NS32FV16 Datasheet, PDF (31/104 Pages) Texas Instruments – Advanced Imaging/Communication Signal Processors

3 0 Functional Description (Continued)
te ^{FIGURE 3-4 Interrupt Dispatch and Cascade Tables}
TL EE 11267 – 13
3 2 2 Returning from an Exception Service Procedure
To return control to an interrupted program one of two in-
structions can be used RETT (Return from Trap) and RETI
(Return from Interrupt)
le ^{RETT is used to return from any trap or non-maskable inter-}
rupt service procedure Since some traps are often used
deliberately as a call mechanism for supervisor mode proce-
dures RETT can also adjust the Stack Pointer (SP) to dis-
card a specified number of bytes from the original stack as
Obso ^{surplus parameter space}
RETI is used to return from a maskable interrupt service
procedure A difference of RETT RETI also informs the on-
chip ICU as well as any external interrupt control logic that
interrupt service has completed Since interrupts are gener-
ally asynchronous external events RETI does not discard
parameters from the stack
Both of the above instructions always restore the Program
Counter (PC) and the Processor Status Register from the
interrupt stack If the Direct-Exception mode is disabled
they also restore the MOD and SB register contents Fig-
ures 3-7 and 3-8 show the RETT and RETI instruction flows
when the Direct-Exception mode is disabled
30