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Labware/Lab2_4C123/osasm.s

+;/*****************************************************************************/
+; OSasm.s: low-level OS commands, written in assembly                       */
+; Runs on LM4F120/TM4C123/MSP432
+; Lab 2 starter file
+; February 10, 2016
+;
+
+
+        AREA |.text|, CODE, READONLY, ALIGN=2
+        THUMB
+        REQUIRE8
+        PRESERVE8
+
+        EXTERN  RunPt            ; currently running thread
+        EXPORT  StartOS
+        EXPORT  SysTick_Handler
+        IMPORT  Scheduler
+
+
+SysTick_Handler                ; 1) Saves R0-R3,R12,LR,PC,PSR
+    CPSID   I                  ; 2) Prevent interrupt during switch
+    PUSH    {R4-R11}           ; 3) Save remaining regs r4-11
+    LDR     R0, =RunPt         ; 4) R0=pointer to RunPt, old thread
+    LDR     R1, [R0]           ;    R1 = RunPt
+    STR     SP, [R1]           ; 5) Save SP into TCB
+    PUSH    {R0,LR}
+    BL      Scheduler
+    POP     {R0,LR}
+    LDR     R1, [R0]           ; 6) R1 = RunPt, new thread
+    LDR     SP, [R1]           ; 7) new thread SP; SP = RunPt->sp;
+    POP     {R4-R11}           ; 8) restore regs r4-11
+    CPSIE   I                  ; 9) tasks run with interrupts enabled
+    BX      LR                 ; 10) restore R0-R3,R12,LR,PC,PSR
+
+StartOS
+    LDR     R0, =RunPt         ; currently running thread
+    LDR     R2, [R0]           ; R2 = value of RunPt
+    LDR     SP, [R2]           ; new thread SP; SP = RunPt->stackPointer;
+    POP     {R4-R11}           ; restore regs r4-11
+    POP     {R0-R3}            ; restore regs r0-3
+    POP     {R12}
+    ADD     SP,SP, #4          ; discard LR from initial stack
+    POP     {LR}               ; start location
+    ADD     SP,SP, #4          ; discard PSR
+    CPSIE   I                  ; Enable interrupts at processor level
+    BX      LR                 ; start first thread
+
+    ALIGN
+    END