NS32CG16-10 Datasheet, PDF (28/82 Pages) National Semiconductor (TI) – High-Performance Printer / Display Processor

3 0 Functional Description (Continued)
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)
RETT is used to return from any trap or a non-maskable
interrupt service procedure Since some traps are often
used deliberately as a call mechanism for supervisor mode
procedures RETT can also adjust the Stack Pointer (SP) to
discard a specified number of bytes from the original stack
as surplus parameter space
RETI is used to return from a maskable interrupt service
procedure A difference of RETT RETI also informs any
external interrupt control units that interrupt service has
completed Since interrupts are generally asynchronous ex-
ternal events RETI does not discard parameters from the
stack
Both of the above instructions always restore the PSR
MOD PC and SB registers to their previous contents
3 2 3 Maskable Interrupts
The INT pin is a level-sensitive input A continuous low level
is allowed for generating multiple interrupt requests The in-
put is maskable and is therefore enabled to generate inter-
rupt requests only while the Processor Status Register I bit
is set The I bit is automatically cleared during service of an
INT or NMI request and is restored to its original setting
upon return from the interrupt service routine via the RETT
or RETI instruction
The INT pin may be configured via the SETCFG instruction
as either Non-Vectored (CFG Register bit Ie0) or Vectored
(bit Ie1)
3 2 3 1 Non-Vectored Mode
In the Non-Vectored mode an interrupt request on the INT
pin will cause an Interrupt Acknowledge bus cycle but the
CPU will ignore any value read from the bus and use instead
a default vector of zero This mode is useful for small sys-
tems in which hardware interrupt prioritization is unneces-
sary
FIGURE 3-6 Return from Trap (RETT n) Instruction Flow
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