Minimal STM32 F4 task sheduler
NanoOS: "Make less makes no sense" =)
Register frame description
/*
* ABI: r0..r3 are caller-saved (scratch registers), R4..R12 are callee-saved.
* It is appropriate to use R12 for a system call opcode (saved by NVIC). The
* stack pointer points to the current extent of the stack -- it is decremented
* before being used as index in a store. The stack grows downwards, to lower
* addresses. When an interrupt is processed, 8 registers are stored. LR is set
* to a special value that makes an ordinary function return into a return from
* interrupt. The LR value indicates which stack is going to be used (process
* or main) and can be modified before return.
*
* ____________________
* Stack | |
* | |
* higher | R4 | <-- SP saved in TCB (64B context)
* addresses | R5 | ^
* | ^ | R6 | |
* | | | R7 | | 8 registers pushed by handler:
* | | | R8 | | R4..R11
* | | | R9 | | Full task context is now stored
* V | | R10 | |
* | | R11 | |
* direction | R0 | <-- SP when SVC handler gets control
* of growth | R1 | ^
* | R2 | |
* | R3 | | 8 registers are pushed by
* | R12 | | the NVIC hardware:
* | LR (R14) | | xPSR, PC, LR, R12, R3..R0
* | PC (R15) | |
* | xPSR | |
* | | <-- SP before SVC
* | (stuff) |
* Stack + | |
* StackSize |____________________|
*
*/