dsp56800 Freescale Semiconductor, Inc, dsp56800 Datasheet - Page 179

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

Figure 7-5 demonstrates the interrupt pipeline. The point at which interrupts are re-enabled and subsequent
interrupts are allowed is shown to illustrate the non-interruptible nature of the early instructions in the long
interrupt service routine.
Reset is a special exception, which will normally contain only a JMP instruction at the exception start
address.
There is only one case in which the stacked address will not point to the illegal instruction. If the illegal
instruction follows an REP instruction (see Figure 7-6), the processor will effectively execute the illegal
instruction as a repeated NOP, and the interrupt vector will then be inserted in the pipeline. In this
illustration, the first instruction (n7 in Figure 7-6) following an illegal instruction (n6) is lost as a
consequence of the illegal opcode. The second instruction following an illegal instruction will be the next
instruction that will be fetched, decoded, and executed normally (n8).
Freescale Semiconductor
i = Interrupt
ii = Interrupt Instruction Word
n = Normal Instruction Word
Interrupt Control Cycle 1
Interrupt Control Cycle 2
Fetch
Decode
Execute
Instruction Cycle Count
Synchronized
Recognized
as Pending
Interrupt
and
Program
n1
1
i
Main
n1
n2
n2
n1 JSR JSR JSR JSR
2
i
(a) Instruction Fetches from Memory
n1
—
3
Figure 7-5. Interrupt Service Routine
Interrupt Synchronized and
Interrupts and the Processing States
(b) Program Controller Pipeline
Recognized as Pending
JSR JSR JSR JSR
Adr
4
(Should Be RTI)
—
5
Return From
Jump Address
Interrupt
Vector Table
Explicit
ii2
6
Interrupt
JSR
ii3
ii2
7
ii4
ii3
ii2
8
ii5
ii4
ii3
9
10
iin RTI
ii5
ii4
Interrupts Re-enabled
11
iin
ii5
Subroutine
Interrupt
RTI RTI RTI RTI RTI n2
Interrupt
Routine
12
—
iin
RTI
ii2
ii3
ii4
iin
RTI RTI RTI RTI RTI n2
13
—
Exception Processing State
14
—
15
—
PC Resumes
Operation
Interrupts
Re-enabled
n2
16
17
—
AA0069
18
—
—
7-15