masters in the system. Both caches are organized as four-way set associative with 64
sets of four lines. Each line contains four long words for a storage capability of 4 Kbytes
for each cache (8 Kbytes total). Each cache and corresponding MMU is allocated
separate internal address and data buses, allowing simultaneous access to both. The
data cache provides write-through or copyback write modes that can be configured on a
page-by-page basis. The caches are physically mapped, reducing software support for
multitasking operating systems, and support external bus snooping to maintain cache
coherency in multimaster systems.
The bus snoop logic provides cache coherency in multimaster applications. The bus
controller executes bus transfers on the external bus and prioritizes external memory
requests from each cache. The M68040 bus controller supports a high-speed,
nonmultiplexed, synchronous, external bus interface supporting burst accesses for both
reads and writes to provide high data transfer rates to and from the caches. Additional bus
signals support bus snooping and external cache tag maintenance.
The MC68040 contains an on-chip FPU, which is user object-code compatible with the
MC68881/MC68882 floating-point coprocessors. The FPU has pipelined instruction
execution. Floating-point instructions in the FPU execute concurrently with integer
instructions in the IU.
The processor is always in one of three states: normal processing, exception processing,
or halted. It is in the normal processing state when executing instructions, fetching
instructions and operands, and storing instruction results.
Exception processing is the transition from program processing to system, interrupt, and
exception handling. Exception processing includes fetching the exception vector, stacking
operations, and refilling the instruction pipe caused after an exception. The processor
enters exception processing when an exceptional internal condition arises such as tracing
an instruction, an instruction results in a trap, or executing specific instructions. External
conditions, such as interrupts and access errors, also cause exceptions. Exception
processing ends when the first instruction of the exception handler begins to execute.
The processor halts when it receives an access error or generates an address error while
in the exception processing state. For example, if during exception processing of one
access error another access error occurs, the MC68040 is unable to complete the
transition to normal processing and cannot save the internal state of the machine. The
processor assumes that the system is not operational and halts. Only an external reset
can restart a halted processor. Note that when the processor executes a STOP
instruction, it is in a special type of normal processing state, one without bus cycles. The
processor stops, but it does not halt.
The MC68040 programming model is separated into two privilege modes: supervisor and
user. The S-bit in the status register (SR) indicates the privilege mode that the processor
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