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by 7bit
241 days ago
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I'm currently trying to learn more about how the boot process works, specifically with UEFI. Yesterday I watched a video on how to program a UEFI bootloader so I was happy when I saw this article this morning. The article jumps over so many steps and leaves out so many mechanics that I had to stop reading, because it just opened more and more questions without closing any. For example, right at the beginning: > Right after reset the CPU jumps to a special address called the reset vector at 0xFFFFFFF0. Think of it as a permanent bookmark that says “start here.” There is room for almost nothing at that address, so manufacturers put a short jump there that passes control to the firmware on your motherboard. What does that even mean? It jumps there means that it executed whatever is at that address, but it failed to explain how the instruction at that location is even put there in the first place. Reading a little more I truly am confused for who this was even written. It seems you need a lot of prior knowledge to understand what the author talks about and then it also makes me wonder what additional information this article conveys if you already can fill in the gaps. |
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More seriously, at least for modern systems: the engineers who designed the computer created a memory map that defined which hardware can be found where. They simply ensured that the reset vector would point to an area of non-volatile memory that can be read directly (e.g. an EEPROM would work since you feed it an address and it returns data, much like RAM). That jump would be to somewhere else in non-volatile memory, an area that would have enough space for the code to continue the initialization process. You would have to ask someone more familiar with CPU architecture why they use an indirect approach. My guess is that many CPU architectures define a vector table, which is a list of places the CPU will jump to when various contditions are met (e.g. an interrupt, or reset).