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What happens when a CPU starts

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Re: What happens when a CPU starts

#81

Post by shash »

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…

Chiming in with everyone else - RPi is way too complex.

But another suggestion if you want a modern alternative to understand - RISC-V has an open boot ecosystem. You can just try it in QEmu and maybe buy a board if you get more advanced?

Re: What happens when a CPU starts

#82

Post by trissylegs »

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…

I think OpenPower has a fully open bootloader. But it's not exactly cheap to try.

Those systems startup into "Cache Contained" mode. Where the Boot ROM is copied to CPU Cache and there's no main memory yet. The code in the ROM has to initialized main memory before it can use it.

Re: What happens when a CPU starts

#83

Post by gdprrrr »

Rimintil wrote: post #16

> The memory chips respond by sending the contents of the selected memory cell over the data bus to the CPU. What does that ROM memory cell _physically look like_? How do we physically manipulate it to contain a 1 or a 0 (absence of something)?

Ben eater to the rescue https://www.youtube.com/watch?v=FnxPIZR1ybs

Re: What happens when a CPU starts

#84

Post by psychphysic »

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…

Yeah better off just getting QEMU to boot. Target 286 or something.

Re: What happens when a CPU starts

#85

Post by ruslan »

Starting procedure on MOS6502 is not that simple as it is told in the article. It spends some 7 clocks on doing internal initialization and only then fetches address of first jump from reset vector (0xFFFC/0xFFFD). I find Ben Eater's series on building simple 8-bit breadboard computer using 6502 CPU very educating and entertaining.

https://www.youtube.com/watch?v=HyznrdDSSGM

Re: What happens when a CPU starts

#86

Post by msk-lywenn »

MuffinFlavored wrote: post #32
Gigachad wrote: post #5

Earlier quoted context omitted.

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…

> But they run no OS, your program runs directly on the hardware How is threading/time scheduling/interrupt handling/context switching usually done?

No threads. Well, there’s two CPUs so you can count that as two threads. They run completely independently, operating on two different codes with different instruction sets. (ARM9 and ARM7TDMI)

I don’t know what you mean exactly by time scheduling. There are several timers that you can configure and you can set them to raise an interrupt when they finish. They can restart automatically.

There is an interrupt vector for both software and hardware interrupts. Software ones are raised by the swi assembly instruction. Hardware ones are raised by the aforementioned timers but also the display (vertical and horizontal blank), the sound system, wifi, etc. They can be enabled/disabled by setting a specific bit in a specific memory location (IE interrupt enable). Your interrupt handler is supposed to restore registers and clear the interrupt flag at the end.

Context switching is done manually.

I think libnds has an implementation of software threads. I don’t know how they work.

Re: What happens when a CPU starts

#87

Post by im3w1l »

toast0 wrote: post #40
MuffinFlavored wrote: post #32

Earlier quoted context omitted.

> But they run no OS, your program runs directly on the hardware How is threading/time scheduling/interrupt handling/context switching usually done?

I dunno about the DS, which doesn't really qualify as that old to me; it's got a pretty decent sized rom for when you don't have a cartridge in, and I'd guess the SDK gives you something approaching an OS, maybe even with a threading library. But on real old hardware, you're not going to run threads or a scheduler, you're going to run one iteration of your game loop, then wait for a sign that it's time to do your gra…

One thing I've always wondered about is what if you get interrupted again while you are halfway done saving registers to the stack? Like I heard something about disabling interrupts during sensitive operations like that, but wouldn't that then risk missing an event entirely instead?

Re: What happens when a CPU starts

#88

Post by swuesqe »

im3w1l wrote: post #87
toast0 wrote: post #40

Earlier quoted context omitted.

I dunno about the DS, which doesn't really qualify as that old to me; it's got a pretty decent sized rom for when you don't have a cartridge in, and I'd guess the SDK gives you something approaching an OS, maybe even with a threading library. But on real old hardware, you're not going to run threads or a scheduler, you're going to run one iteration of your game loop, then wait for a sign that it's time to do your gra…

One thing I've always wondered about is what if you get interrupted again while you are halfway done saving registers to the stack? Like I heard something about disabling interrupts during sensitive operations like that, but wouldn't that then risk missing an event entirely instead?

When interrupts are disabled events that occur during execution are usually buffered to the next possible moment.

Re: What happens when a CPU starts

#89

Post by snvzz »

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.

Similarly, I have come to appreciate rehsd's[0] efforts in building a 80286 computer.

0. https://www.youtube.com/@rehsd/videos

Re: What happens when a CPU starts

#90

Post by 867-5309 »

Gigachad wrote: post #25
eismcc wrote: post #24

Earlier quoted context omitted.

Just looked in eBay and had no idea there were so many DS systems available.

The console sold 154 million units. And since it’s so old, they are all finding they way to eBay. It’s honestly more interesting to play with and easier to buy than a raspberry pi.

wondering if they meant variants
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