8 ms·

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 w

by billti 4y ago

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! 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-does-the-raspberry-pis-gpu-control-the-first-stages-of-the-boot-process 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).

pabs3 4y ago
This keynote about hardware vs operating systems knowledge of hardware was enlightening: https://www.youtube.com/watch?v=36myc8wQhLo https://www.youtube.com/watch?v=36myc8wQhLo
_448 4y ago
> Turns out the Raspberry Pi (and I'm guessing many other systems) are pretty confusing to understand at boot time. That is because how ARM ecosystem functions. There is no standard way of integrating ARM CPU into a product as Arm, the company, just sells base IP and not the complete CPU. Every ARM licensee, from Qualcomm to Apple to Nvidia are free to design their own extensions and integrations into their SoC. There is no standard for this. This creates a lot of problems for writing a generic tutorial that you see in the x86 world.
Dork1234 4y ago
Might want to look at MicroPython. The ports have various init functions for various chips /system. It might not be a full OS, but it is pretty low level code you can compare to various other systems. https://github.com/micropython/micropython/tree/master/ports https://github.com/micropython/micropython/tree/master/ports
P_I_Staker 4y ago
This is very common in embedded systems, and honestly I'm not surprised to see this, even with projects like Raspberry Pi... although, I do think it's a big shortcoming for a project like that. In a "real project" you would be relying on your suppliers and other teams / colleagues for many of the "hard questions". Chances are you'd have a contact with the silica manufacturer, just as a major example. There would be a boot team, or sometimes just one boot developer. If anything goes wrong, or you want to change anything, you would be pretty helpless without them. You can go to the various specs and code, but this can be quite in depth if you're starting from scratch. Change project or micro? Chances are that all goes out the window, and you have to start from scratch.
pabs3 4y ago
Check out the LibreRPi project, they are aiming to reverse engineer, document and replace the GPU firmware boot blob and other blobs. https://github.com/librerpi/ https://github.com/librerpi/
Gigachad 4y ago
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.
- eismcc 4y ago
  Just looked in eBay and had no idea there were so many DS systems available.
  - Gigachad 4y ago
    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.
    - 867-5309 4y ago
      wondering if they meant variants
- MuffinFlavored 4y ago
  > But they run no OS, your program runs directly on the hardware How is threading/time scheduling/interrupt handling/context switching usually done?
pcwalton 4y ago
> 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 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.
- ajross 4y ago
  The PMC also needs to be online before the main cores. And many machines there's still an external EC on the board responsible for sequencing power state external to the chip. The x86 application cores actually start up quite late in the process, long after the SPI has been read out, memory and cache controllers initialized, etc...
userbinator 4y ago
Look at the IBM PC/XT/AT if you want something more documented. IBM supplied schematics and even BIOS listings for those machines, the newest being a 286 (AT). Thanks to doing some BIOS RE work many years ago, the address F000:E05B still remains in my mind...
retrac 4y ago
As early as the mid-70s, minicomputers started including microcontrollers to manage the boot environment. I had a Sun machine once that included what they called Lights Out Management. A little microcontroller that had control over the system power and etc. Always available via its dedicated serial port, even when the machine was shut off. Everything is like that now. A smartphone will have multiple processors. Some doing IO interfacing. One to manage the battery. The radio hardware will have a general-purpose processor. Most of these processors are fixed-in-ROM sort of machines, so the story for booting them individually is pretty simple. Much like a late 20th century PC, when switched on (either by the power supply or by another processor), start running BIOS code from the hardwired start address. Some need to have more software transferred to them after that. Modern machines are really networks of computers in themselves. Networking and bringing all these parts together to support the main processors, at the low level, is not only poorly or completely undocumented, but it's probably impossible for one person to fit it all in their head these days.
- rmckayfleming 4y ago
  A recurring question I have is: how many microcontrollers/CPUs are in a modern personal computer? There are clearly a lot, but just how many?
  - tenebrisalietum 4y ago
    - the embedded controller (EC) - the CPU core in the chipset that runs the ME/PSP - if the TPM is not an fTPM, it has a CPU I'm sure. - if your NIC has offload engines, it has a CPU or two. - each storage device has a CPU. - each Wi-Fi device has a CPU. - thunderbolt controller takes firmware, it has a CPU. I'd bet USB3 and 4 do too. - any USB device has a CPU on the other end accepting and interpreting commands. - same with any SCSI device. - monitors have a CPU or two, one for the OSD settings and another to drive the display I'm sure. - I think any nVidia or AMD graphics card has a CPU in there (in addition to the GPU). - The following portable media has microcontroller firmware: SD cards, Memory Stick. (The now defunct SmartMedia and Olympus XD were raw NAND). - your optical mouse has a CPU as well to process optical data. - obviously printers have CPUs and firmware, probably separate ones for the web UI and the part that drives the actual print mechanism, and I'd bet a separate one for scanning and image processing. - any keyboard has a microcontroller and firmware (there are open source keyboard firmwares) - SIM cards for cellular connections have their own CPU and firmware.
wiml 4y ago
If you want something you can almost fully understand, I'd go another step or two lower, to a microcontroller like an AVR or one of the old Motorola chips (68HC11 or something). These chips were actually designed by hand and expected to be programmed by hand, and their documentation reflects this. They also ave much less hidden microarchitectural state than a modern CPU. Once you're familiar with that move to a more modern microcontroller like an Arm Cortex-M0, and after that maybe something with off-chip memory, a MMU, etc.
- LeifCarrotson 4y ago
  Those 8-bit controllers with (exclusively) on-chip SRAM and Flash are indeed quite simple - they can be summed up as "Jump to 0x0000", but really are surprisingly complex once you get into bootloaders and interrupt service routines and so on. The proprietary blobs and GPU on the Raspberry Pi make basically impossible to have a full understanding of what's happening. Instead, I'd recommend learning with the TI Beaglebone Black, which has an ARM Cortex-A8 with an MMU and lots of open documentation.
sgtnoodle 4y ago
You could start with a simple microcontroller like an AVR. Pick up an Arduino Uno or a Mega and an ICSP programmer, then write your own bootloader for it.
bsder 4y ago
RPi's unfortunately have lousy documentation for the low level stuff. Just about anything else (including dodgy Chinese substitutes) is better, sadly.
c0wb0yc0d3r 4y ago
Would you mind sharing some of your favorite youtube videos that you've come across?
epigramx 4y ago
the "whole computer" is also thought to start on the memory, because you can have memory on a computer and a manual operator of it, but you can't have a computer with an operator of it and no memory.
saagarjha 4y ago
Macs with a T2 chip first start execution there before pulling the Intel CPU out of reset.
hot_gril 4y ago
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.
shash 4y ago
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?
trissylegs 4y ago
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.
psychphysic 4y ago
Yeah better off just getting QEMU to boot. Target 286 or something.
ilyt 4y ago
Yup, in bigger systems you also often need to initialize/train DRAM which means your first code essentially runs off L2/L3 cache Here is a presentation about open firmware with a lot of stuff about boot process: https://www.youtube.com/watch?v=fTLsS_QZ8us https://www.youtube.com/watch?v=fTLsS_QZ8us

