# Context switch # # void swtch(struct context **old, struct context *new); # # Save current register context in old # and then load register context from new. # The stack is as r4_svc-r12_svc, lr_svc, sp_usr, lr_usr, and pc_usr .global swtch swtch: STMFD r13!, {r4-r12, lr} // push svc r4-r12, lr to the stack # switch the stack STR r13, [r0] // save current sp to the old PCB (**old) MOV r13, r1 // load the next stack # load the new registers. pc_usr is not restored here because # LDMFD^ will switch mode if pc_usr is loaded. We just simply # pop it out as pc_usr is saved on the stack, and will be loaded # when we return from kernel to user space (swi or interrupt return) LDMFD r13!, {r4-r12, lr} // pop svc r4-r12, lr # return to the caller bx lr