dsp56800 Freescale Semiconductor, Inc, dsp56800 Datasheet - Page 180

• Manufacturer Part Number: dsp56800
• Description: 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

Interrupts and the Processing States
In DO loops, if the illegal instruction is in the loop address (LA) location and the instruction preceding it
(that is, at LA-1) is being interrupted, the loop counter (LC) will be decremented as if the loop had reached
the LA instruction. When the interrupt service ends and the instruction flow returns to the loop, the
instruction after the illegal instruction will be fetched (since it is the next sequential instruction in the
flow).
7.3.8
Interrupt latency represents the time between when an interrupt request first appears and when the first
instruction in an interrupt service routine is actually executed. The interrupt can only take place on
instruction boundaries, and so the length of execution of an instruction affects interrupt latency.
There are some special cases to consider. The SWI, STOP, and WAIT instructions are not interruptible.
Likewise, the REP instruction and the instruction it repeats are not interruptible.
A REP instruction and the instruction that follows it are treated as a single two-word instruction, regardless
of how many times it repeats the second instruction of the pair. Instruction fetches are suspended and will
be reactivated only after the LC is decremented to one (see Figure 7-7). During the execution of n2 in
Figure 7-7, no interrupts will be serviced. When LC finally decrements to one, the fetches are re-initiated,
and pending interrupts can be serviced.
7-16
i = Interrupt
ii = Interrupt Instruction Word
II = Illegal Instruction
n = Normal Instruction Word
Interrupt Control Cycle 1
Interrupt Control Cycle 2
Fetch
Decode
Execute
Instruction Cycle Count
Interrupt Latency
1
n1
2
Figure 7-6. Repeated Illegal Instruction
n2
n1
3
DSP56800 Family Manual
n3
n2
n1
4
n4
n3
n2
5
REP
n4
n3
6
Illegal Instruction Interrupt
Recognized as Pending
REP
n6
n4
7
REP REP REP
n7
II
8
—
—
9
10
—
—
i
—
—
11
II
i
12
ii1
—
—
Freescale Semiconductor
13
ii2
ii1
—
n8
ii2
ii1
14
n8
15
ii2
AA0070
n8
16