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Revision as of 05:22, 30 August 2016 by Bond697 (talk | contribs)
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Size : 0x1000 bytes (1 page)

Each thread is allocated a 0x1000-byte page in this region for the thread context. Each thread has its context address stored as 2 members:

Offset Type Description
0x88 u32* Ptr to svc mode register storage for KThread inside the thread context.
0x8C u32* End-address of the page for this thread context allocated in the 0xFF4XX000 region.

KThread offset 0x88 points to offset 0xF50 in the thread context and 0x8C points to the end of the thread context page. The kernel refers to the thread context page by subtracting from the end instead of adding from the beginning.

Offset Type Description
0xF38 (page_end - 0xC8) u32[4] SVC access control data copied from KThread's owner process
0xF50 (page_end - 0xB0) u32[10] SVC mode registers, r4-r11, r13, r14
0xFF8 u32 FPEXC, floating point exception register for thread- stored and loaded on context switches

When switching thread contexts the kernel does, in order:

  • Load FPEXC
  • Save the LR to r1- this LR is the return back to the main scheduling and context switching function
  • Load r4-r11, SP, LR
  • Branch back to r1, preserving the LR which was just reloaded, back to the main scheduling function, but in the context of the newly switched-to thread
  • Disable Interrupts
  • Check if that core's KScheduler object indicates that it needs switching again and exit if it does not
  • Otherwise, run the thread scheduler again and repeat all the steps to switch contexts again