3.3.4. Merged I-S cycles

Where possible, the processor performs an optimization on the bus to allow extra time for memory decode. When this happens, the address of the next memory cycle is broadcast during an internal cycle on this bus. This allows the memory controller to decode the address, but it must not initiate a memory access during this cycle. In a merged I-S cycle, the next cycle is a sequential cycle to the same memory location. This commits to the access, and the memory controller must initiate the memory access. This is shown in Figure 3.5.

Figure 3.5. Merged I-S cycle


When designing a memory controller, make sure that the design also works when an I cycle is followed by an N cycle to a different address. This sequence might occur during exceptions, or during writes to the PC. It is essential that the memory controller does not commit to the memory cycle during an I cycle.

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