View source for KThreadContext

[[Category:Kernel objects]]

Size : 0x1000 bytes (1 page)

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


{| class="wikitable" border="1"
|-
!  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 [[Memory_layout|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.


{| class="wikitable" border="1"
|-
!  Offset
!  Type
!  Description
|-
| 0xF38 (page_end - 0xC8)
| u32[4]
| SVC access control data copied from KThread's owner process
|-
| 0xF48 (page_end - 0xB8)
| u8[4]
| Debug info (read by the SVC handler): 
0: Bit0: reschedule.  Bit1: switch context. If non zero, and depending on <code>currentThread->shallTerminate</code>, send thread exit debug events after handling valid/authorized SVCs (except 0xFF). Cleared to zero after handling. Bit2: lock debug features

1: "Allow debug" flag from the process's exheader. If set, <code>{r0-r7, r12, lr_svc}</code> will be pushed (they'll always be when handling invalid/unauthorized SVCs/SVC 0xFF). Needed for svcGetThreadContext and svcSetThreadContext to work properly in contexts other than exception handling (incl. hardware breakpoints). If lr_svc is set to one in this frame, the registers are reloaded from it (used with svcBreak, svc 0xff, etc).

2: ?

3: ID of the SVC being handled (reset to 0 when done).
|-
| 0xF50 (page_end - 0xB0)
| u32[10]
| SVC mode registers, r4-r11, r13, r14
|-
| 0xF78 
| f64[16]
| VFP registers aliased as 16 double precision, 64-bit registers
|-
| 0xFF8
| u32
| FPEXC, floating point exception register for thread- stored and loaded on context switches
|-
| 0xFFC
| u32
| FPSCR, floating point status and control register
|}

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