Here’s a discussion on operating (or addressing) modes in x86 architecture, and on segment selectors, descriptors, paging and multitasking (rather, TSS). Readers trying to program kernels/bootloaders, or thinking of writing their own toy OS, will find it

Because paging is not active in real modeI the linear addresses generated are the same as the physical addresses. Segment registers hold the starting address of the active segments in the entire memory. oefer to cigure 1 for clarity.

In protected modeI neverthelessI memory larger than 1 MB can be accessed. All modern operating systems work in protected mode and provide advanced features like pagingI virtual memory and multi-taskingI and thus have a more controlled environment.

te can now focus on the next level— protected mode, or even betterI protected virtual address mode.