Diff: source/asm/CortexContextSwitch.s

Revision:

14:6dcf65f61a11

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/source/asm/CortexContextSwitch.s	Fri Apr 08 15:18:44 2016 +0100
@@ -0,0 +1,293 @@
+; The MIT License (MIT)
+
+; Copyright (c) 2016 British Broadcasting Corporation.
+; This software is provided by Lancaster University by arrangement with the BBC.
+
+; Permission is hereby granted, free of charge, to any person obtaining a
+; copy of this software and associated documentation files (the "Software"),
+; to deal in the Software without restriction, including without limitation
+; the rights to use, copy, modify, merge, publish, distribute, sublicense,
+; and/or sell copies of the Software, and to permit persons to whom the
+; Software is furnished to do so, subject to the following conditions:
+
+; The above copyright notice and this permission notice shall be included in
+; all copies or substantial portions of the Software.
+
+; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+; THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+; FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+; DEALINGS IN THE SOFTWARE.
+
+   AREA asm_func, CODE, READONLY
+
+; Export our context switching subroutine as a C function for use in mbed
+    EXPORT swap_context
+    EXPORT save_context
+    EXPORT save_register_context
+    EXPORT restore_register_context
+
+    ALIGN
+
+; R0 Contains a pointer to the TCB of the fibre being scheduled out.
+; R1 Contains a pointer to the TCB of the fibre being scheduled in.
+; R2 Contains a pointer to the base of the stack of the fibre being scheduled out.
+; R3 Contains a pointer to the base of the stack of the fibre being scheduled in.
+
+swap_context
+
+    ; Write our core registers into the TCB
+    ; First, store the general registers
+
+    ; Skip this is we're given a NULL parameter for the TCB
+    CMP     R0, #0
+    BEQ     store_context_complete
+
+    STR     R0, [R0,#0]
+    STR     R1, [R0,#4]
+    STR     R2, [R0,#8]
+    STR     R3, [R0,#12]
+    STR     R4, [R0,#16]
+    STR     R5, [R0,#20]
+    STR     R6, [R0,#24]
+    STR     R7, [R0,#28]
+
+    ; Now the high general purpose registers
+    MOV     R4, R8
+    STR     R4, [R0,#32]
+    MOV     R4, R9
+    STR     R4, [R0,#36]
+    MOV     R4, R10
+    STR     R4, [R0,#40]
+    MOV     R4, R11
+    STR     R4, [R0,#44]
+    MOV     R4, R12
+    STR     R4, [R0,#48]
+
+    ; Now the Stack and Link Register.
+    ; As this context is only intended for use with a fiber scheduler,
+    ; we don't need the PC.
+    MOV     R6, SP
+    STR     R6, [R0,#52]
+    MOV     R4, LR
+    STR     R4, [R0,#56]
+
+store_context_complete
+    ; Finally, Copy the stack. We do this to reduce RAM footprint, as stack is usually very small at the point
+    ; of scheduling, but we need a lot of capacity for interrupt handling and other functions.
+
+    ; Skip this is we're given a NULL parameter for the stack.
+    CMP     R2, #0
+    BEQ     store_stack_complete
+
+    LDR     R4, [R0,#60]         ; Load R4 with the fiber's defined stack_base.
+store_stack
+    SUBS    R4, #4
+    SUBS    R2, #4
+
+    LDR     R5, [R4]
+    STR     R5, [R2]
+
+    CMP     R4, R6
+    BNE     store_stack
+
+store_stack_complete
+
+    ;
+    ; Now page in the new context.
+    ; Update all registers except the PC. We can also safely ignore the STATUS register, as we're just a fiber scheduler.
+    ;
+    LDR     R4, [R1, #56]
+    MOV     LR, R4
+    LDR     R6, [R1, #52]
+    MOV     SP, R6
+
+    ; Copy the stack in.
+    ; n.b. we do this after setting the SP to make comparisons easier.
+
+    ; Skip this is we're given a NULL parameter for the stack.
+    CMP     R3, #0
+    BEQ     restore_stack_complete
+
+    LDR     R4, [R1,#60]         ; Load R4 with the fiber's defined stack_base.
+
+restore_stack
+    SUBS    R4, #4
+    SUBS    R3, #4
+
+    LDR     R5, [R3]
+    STR     R5, [R4]
+
+    CMP     R4, R6
+    BNE     restore_stack
+
+restore_stack_complete
+    LDR     R4, [R1, #48]
+    MOV     R12, R4
