Operating System Development from Scratch – Real Mode – Part 3

in operating-system •  10 months ago 

Hello Everybody, it’s me, Lovelace! In this post, we will talk a little about Real Mode.

Real Mode

In previous entries, I’ve explained that ‘Real Mode' is a compatibility mode that all modern intel processes have, and this is the mode in which they start, it mimics the processor from several years ago.

In Real Mode we can only access 1 MiB of RAM, it doesn’t matter if you have four or more gigabytes plugged into your computer, only 1 MiB will be accessible, also, memory is accessed through the segmentation memory model, simply put, memory can be accessed through the use of segments and offsets.

Real Mode is based on the original x86 design, it acts like older intel processors from the 70s (such as the 8086) and we also have the same limitations as these. All the code we write while in real mode, is required to be written in 16 bits.

When the processor is in Real Mode, there is no memory nor hardware security, which means, programs that are run could destroy our computer with no way for us to stop them, the reason why in modern operating systems a process can’t just destroy your computer is that the kernel tells the processor (this is made in the processor itself) which programs cannot access certain parts of memory or directly the hardware.

And finally, when we are in Real Mode, only 8-bit and 16-bit registers are accessible, which means, we can only request memory address offsets up to 65535 for our given segment, there can’t be decimal numbers larger than 65535.

Segmentation Memory Model

Memory segmentation is an operating system memory management technique of division of a computer’s primary main memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset (memory location) within that segment. Segments or sections are also used in object files of compiled programs when they are linked together into a program image and when the image is loaded into memory.
Segments usually correspond to natural divisions of a program such as individual routines or data tables so segmentation is generally more visible to the programmer than paging alone. Different segments may be created for different program modules, or for different classes of memory usage such as code and data segments. Certain segments may be shared between programs.
Segmentation was originally invented as a method by which system software could isolate different tasks and data they are using. It was intended to increase the reliability of the systems running multiple processes simultaneously. In an x86-64 architecture, it is considered legacy and most x86-64 based modern system software don’t use memory segmentation. Instead, they handle programs and their data by utilising memory paging which also serves as a way of memory protection. However, most x86-64 implementations still support it for backward compatibility reasons.

Source: https://en.wikipedia.org/wiki/Memory_segmentation.

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