What happens when a CPU starts
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What happens when a CPU starts
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Re: What happens when a CPU starts
#2"It starts at 0 and executes instructions" is a funny, but mostly true way to express this. Some people are shocked that no magic happens before the instructions start.
Re: What happens when a CPU starts
#3Amongst the first sentences...
> It may be thought of as what happens when a whole computer starts, since the CPU is the center of the computer and the place where the action begins.
I thought that too. Last year I spent a while getting as low-level as I could and trying to understand how to write a boot loader, a kernel, learn about clocks and pins and interrupts, etc. I thought, "I know, I'll get a Raspberry Pi! That way even if I brick it I didn't waste too much money".
Turns out the Raspberry Pi (and I'm guessing many other systems) are pretty confusing to understand at boot time. For one, it's the GPU that actually does the initial boot process, and much of that is hard to find good info on. (https://raspberrypi.stackexchange.com/questions/14862/why-do...)
I spent many many hours reading various specs & docs and watching tons of low-level YouTube videos. Compared to software development higher up the stack (my usual area), I found the material surprisingly sparse and poor for the most part. (Maybe that's reflective of the size of the audience and the value in producing it).
Re: What happens when a CPU starts
#4Amazing to see that tripod sites are still around.
Re: What happens when a CPU starts
#5Amongst the first sentences... > It may be thought of as what happens when a whole computer starts, since the CPU is the center of the computer and the place where the action begins. I thought that too. Last year I spent a while getting as low-level as I could and trying to understand how to write a boot loader, a kernel, learn about clocks and pins and interrupts, etc. I thought, "I know, I'll get a Raspberry Pi! Th…
Yeah modern hardware is crazy hard to understand and a lot is proprietary/trademark stuff like you saw on the rpi. Arduino is a good start but it's just so low level that it doesn't get you much further to understanding how a modern system works.
One slightly weird but fascinating path I have been playing with is writing programs for old game consoles, particularly the Nintendo DS. You can get full and comprehensive hardware documentation for these consoles and there are easy ways to run your own code on them now as well as libraries/tooling around it. But they run no OS, your program runs directly on the hardware, so you get a good feel for low level programming while not being down to the level of atmel chips.
It can be a little hard to work out how to get started but it's really as simple as setting up `libnds` from devkitpro and then either hacking a dsi to run the twilight firmware, or buying a cheap flashcard from ebay to run your own programs. Read the example programs from the devkitpro github and some posts on the hardware and you'll get the hang of it.
Re: What happens when a CPU starts
#6Amongst the first sentences... > It may be thought of as what happens when a whole computer starts, since the CPU is the center of the computer and the place where the action begins. I thought that too. Last year I spent a while getting as low-level as I could and trying to understand how to write a boot loader, a kernel, learn about clocks and pins and interrupts, etc. I thought, "I know, I'll get a Raspberry Pi! Th…
> For one, it's the GPU that actually does the initial boot process, and much of that is hard to find good info on. (https://raspberrypi.stackexchange.com/questions/14862/why-do...)
x86 is similar, since Intel ME (part of the PCH, whether in the chip or not) is needed to boot the CPU.
Re: What happens when a CPU starts
#7"It starts at 0 and executes instructions" is a funny, but mostly true way to express this. Some people are shocked that no magic happens before the instructions start.
It's easy to overlook for those who program mostly on higher level systems. There was a recent HN thread that I think pointed to this article on what happens before main() in C programs:
https://embeddedartistry.com/blog/2019/04/08/a-general-overv...
On microcontrollers, there are often some preliminaries that are programmed into nonvolatile settings of the hardware, such as the type of clock oscillator. On a microprocessor like the Z80, your circuit was supposed to ensure that things like the clock oscillator and power supplies were stable before releasing the RESET pin.
Re: What happens when a CPU starts
#8Is it me or the article seems incomplete? It kinda finishes for me after:
> The following are the memory ranges you get with a 2-to-4 converter on an 8-bit address bus:
And that's it. It looks truncated, or incomplete :thinking:
Re: What happens when a CPU starts
#9"It starts at 0 and executes instructions" is a funny, but mostly true way to express this. Some people are shocked that no magic happens before the instructions start.
This is pretty old. There's quite a lot that happens before instructions start on modern CPUs.
Just one small example: CPUs have many small SRAM arrays for micro-architecture features like branch predictors. Some of these need to be initialized after reset by a state machine that takes many cycles.
I have even heard of a large chip that pushes initialization vectors through the scan chains so that all flops begin in an initial state, without requiring a reset network.
Re: What happens when a CPU starts
#10"It starts at 0 and executes instructions" is a funny, but mostly true way to express this. Some people are shocked that no magic happens before the instructions start.
As long as things like embedded coprocessors (IME, TXT, PSP, etc) are kept out of view.