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5- 14 M68040 USER’S MANUAL MOTOROLA
The following examples are for PSTx encodings:
1. An access error terminates an instruction such that the instruction execution stage is
not reached. In this case, an ‘end current instruction’ is not indicated. Exception
processing starts, the exception stacking status is indicated, and then the virtual
JMP causes the ‘supervisor, branch taken/end current instruction’ encoding.
2. An FTRAPcc that does not take an exception ending with the ‘end current
instruction’ encoding. The exception stacking status is indicated and then reaches
the ‘supervisor, branch taken/end current instruction’ encoding if the FTRAPcc ends
in an exception.
3. Two simultaneous interrupt exception processing sequences follow an ADD
instruction. The ADD instruction ends with ‘end current instruction’, followed by
exception stacking, followed by ‘branch taken/end current instruction’, followed by
exception stacking, followed by ‘branch taken/end current instruction’.
4. An RTE instruction follows an ADD instruction. The ‘end current instruction’ is
followed by RTE executing followed by a branch taken/end current instruction.
5.9.2 Bus Clock (BCLK)
This input signal is used as a reference for all bus timing. It is a TTL-compatible signal and
cannot be gated off. Refer to Section 11 MC68040 Electrical and Thermal
Characteristics for electrical specifications.
5.9.3 Processor Clock (PCLK)—Not on MC68040V and MC68EC040V
PCLK is used to derive all internal timing. This clock is also TTL compatible and cannot be
gated off. Refer to Section 11 MC68040 Electrical and Thermal Characteristics for
electrical specifications.
5.10 MMU DISABLE ( )—NOT ON MC68EC040
The MMU disable signal dynamically disables the translation of addresses by the MMUs.
The assertion of MDIS does not flush the address translation caches (ATCs); ATC entries
become available again when MDIS is negated. During a processor reset, the level on
MDIS is latched and used to select the normal data latch mode (MDIS high) or DLE mode
(MDIS low). Refer to Section 3 Memory Management Unit (Except MC68EC040 and
MC68EC040V) for a description of address translation and to Section 7 Bus Operation
for information about DLE mode.
5.11 DATA LATCH ENABLE (DLE)—ONLY ON MC68040
This input signal is used in DLE mode to latch the input data bus on read transfers. DLE
mode can be used to support asynchronous memory interfaces by allowing the interface
to specify when data should be latched instead of requiring data to be valid on the rising
edge of BCLK.
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