#protected mode

In the last section of this tutorial for writing your own toy operating system, we discussed memory and focused on the 21st address line (the “A20 line”) that must be enabled before we can have access to the full 4GB of memory, which is a prerequisite to entering protected mode. Now it’s time to jump to protected mode.

In fact, all we’ve done in the last few articles is prepare for entering protected mode. We’ve set up a global descriptor table (GDT), an interrupt descriptor table (IDT) and enabled the A20 line. All that remains is actually jumping to protected mode where we’ll finally be able to execute 32-bit code so we can focus on our kernel.

Here we are again, with yet another step in the process of building a second-stage bootloader for our own toy operating system. In the previous sections (here and here) we learned how to create and set up the global descriptor table (GDT) and the interrupt descriptor table (IDT), which the CPU uses to access and control memory and execute interrupts calls. Both of these structures are necessary to switch the CPU to protected mode. In this tutorial, we’ll see how to enable the A20 line.

We are now actually close to reaching that holy grail now. Only a few more preparatory steps are necessary to switch to protected mode, which will finish off our second-stage boot loader and allow us to start writing our kernel in C (which will be a relief from all the assembler code).

In this article, we will talk a little more about memory (we had already done that extensively in the past), but there’s a final hoop we need to jump through in order to have the the full 4GB of memory at our kernel’s disposal.