cfenollosa_os-tutorial/09-32bit-gdt

Concepts you may want to Google beforehand: GDT

Goal: program the GDT

Remember segmentation from lesson 6? The offset was left shifted to address an extra level of indirection.

In 32-bit mode, segmentation works differently. Now, the offset becomes an index to a segment descriptor (SD) in the GDT. This descriptor defines the base address (32 bits), the size (20 bits) and some flags, like readonly, permissions, etc. To add confusion, the data structures are split, so open the os-dev.pdf file and check out the figure on page 34 or the Wikipedia page for the GDT.

The easiest way to program the GDT is to define two segments, one for code and another for data. These can overlap which means there is no memory protection, but it's good enough to boot, we'll fix this later with a higher language.

As a curiosity, the first GDT entry must be 0x00 to make sure that the programmer didn't make any mistakes managing addresses.

Furthermore, the CPU can't directly load the GDT address, but it requires a meta structure called the "GDT descriptor" with the size (16b) and address (32b) of our actual GDT. It is loaded with the lgdt operation.

Let's directly jump to the GDT code in assembly. Again, to understand all the segment flags, refer to the os-dev.pdf document. The theory for this lesson is quite complex.

In the next lesson we will make the switch to 32-bit protected mode and test our code from these lessons.