How are applications and data accessed by the CPU from RAM

1
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I am having a bit of trouble understanding how applications and data are accessed by the CPU from RAM after the application has been loaded into RAM and a file opened (thus data for the file also stored in RAM).

By my understanding, a CPU just gets instructions from RAM as the program counter ticks or carries out tasks after an interrupt. How then does it access the application and data. Is it that it doesn't and still just gets instructions (for example to load a file on the hard drive to be opened in the application) and processes any requests made by the application which are stored in RAM as instructions thereafter (like saving a file). Or does the application and data relating to an opened file (for example) just stay in RAM and not get accessed by the CPU at all.

Similarly, after reading an article, it said that a copy of the operating system is stored in RAM. The CPU can then access the operating system. (I thought the CPU just worked with instructions from RAM). How does it then communicate with the operating system and how are interrupts sent to the CPU, from the copy of the OS in RAM or from the OS in the hard drive.

Sorry if this is really confusing, alot i didn't understand.

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operating-systemcpuinterruptraminstructions

4 Answers

4
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Root of your question: Lack of clear differentiation between Computer's Hardware and Computer's Software.

Components of a Computer System

Just so that we are clear about both of them and that we understand their nature, let me state as follows:

  • Hardware: It includes CPU, RAM, Disk, Register, Graphics Card, Network Card, Memory BUS and everything that you can touch and call to be the 'Computer'. It is the body.
  • Software: It includes Operating System, Program, CPU instruction, Compiler, Programming Language and almost everything intangible about the computer. It is the soul.
  • Firmware: It is that basic code which is absolutely essential for hardware's working. This is stored on a Read Only Memory installed in the hardware itself. This piece of software is vital for hardware therefore is considered in the mid of hardware and software and hence called Firmware.

We will start with understanding from the time when we say that the computer is up and running and is properly executing our instructions. But at that time you will say - How did I reach here? So I will mention a few points about the startup of the computer.

When the power button is pressed...

...the most primitive and basic input output system (therefore called BIOS), which is hard written on the computer hardware begins execution. This is written on Read Only Memory and this starts the process to get the machine to stand on its own. And it loads the software (Operating System) from one piece of hardware (disks) into another piece of hardware (RAM and CPU registers) enabling the software to work properly with hardware.

Now the body and soul are together and the individual (machine) can work.

Until now, OS is already in RAM and CPU. (Read When the power button is pressed if you doubt it.) Let's handle your question paragraph by paragraph now -

First Paragraph

I am having a bit of trouble understanding how applications and data are accessed by the CPU from RAM after the application has been loaded into RAM and a file opened (thus data for the file also stored in RAM).

The explanation is as follows:

  1. The exact issue here is your thinking that it is CPU and RAM that access the data. CPU and RAM are only executing units.
  2. It is OS (software) that accesses the data by means of CPU and RAM (hardware). It is in the realm of OS where applications are executed.
  3. This is why you can install Linux and Windows on same hardware but cannot execute .exe files in Linux because OS does the execution and not RAM/CPU.
  4. Further, how do CPU and RAM and disk physically interact to bring in the data, execute it, save it back etc. is in the domain of hardware. That would require explanation which involves logic gates (AND, OR, NOT...), diodes, circuitry and a hell lot of other things which an Electronics guy can explain.

Second Paragraph

By my understanding, a CPU just gets instructions from RAM as the program counter ticks or carries out tasks after an interrupt. How then does it access the application and data. Is it that it doesn't and still just gets instructions (for example to load a file on the hard drive to be opened in the application) and processes any requests made by the application which are stored in RAM as instructions thereafter (like saving a file).

As you have guessed it - CPU doesn't get instructions, Operating System does it through CPU. Also, just the way brain doesn't directly instruct the hands and legs to move and instead uses nerves for interaction, the CPU doesn't tell the disks to give/take the data. CPU works with RAM and registers only. Multiple units of hardware work in conjunction to provide a path for data and instruction to travel. The important pieces of involved hardware are:

  1. Processor (CPU and registers built in the CPU)
  2. Cache
  3. Memory (RAM)
  4. Disk
  5. Tape

I like the image provided in this answer. This image not only lists the hardware pieces but also illustrates the mammoth difference in the execution speed of these pieces.

Let's move on to the...

Third Paragraph

Similarly, after reading an article, it said that a copy of the operating system is stored in RAM. The CPU can then access the operating system. (I thought the CPU just worked with instructions from RAM). How does it then communicate with the operating system and how are interrupts sent to the CPU, from the copy of the OS in RAM or from the OS in the hard drive.