by billti 4y ago

pabs3 4y ago

This keynote about hardware vs operating systems knowledge of hardware was enlightening: https://www.youtube.com/watch?v=36myc8wQhLo https://www.youtube.com/watch?v=36myc8wQhLo

_448 4y ago

> Turns out the Raspberry Pi (and I'm guessing many other systems) are pretty confusing to understand at boot time. That is because how ARM ecosystem functions. There is no standard way of integrating ARM CPU into a product as Arm, the company, just sells base IP and not the complete CPU. Every ARM licensee, from Qualcomm to Apple to Nvidia are free to design their own extensions and integrations into their SoC. There is no standard for this. This creates a lot of problems for writing a generic tutorial that you see in the x86 world.

Dork1234 4y ago

Might want to look at MicroPython. The ports have various init functions for various chips /system. It might not be a full OS, but it is pretty low level code you can compare to various other systems. https://github.com/micropython/micropython/tree/master/ports https://github.com/micropython/micropython/tree/master/ports

P_I_Staker 4y ago

This is very common in embedded systems, and honestly I'm not surprised to see this, even with projects like Raspberry Pi... although, I do think it's a big shortcoming for a project like that. In a "real project" you would be relying on your suppliers and other teams / colleagues for many of the "hard questions". Chances are you'd have a contact with the silica manufacturer, just as a major example. There would be a boot team, or sometimes just one boot developer. If anything goes wrong, or you want to change anything, you would be pretty helpless without them. You can go to the various specs and code, but this can be quite in depth if you're starting from scratch. Change project or micro? Chances are that all goes out the window, and you have to start from scratch.

pabs3 4y ago

Check out the LibreRPi project, they are aiming to reverse engineer, document and replace the GPU firmware boot blob and other blobs. https://github.com/librerpi/ https://github.com/librerpi/

Gigachad 4y ago

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.

eismcc 4y ago
Just looked in eBay and had no idea there were so many DS systems available.
- Gigachad 4y ago
  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.
  - 867-5309 4y ago
    wondering if they meant variants
MuffinFlavored 4y ago
> But they run no OS, your program runs directly on the hardware How is threading/time scheduling/interrupt handling/context switching usually done?

pcwalton 4y ago

> 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 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.

