From the PDF: "It is true: in our quest to make full use of the CPU, we must abandon all of those helpful routines provided by BIOS. As we will see when we look in more detail at the 32-bit protected mode switch-over, BIOS routines, having been coded to work only in 16-bit real mode, are no longer valid in 32-bit protected mode; indeed, attempting to use them would likely crash the machine.

"So what this means is that a 32-bit operating system must provide its own drivers for all hardware of the machine (e.g. the keybaord, screen, disk drives, mouse, etc). Actually, it is possible for a 32-bit protected mode operating system to switch temporarily back into 16-bit mode whereupon it may utilise BIOS, but this teachnique can be more trouble than it is worth, especially in terms of performance."

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In the toy 32-bit OS I am currently writing, having easy disk access and text output was more important to me than performance, so I decided to implement this technique to access the disk and screen via the BIOS instead of writing an ATA disk driver.

Although I could not find any minimal yet complete working examples of dropping to 16 bit and later resuming 32 bit mode, I was able to piece it together and write assembly functions called enter_16bit_real and resume_32bit_mode. See https://github.com/ironmeld/builder-hex0/blob/main/builder-h.... Those routines are working well but beware the project is a volatile work in progress and is coded in hex for bootstrapping reasons.