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1 # Context switch
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2 #
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3 # void swtch(struct context **old, struct context *new);
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4 #
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5 # Save current register context in old
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6 # and then load register context from new.
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7 # The stack is as r4_svc-r12_svc, lr_svc, sp_usr, lr_usr, and pc_usr
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8 .global swtch
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9
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10 swtch:
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11 STMFD r13!, {r4-r12, lr} // push svc r4-r12, lr to the stack
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12
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13 # switch the stack
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14 STR r13, [r0] // save current sp to the old PCB (**old)
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15 MOV r13, r1 // load the next stack
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16
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17 # load the new registers. pc_usr is not restored here because
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18 # LDMFD^ will switch mode if pc_usr is loaded. We just simply
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19 # pop it out as pc_usr is saved on the stack, and will be loaded
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20 # when we return from kernel to user space (swi or interrupt return)
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21
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22 LDMFD r13!, {r4-r12, lr} // pop svc r4-r12, lr
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23
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24 # return to the caller
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25 bx lr
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