Sign up using Google. Sign up using Facebook. It is a digital counter needed for faster execution of tasks as well as for tracking the current execution point. A program counter is also known as an instruction counter, instruction pointer, instruction address register or sequence control register. All instructions as well as data in memory have a specific address.
As each instruction is processed, the software application responsible updates the program counter with the upcoming instructions' address which needs to be fetched. The program counter increases the stored value by one as the next instruction is fetched. If the computer is reset or restarts, the program counter usually reverts to the value of zero. Similar to other process registers of the computer, the program counter is like a bank of binary latches, with each representing one bit of value.
The program counter works in combination with other registers to identify the current instruction. It can be modified or accessed with the help of access or jump instructions.
Therefore, the destination address can be loaded to the program counter via branch instructions. The program counter can also be loaded with the address using the data processing instructions.
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