What is the structure and function of memory | Main memory structure

Analyse de l’architecture interne de l’ordinateur

Main memory structure

In memory of size N, there are N memory locations, numbered from 0 to N-1. Each location is identified by its number, called an address. The address is most often written in hexadecimal. The capacity (size) of memory is the number of locations, usually expressed in kilobytes or megabytes, or more. Recall that the computer kilo is worth 1024 and not 1000 (2 10 = 1024 ¡1000). Here are the most used multiples:

 1 K (Kilo) 2 10 = 1024.

 1 M (Mega) 2 20 = 1048 576.

 1 Giga = 1 073 741 824.

 1 T (Tera) 2 40 = 1 099 511 627 776.

 Priming sequence (POST):

 The boot sequence is all the steps that will be executed from the moment you start your computer. There are several elements that come into play during this sequence. What is the use of knowing this sequence? Quite simply in the event of a problem, to better identify it and to better resolve it. And of course to get to know your computer better. To start, from the moment you press the power button on your PC, an electrical pulse will be sent to the power supply from the motherboard. Which will then produce current, current which will ignite the Bios!

 The BIOS:

The BIOS takes care of testing and initializing all the hardware. This part is also called the POST sequence (Power On Self Test) or even the preboot sequence. This is the sequence during which all the components will be tested as well as their compatibility. If the POST sequence does not pass, the system will not go any further and your OS will not be launched.

It will start by checking the system bus and will then check all the expansion connectors. It will continue by checking the memory of the graphics card and the signals controlling the display. Then it will interrogate the BIOS of the video card and add its recognition code. It is from this moment that the first displays arrive on the screen. It will test the RAM, for that, it tries writing on each memory zone and tries to read them to compare them with what it wrote. It checks if the keyboard and the mouse are well connected.

Then it sends signals to all storage devices (floppy, cd, HDD, USB, #) to define which different drives are. All the results are compared on the CMOS, which lets the BIOS know if the hardware configuration has changed since the last boot or not. Then, it integrates the identifiers of all the components having a BIOS. Then, the hardware tests validated, will try to boot into memory the main boot sector of the hard drive also called MBR.

There are a lot of errors that can occur during this phase, they are most often of a hardware nature, for example, a badly connected bar or a missing component, or an incompatibility between 2 materials. These errors are independent of the operating system.

The MBR:

The MBR (Master Boot Record) or partition table in French, allows you to find the active partition of the disk.

Once this partition is identified, the MBR will load the corresponding boot sector and then transfer the execution to it.

The errors that can occur at this stage of a startup are often of storage order. That is, there can be multiple active partitions or no valid storage media. Or it may happen that the partition table is corrupted.

The boot sector:

Once the MBR has given its control, the boot sector will load the 15 sectors that follow it on the disk and will then transfer control to a program present in these sectors. These first 15 sectors are called "Boostrap Code" and are responsible for locating and then transferring the execution to the NTLDR file.

The errors that can occur at this level are once again mainly hardware problems. That is to say that for example one of the sectors which it must load is missing. Or that the disk on which we boot has no NTLDR, so we cannot boot on it. Or there may be a problem with the NTLDR file.


We will now go to the NTLDR, which this time marks the first part of Windows execution. It is the Windows boot loader. It is he who will know which windows are installed and which one to launch. It starts by loading the drivers for the appropriate file system.

Then, according to the Boot.ini file, it will define which operating systems it can launch and if there are several, it will display them on the screen and ask the user to choose one. It loads the NTDETECT program which will then detect the hardware of the pc. It loads several DLLs which will allow the rest of the work to be carried out. It loads the majority of the registry (the rest being loaded later by the operating system). And finally, it gives control to NTOSKRNL.exe.

The problems that can happen here are mainly problems related to the files which must be launched, for example, a file that is missing or then a problem of access to one of the files.


Here we are at the end of the PC boot sequence, this time, the NT kernel will launch definitively and load the login program and we will end up on our good old Windows.

The operating system :

The goals of an operating system

The operating system is a software offering:

• an interface adapted to the needs of users.

• an interface relatively independent of the machine.

• optimal management of physical resources.

• user control.

Interface adapted to user needs:

Depending on what the machine is intended for, the operating system offers tools adapted to users:

• Development tools

• Or data management

• Or extended possibilities of connections to external devices

Relatively machine-independent interface :

• By the use of advanced symbolic languages.

• By the manipulation of logical resources at the user level, the logical resource and physical resource correspondence is ensured by the system.

• Possibly by the realization of standard systems presenting the same logical machine whatever the physical machine.

Optimal user management :

• By sharing resources between users of a system

• By using the possible parallelism of operation at the hardware level

• By recovering from errors

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