Simultaneous Write/Read to on chip dual port RAM

From the cyclone II datasheet (same rules apply for other families):

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Mixed-Port Read-During-Write Mode

This mode applies to a RAM in simple or true dual-port mode, which has

one port reading and the other port writing to the same address location

with the same clock.

In this mode, you also have two output choices: old data or don't care. In

Old Data Mode, a read-during-write operation to different ports causes

the RAM outputs to reflect the old data at that address location. In Don't

Care Mode, the same operation results in a "don't care" or unknown value

on the RAM outputs.

--- Quote End ---

There are accompanying waveforms as well. Unknown means unknown folks. It does not mean "New Data." If you are going to write and read from the same address at the same time, it had better be because you "Don't Care" what the read data is.

Jake