Two directional memory blocks

• supporting bidirectional transfers on low level IO devices takes considerable amount of resources and is mostly unnecessary. It also slows down the communication.

• use only for testers, configuration tools, etc. Do not use for normal IO communication, reliable recovery from exception situations gets complicated. Stick with one directional memory blocks.

• Both source and target buffers exist memory block in higher and in lower level.

• Source and target buffer tracks invalidated addresses at byte precision. This is done by using extra memory for mark bits.

• Invalidated data is transferred, again at byte precision.

• Received changes coming from lower hierarchy level are not echoed back down to same memory block.