By now you already know that indeed OS is present in RAM and CPU registers. That is where it lives. That is from where it tells the CPU how to work. If OS would be small enough (or if Registers and Caches would be big enough), the OS would live even closer to CPU.

  • The CPU does not communicate with the OS. It can't. It is the worker that is controlled by a boss. OS is that boss.
  • CPU cannot access Operating System. CPU is the body, OS is the soul. Soul tells the body what to do, not vice-versa.
  • CPU doesn't work with instructions from RAM. It merely executes the instructions given by the Operating System (which may be living in RAM). So even when there is an instruction to load some module of OS into the RAM, it is not RAM/CPU but OS itself that issues that instruction.
  • Interrupts are of two types - Hardware and Software - and your query is about the software interrupts. Since the executive part of OS is in the RAM, in simple words we can say that interrupts are sent to CPU from OS living in RAM.

Conclusions

The lack of distinction between hardware and software is the basic cause of your confusions. Take some course about Operating Systems on Coursera or Academic Earth for deeper understanding.

3
votes

It is confusing indeed. Let me try to explain.

CPU and RAM

The CPU is hardwired to the RAM via the 'motherboard', and they work together. The CPU can perform many instructions, but it has to be told what to do by instructions in RAM. The CPU is basically in a loop: all it does it fetch the next instruction from RAM and execute it, over and over.

So how does this RAM get filled with instructions?

BIOS (basic input/output system)

When the computer first boots up, a portion of RAM is filled with data from a chip on the motherboard (the BIOS chip), and the CPU is turned on and starts processing. These are the factory settings.

The data from the BIOS chip that is copied to RAM consists of a library of instructions to access hardware devices (hard disks, CD/ROM, USB storage, network cards etc.), and a program using that library to load what is called the bootsector, the first sector on the boot device, into RAM, and transfer control to it (with a jump instruction).

BOOTLOADER

The bootsector data that the BIOS program loaded from the boot device is very small - only 440 bytes - but with the help of the BIOS library, this is enough to be able to load more sectors and execute these. The bootsector and the data it loads is called the bootloader, which is in charge of loading the Operating System.

In effect, the bootloader is a more dynamic version of the BIOS: the BIOS program resides in flash memory, whereas the bootloader resides on hard disks, USB sticks, SSD drives etc., and thus can be larger and more complex.

OPERATING SYSTEM

In it's turn, The operating system (OS) is simply a more advanced version of the bootloader, as it can load and run multiple programs from multiple locations at the same time.

--

The BIOS knows about drives. The Bootloader knows about drives and partitions. The OS knows about drives, partitions, and file systems.

1
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CPU,as you've noticed, reads the program from RAM, instruction by instruction. When an instruction is executed, it might refer to data stored in memory, which it either fetches explicitly to the registers (internal storage of the CPU, quite small - on x86_64 that's like several 64-bit registers + other stuff like segment registers, IP, SP etc) with a separate instruction, or the data read from the memory (we are talking about small amount of data). That's all it really does.

Loading a file from a disk would be done by asking the appropriate controller to fetch the data into a specific place in memory. CPU is connected to buses which will carry instructions to appropriate controllers.

As to interrupts these are special things - CPU has several interrupt lines which can be activated by various devices, for example your network card. When it receives such an interrupt, it is usually handled by an interrupt handler, which is just a program located in a well-known place in memory. They can be registered by, for example, operating system. Each interrupt line has its own interrupt handler. When interrupt happens, the CPU saves the current state of the program it happens to be executing, handles interrupt, restores the state and resumes the program.

1
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You seem to be asking about addressing modes. At the risk of gross oversimplification (ignoring caching, segments, and logical memory), memory stored as a sequential array accessed by an integer address.

The CPU has a number of internal storage areas called registers. We will call them R0 to Rn. The processor assigns some registers dedicated purposes. One of those registers is the PC.

One common addressing mode is deferred. I indicate this mode as (Rn). An instruction like this:

MOV (R0), R1

uses the value contained in R0 as a memory address, fetches the value stored that memory location, and stores a copy of that value in R1.

An instruction sequence like this:

MOV (R0), R1 MOV (R2), R3

is stored in memory as data (ignoring protection), code, data, and variables all use the same type of memory. In other words, any memory location can be interpreted as code, data, or variable.

The CPU executes the next instruction located at (PC). After executing the instruction, the CPU automatically increments the PC to point to the next instruction.