ajross 4y ago
The PMC also needs to be online before the main cores. And many machines there's still an external EC on the board responsible for sequencing power state external to the chip. The x86 application cores actually start up quite late in the process, long after the SPI has been read out, memory and cache controllers initialized, etc...

userbinator 4y ago

Look at the IBM PC/XT/AT if you want something more documented. IBM supplied schematics and even BIOS listings for those machines, the newest being a 286 (AT). Thanks to doing some BIOS RE work many years ago, the address F000:E05B still remains in my mind...

retrac 4y ago

As early as the mid-70s, minicomputers started including microcontrollers to manage the boot environment. I had a Sun machine once that included what they called Lights Out Management. A little microcontroller that had control over the system power and etc. Always available via its dedicated serial port, even when the machine was shut off. Everything is like that now. A smartphone will have multiple processors. Some doing IO interfacing. One to manage the battery. The radio hardware will have a general-purpose processor. Most of these processors are fixed-in-ROM sort of machines, so the story for booting them individually is pretty simple. Much like a late 20th century PC, when switched on (either by the power supply or by another processor), start running BIOS code from the hardwired start address. Some need to have more software transferred to them after that. Modern machines are really networks of computers in themselves. Networking and bringing all these parts together to support the main processors, at the low level, is not only poorly or completely undocumented, but it's probably impossible for one person to fit it all in their head these days.

rmckayfleming 4y ago
A recurring question I have is: how many microcontrollers/CPUs are in a modern personal computer? There are clearly a lot, but just how many?
- tenebrisalietum 4y ago
  - the embedded controller (EC) - the CPU core in the chipset that runs the ME/PSP - if the TPM is not an fTPM, it has a CPU I'm sure. - if your NIC has offload engines, it has a CPU or two. - each storage device has a CPU. - each Wi-Fi device has a CPU. - thunderbolt controller takes firmware, it has a CPU. I'd bet USB3 and 4 do too. - any USB device has a CPU on the other end accepting and interpreting commands. - same with any SCSI device. - monitors have a CPU or two, one for the OSD settings and another to drive the display I'm sure. - I think any nVidia or AMD graphics card has a CPU in there (in addition to the GPU). - The following portable media has microcontroller firmware: SD cards, Memory Stick. (The now defunct SmartMedia and Olympus XD were raw NAND). - your optical mouse has a CPU as well to process optical data. - obviously printers have CPUs and firmware, probably separate ones for the web UI and the part that drives the actual print mechanism, and I'd bet a separate one for scanning and image processing. - any keyboard has a microcontroller and firmware (there are open source keyboard firmwares) - SIM cards for cellular connections have their own CPU and firmware.

wiml 4y ago

If you want something you can almost fully understand, I'd go another step or two lower, to a microcontroller like an AVR or one of the old Motorola chips (68HC11 or something). These chips were actually designed by hand and expected to be programmed by hand, and their documentation reflects this. They also ave much less hidden microarchitectural state than a modern CPU. Once you're familiar with that move to a more modern microcontroller like an Arm Cortex-M0, and after that maybe something with off-chip memory, a MMU, etc.

LeifCarrotson 4y ago
Those 8-bit controllers with (exclusively) on-chip SRAM and Flash are indeed quite simple - they can be summed up as "Jump to 0x0000", but really are surprisingly complex once you get into bootloaders and interrupt service routines and so on. The proprietary blobs and GPU on the Raspberry Pi make basically impossible to have a full understanding of what's happening. Instead, I'd recommend learning with the TI Beaglebone Black, which has an ARM Cortex-A8 with an MMU and lots of open documentation.

sgtnoodle 4y ago

You could start with a simple microcontroller like an AVR. Pick up an Arduino Uno or a Mega and an ICSP programmer, then write your own bootloader for it.

bsder 4y ago

RPi's unfortunately have lousy documentation for the low level stuff. Just about anything else (including dodgy Chinese substitutes) is better, sadly.

c0wb0yc0d3r 4y ago

Would you mind sharing some of your favorite youtube videos that you've come across?

epigramx 4y ago

the "whole computer" is also thought to start on the memory, because you can have memory on a computer and a manual operator of it, but you can't have a computer with an operator of it and no memory.

saagarjha 4y ago

Macs with a T2 chip first start execution there before pulling the Intel CPU out of reset.

hot_gril 4y ago

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.

shash 4y ago

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?

trissylegs 4y ago

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.

psychphysic 4y ago

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

ilyt 4y ago

Yup, in bigger systems you also often need to initialize/train DRAM which means your first code essentially runs off L2/L3 cache Here is a presentation about open firmware with a lot of stuff about boot process: https://www.youtube.com/watch?v=fTLsS_QZ8us https://www.youtube.com/watch?v=fTLsS_QZ8us