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So above I have a memory design including a 74LS138 3-to-8 multiplexer which enables 8 memory banks comprised of 2732s which are 4Kx8 in size.

My question is why the memory addressing starts F8000 instead of F0000?

  • 1
    $\begingroup$ Because all of A19 through A15 must be high in order to enable the decoder. That's what the big NAND gate is doing. $\endgroup$
    – Dave Tweed
    Mar 8 '16 at 23:34
  • $\begingroup$ @DaveTweed - thanks for the edit - I seem to have had a stutt0000er in my address generatio0000n unit :-). $\endgroup$ Mar 10 '16 at 14:37

Addresses are: A19 18 17 16 15 ....

This expands on Dave Tweed's correct but brief comment:

F8000 = 1111 1000 0000 0000 0000 <- A15 = 1
F0000 = 1111 0000 0000 0000 0000 <- A15 = 0

For F8000, A15 is high
For F0000, A15 is low

As Dave Tweed notes, the decoder is only selected when all inputs to the '30 NAND gate are high.
As A15 is one of the inputs it must be high for the decoder to be selected.
As A19 18 17 16 15 must be high the applicable address is F8xxx as above .

  • $\begingroup$ Thank you so much for helping. You actually replied faster than my professor to an e-mail I sent him with the same question. $\endgroup$
    – Mourneris
    Mar 9 '16 at 12:29

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