+    LDR     R4, [R1, #44]
+    MOV     R11, R4
+    LDR     R4, [R1, #40]
+    MOV     R10, R4
+    LDR     R4, [R1, #36]
+    MOV     R9, R4
+    LDR     R4, [R1, #32]
+    MOV     R8, R4
+
+    LDR     R7, [R1, #28]
+    LDR     R6, [R1, #24]
+    LDR     R5, [R1, #20]
+    LDR     R4, [R1, #16]
+    LDR     R3, [R1, #12]
+    LDR     R2, [R1, #8]
+    LDR     R0, [R1, #0]
+    LDR     R1, [R1, #4]
+
+    ; Return to caller (scheduler).
+    BX      LR
+
+
+; R0 Contains a pointer to the TCB of the fibre to snapshot
+; R1 Contains a pointer to the base of the stack of the fibre being snapshotted
+
+save_context
+
+    ; Write our core registers into the TCB
+    ; First, store the general registers
+
+    STR     R0, [R0,#0]
+    STR     R1, [R0,#4]
+    STR     R2, [R0,#8]
+    STR     R3, [R0,#12]
+    STR     R4, [R0,#16]
+    STR     R5, [R0,#20]
+    STR     R6, [R0,#24]
+    STR     R7, [R0,#28]
+
+    ; Now the high general purpose registers
+    MOV     R4, R8
+    STR     R4, [R0,#32]
+    MOV     R4, R9
+    STR     R4, [R0,#36]
+    MOV     R4, R10
+    STR     R4, [R0,#40]
+    MOV     R4, R11
+    STR     R4, [R0,#44]
+    MOV     R4, R12
+    STR     R4, [R0,#48]
+
+    ; Now the Stack and Link Register.
+    ; As this context is only intended for use with a fiber scheduler,
+    ; we don't need the PC.
+    MOV     R6, SP
+    STR     R6, [R0,#52]
+    MOV     R4, LR
+    STR     R4, [R0,#56]
+
+    ; Finally, Copy the stack. We do this to reduce RAM footprint, as stackis usually very small at the point
+    ; of sceduling, but we need a lot of capacity for interrupt handling and other functions.
+
+    LDR     R4, [R0,#60]         ; Load R4 with the fiber's defined stack_base.
+
+store_stack1
+    SUBS    R4, #4
+    SUBS    R1, #4
+
+    LDR     R5, [R4]
+    STR     R5, [R1]
+
+    CMP     R4, R6
+    BNE     store_stack1
+
+    ; Restore scratch registers.
+
+    LDR     R7, [R0, #28]
+    LDR     R6, [R0, #24]
+    LDR     R5, [R0, #20]
+    LDR     R4, [R0, #16]
+
+    ; Return to caller (scheduler).
+    BX      LR
+
+
+; R0 Contains a pointer to the TCB of the fiber to snapshot
+save_register_context
+
+    ; Write our core registers into the TCB
+    ; First, store the general registers
+
+    STR     R0, [R0,#0]
+    STR     R1, [R0,#4]
+    STR     R2, [R0,#8]
+    STR     R3, [R0,#12]
+    STR     R4, [R0,#16]
+    STR     R5, [R0,#20]
+    STR     R6, [R0,#24]
+    STR     R7, [R0,#28]
+
+    ; Now the high general purpose registers
+    MOV     R4, R8
+    STR     R4, [R0,#32]
+    MOV     R4, R9
+    STR     R4, [R0,#36]
+    MOV     R4, R10
+    STR     R4, [R0,#40]
+    MOV     R4, R11
+    STR     R4, [R0,#44]
+    MOV     R4, R12
+    STR     R4, [R0,#48]
+
+    ; Now the Stack Pointer and Link Register.
+    ; As this context is only intended for use with a fiber scheduler,
+    ; we don't need the PC.
+    MOV     R4, SP
+    STR     R4, [R0,#52]
+    MOV     R4, LR
+    STR     R4, [R0,#56]
+
+    ; Restore scratch registers.
+    LDR     R4, [R0, #16]
+
+    ; Return to caller (scheduler).
+    BX      LR
+
+
+restore_register_context
+
+    ;
+    ; Now page in the new context.
+    ; Update all registers except the PC. We can also safely ignore the STATUS register, as we're just a fiber scheduler.
+    ;
+    LDR     R4, [R0, #56]
+    MOV     LR, R4
+    LDR     R4, [R0, #52]
+    MOV     SP, R4
+
+    ; High registers...
+    LDR     R4, [R0, #48]
+    MOV     R12, R4
+    LDR     R4, [R0, #44]
+    MOV     R11, R4
+    LDR     R4, [R0, #40]
+    MOV     R10, R4
+    LDR     R4, [R0, #36]
+    MOV     R9, R4
+    LDR     R4, [R0, #32]
+    MOV     R8, R4
+
+    ; Low registers...
+    LDR     R7, [R0, #28]
+    LDR     R6, [R0, #24]
+    LDR     R5, [R0, #20]
+    LDR     R4, [R0, #16]
+    LDR     R3, [R0, #12]
+    LDR     R2, [R0, #8]
+    LDR     R0, [R0, #0]
+    LDR     R1, [R0, #4]
+
+    ; Return to caller (normally the scheduler).
+    BX      LR
+
+    ALIGN
+    END
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