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8 address lines for I/O mapped I/O

Discussion in '8051/8951' started by rameshrai, Dec 15, 2014.

  1. rameshrai

    rameshrai Member

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    hi,

    1. can anybody tell me why 8 address lines are used for I/O mapped I/O in 8085 microprocessor?
    2. 8085 has 16 address lines, and either upper A15 to A8 or A7 to A0 can be used for I/O mapped I/O, so if I used A7 to A0 for I/O mapped I/O what is the values of A15 to A8?
    3. where are opcodes stored in 8085?

    thanks
     
    Last edited: Dec 15, 2014
  2. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    First off!!! The 8085 is not a micro controller so the post is in the wrong section...
    Second..... Why are you working with such an old microprocessor??

    The 8085 is a slightly newer chip than the 8080 and has the same functionality as the Z80 so the IN and OUT commands control the address of the I/O according to the manual....

    The opcodes (commands) are normally stored in code space... Usually an Eeprom attached to the data and address buses...

    I haven't studied the 8085 so I don't know why you can use the high address byte ( I didn't think you could )...
     
  3. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    It has been a long time! My memory ..............
    Most 8085 boards only had 1,2 or three I/O addresses in use. (serial, parallel) I cannot think of a case where any one would need all 256 addresses. It takes MORE parts to decode 64000 addresses. (money) If you made memory mapped (16 bit I/O addressing) then there will be a hole in the memory addressing.
    ALSO
    Instructions: The CPU is 8 bit. So thinking 'do something at I/O at 5' is simple. If you need to do a 16 bit address for every I/O the CPU has a hard time: Get the high 1/2 of a address and put it in a register pair, get the lower 1/2 of a address and put it in a register pair, now do a I/O using the register pair as a pointer.

    The CPU is 8 bit. If you want fast math you do it in 8 bits. Math in 16 bit is (much + much, test for carry, add the carry) slower.

    I know my examples are bad/funny but don't do anything in 16 bits if you can find a 8 bit way. (hardware and software)
     
  4. dave

    Dave New Member

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  5. Wade_Hassler

    Wade_Hassler New Member

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    8085 has no on-board memory. Up to 16 address-lines can be used for external memory, wherein opcodes are stored.
     

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