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Service Calls from a Handler That Caused an Interrupt during Handler Execution 
Let us think of a situation in which an interrupt occurs during handler execution (this handler is hereinafter referred to as 
handler A for explanation purposes). When task switching is called for as a handler (hereinafter referred to as handler B) 
that caused an interrupt during handler A execution issued a service call, task switching does not take place during the exe-
cution of the service call (ret_int service call) returned from handler B, but is effected by the ret_int service call from han-
dler A (See Figure 3.13). 

 

ret_int 

ret_int 

iset_flg 

Interrupt 

Interrupt 

TaskA 

Interrupt handler A 

Save Registers 

Service call processing

OS 

Restore Register

SP <= User 

Restore Registers

Task selection 

TaskB 

Save Registers 

SP <= System 

Interrupt handler A 

Restore Register