Renesas SuperH SH-4A, Software Manual

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Rev. 1.50, 10/04, page 73 of 448
5.5.2 Exception Source Acceptance
A priority ranking is provided for all exceptions for use in determining which of two or more
simultaneously generated exceptions should be accepted. Five of the general exceptions—general
illegal instruction exception, slot illegal instruction exception, general FPU disable exception, slot
FPU disable exception, and unconditional trap exception—are detected in the process of
instruction decoding, and do not occur simultaneously in the instruction pipeline. These
exceptions therefore all have the same priority. General exceptions are detected in the order of
instruction execution. However, exception handling is performed in the order of instruction flow
(program order). Thus, an exception for an earlier instruction is accepted before that for a later
instruction. An example of the order of acceptance for general exceptions is shown in figure 5.2.
I1
I1
ID
ID
E3
WB
WB
TLB miss (data access) Pipeline flow:
Order of detection:
Instruction n
Instruction n + 1
General illegal instruction exception (instruction n + 1) and
TLB miss (instruction n + 2) are detected simultaneously
Order of exception handling:
TLB miss (instruction n)
Program order
1
Instruction n + 2
General illegal instruction exception
I1
ID WB
I2 ID WB
TLB miss (instruction access)
2
3
4
I1, I2: Instruction fetch
ID : Instruction decode
E1, E2, E3: Instruction execution
(E2, E3 Memory access)
WB Write-back
Instruction n + 3
TLB miss (instruction n)
Re-execution of instruction n
General illegal instruction exception
(instruction n + 1)
Re-execution of instruction n + 1
TLB miss (instruction n + 2)
Re-execution of instruction n + 2
Execution of instruction n + 3
[Legend]
E3
E3
E2
E2 E1
E1 I2
I2
E1
E1 E2
E2
E3
I2
I1
Figure 5.2 Example of General Exception Acceptance Order