3 ms·

How Does an Intel Processor Boot? (2018)

49 points by smikhanov 4y ago

userbinator 4y ago
What an ironic choice of header image --- phone SoCs vary widely in how they boot, and the details are often not publicly documented, making them very easily become e-waste far sooner than x86/PC's legacy of backwards compatibility and (relative) openness. This is because on reset, the system is in a “special” Real Mode, where the first 12 address lines are asserted. To be precise, it's in real mode but with the descriptor caches loaded such that the physical address generated is at FFFFFFF0. http://www.rcollins.org/ddj/Aug98/Aug98.html http://www.rcollins.org/ddj/Aug98/Aug98.html These 12 address lines remain asserted until a long JMP is executed, after which they are de-asserted and normal Real Mode addressing calculations resume. ...because such a jump reloads the segment descriptor caches with real-mode-compatible values. Also from a comment there: Prior to UEFI, protected mode and long mode were not enabled by firmware but rather by the OS loader. All BIOSes since the 286 PC/AT will enter protected mode shortly after the reset vector. UEFI boot is what starts the OS in protected mode, while legacy/compatibility boot returns back to realmode before executing the boot sector.
saagarjha 4y ago
See also https://news.ycombinator.com/item?id=3559674 https://news.ycombinator.com/item?id=3559674, which has a reference to this in its comments.
- sixstringtheory 4y ago
  That doesn’t seem like the right link…
  - asicsp 4y ago
    Missing number at the end? They probably meant https://news.ycombinator.com/item?id=35596748 https://news.ycombinator.com/item?id=35596748 with the comment being https://news.ycombinator.com/item?id=35598629 https://news.ycombinator.com/item?id=35598629 >There's a very detailed but less emotional writeup on the same topic from 2018, highly recommended: https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-processor-boot/ https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-...
    - saagarjha 4y ago
      Yes, sorry. Mobile woes…
efitz 4y ago
Where are the microcode patches persistently stored and loaded from? The article didn’t go into much depth there.

49 points by smikhanov 4y ago

What an ironic choice of header image --- phone SoCs vary widely in how they boot, and the details are often not publicly documented, making them very easily become e-waste far sooner than x86/PC's legacy of backwards compatibility and (relative) openness. This is because on reset, the system is in a “special” Real Mode, where the first 12 address lines are asserted. To be precise, it's in real mode but with the descriptor caches loaded such that the physical address generated is at FFFFFFF0. http://www.rcollins.org/ddj/Aug98/Aug98.html http://www.rcollins.org/ddj/Aug98/Aug98.html These 12 address lines remain asserted until a long JMP is executed, after which they are de-asserted and normal Real Mode addressing calculations resume. ...because such a jump reloads the segment descriptor caches with real-mode-compatible values. Also from a comment there: Prior to UEFI, protected mode and long mode were not enabled by firmware but rather by the OS loader. All BIOSes since the 286 PC/AT will enter protected mode shortly after the reset vector. UEFI boot is what starts the OS in protected mode, while legacy/compatibility boot returns back to realmode before executing the boot sector.

saagarjha 4y ago

See also https://news.ycombinator.com/item?id=3559674 https://news.ycombinator.com/item?id=3559674, which has a reference to this in its comments.

sixstringtheory 4y ago
That doesn’t seem like the right link…
- asicsp 4y ago
  Missing number at the end? They probably meant https://news.ycombinator.com/item?id=35596748 https://news.ycombinator.com/item?id=35596748 with the comment being https://news.ycombinator.com/item?id=35598629 https://news.ycombinator.com/item?id=35598629 >There's a very detailed but less emotional writeup on the same topic from 2018, highly recommended: https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-processor-boot/ https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-...
  - saagarjha 4y ago
    Yes, sorry. Mobile woes…

efitz 4y ago

Where are the microcode patches persistently stored and loaded from? The article didn’t go into much depth there.