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What happens when a CPU starts

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Re: What happens when a CPU starts

#71

Post by spijdar »

monocasa wrote: post #18
pclmulqdq wrote: post #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.

Sort of. It's actually fairly common on larger cores (and particular, larger SoCs) for there to exist magic that happens before architectural reset vectors. https://www.bunniestudios.com/blog/?p=5127 > By pre-boot code, I’m not talking about the little ROM blob that gets run after reset to set up your peripherals so you can pull your bootloader from SD card or SSD. That part was a no-brainer to share. I’m talking abo…

Something fun mentioned in a comment on that article, this the majority of this "pre-boot" code is actually FOSS in POWER9. There are a set of "auxiliary processors" called "PPE"s, among which there is one, the "SBE", or "self boot engine", which is a very small and simple PowerPC core that IPLs the big POWER9 cores [0]. These big processors with tons of cache and interconnects need a lot of help to get to executing PC 0x00.

I suspect that almost all the big "application processors" from Intel and AMD, and the exotic ARM/SPARC server chips, have equivalent embedded ICs to jump-start the "big cores".

[0] https://github.com/open-power/sbe/blob/master/src/sbefw/app/...

Re: What happens when a CPU starts

#72

Post by andirk »

broast wrote: post #4

Amazing to see that tripod sites are still around.

Mine's still around altho it looks like I turned it from a The Simpsons fan page in to one sentence https://bartzone.tripod.com/ . And with allowing all scripts, it's >1,000 requests, >10MB of data.

These ad networks on top of ad networks on the oldest IE-compliant code with it's `document.write` fighting it out for eyeballs since 1999. As Lycos' motto says: Battling it out to complete obsolescence and will never give an inch. Go iframe go!

Re: What happens when a CPU starts

#73

Post by NegativeLatency »

spijdar wrote: post #71
monocasa wrote: post #18

Earlier quoted context omitted.

Sort of. It's actually fairly common on larger cores (and particular, larger SoCs) for there to exist magic that happens before architectural reset vectors. https://www.bunniestudios.com/blog/?p=5127 > By pre-boot code, I’m not talking about the little ROM blob that gets run after reset to set up your peripherals so you can pull your bootloader from SD card or SSD. That part was a no-brainer to share. I’m talking abo…

Something fun mentioned in a comment on that article, this the majority of this "pre-boot" code is actually FOSS in POWER9. There are a set of "auxiliary processors" called "PPE"s, among which there is one, the "SBE", or "self boot engine", which is a very small and simple PowerPC core that IPLs the big POWER9 cores [0]. These big processors with tons of cache and interconnects need a lot of help to get to executing…

Reminds me of the pony motor on old large excavating equipment. Both are also “big iron”

Re: What happens when a CPU starts

#74

Post by AceJohnny2 »

santadakota wrote: post #34

Ben Eater's fantastic video series on building a breadboard 6502 based computer and an 8-bit breadboard computer from scratch might be appreciated in this thread. 6502 playlist: https://www.youtube.com/watch?v=LnzuMJLZRdU&list=PLowKtXNTBy... 8-bit build playlist: https://www.youtube.com/watch?v=HyznrdDSSGM&list=PLowKtXNTBy... He also sells kits if one is interested in playing along.

Highly recommend Ben Eater's video, which in this embedded developer's eyes has been the clearest, best explanation of the fundamentals of a CPU as I've ever seen.

Re: What happens when a CPU starts

#75

Post by hot_gril »

billti wrote: post #3

Amongst 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…

Maybe a good place to start is an old video game console. Those have a lot of community info because of interest in retro gaming/emulation, plus they're simpler than modern hardware.

Re: What happens when a CPU starts

#78

Post by derefr »

abbeyj wrote: post #65
derefr wrote: post #49

Earlier quoted context omitted.

If older CPUs have no magic, what is the 6502 doing here ( https://youtu.be/yl8vPW5hydQ?t=706 ) that causes it to put eb60 → ffff → eb60 → 01f7 → 01f6 → 01f5 on the address bus, before it actually reads from memory?

https://www.pagetable.com/?p=410 may go some way toward explaining this. The behavior is slightly different than in the video. I believe this is because the video is using a 65C02 instead of the original NMOS 6502 and the implementation is slightly different.

Brilliant; makes perfect sense of the "magic." It'd be interesting to see a compare-and-contrast of the "dump" of a decode ROM of a 65C02 vs the 6502's decode PLA, to see exactly what the 65C02 is doing with those few added cycles in what is presumably its generic BRK implementation.

Re: What happens when a CPU starts

#80

Post by oharapj »

Dylan16807 wrote: post #57
hhh wrote: post #39

Earlier quoted context omitted.

20 years next year.

It did launch in 04/05, but it's only two console generations older than a switch.

Considering the switch is at the end of its lifespan, I’d say that’s pretty old. It’s like saying 7 years into the PS3 lifespan that the PS1 isn’t old. It’s old
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