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MemoryAccessFlow.md

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Memory access flow

The following is the chain of operations performed by IZ80Processor whenever memory is read, either because opcodes are being fetched or as part of the processing of an instruction.

  1. The MemoryAccess event is triggered with MemoryAccessEventArgs.EventType equal to BeforeMemoryRead and Value equal to FFh. The code handling the event has the opportunity to set MemoryAccessEventArgs.CancelMemoryAccess to true and/or change the value.
  2. If the configured memory access mode for the affected address is ReadAndWrite or ReadOnly, and the CancelMemoryAccess property of the event has not been set to true, the value is read by accessing Memory[address]. Otherwise, the value set on the previous step is used as if it was actually obtained from memory.
  3. The MemoryAccess event is triggered with MemoryAccessEventArgs.EventType equal to AfterMemoryRead. The code handling the event has the opportunity to change the value.
  4. The value in MemoryAccessEventArgs.Value is assumed to be the obtained value and is processed appropriately.

As for the operations performed for a memory write, they are the following:

  1. The MemoryAccess event is triggered with MemoryAccessEventArgs.EventType equal to BeforeMemoryWrite. The code listening the event has the opportunity to either change the value to be written or set MemoryAccessEventArgs.CancelMemoryAccess to true.
  2. If the configured memory access mode for the affected address is ReadAndWrite or WriteOnly, and the CancelMemoryAccess property of the event has not been set to true, the value set on the previous step is written to Memory[address].
  3. The MemoryAccess event is triggered with MemoryAccessEventArgs.EventType equal to AfterMemoryWrite.

The code of the processor class takes care of the extra wait states needed for timing purposes.

The flow for accessing I/O ports is similar, but PortsSpace is used instead of Memory and the 'Port' members of MemoryAccessEventArgs.EventType are used instead of the 'Memory' members.

Note that MemoryAccessEventArgs inherits from ProcessorEventArgs, which defines the LocalUserState property. This property is propagated from the 'before' event to the 'after' event and can be used by the events handling code at its convenience.