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Theoretical question: Missing memory

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metertech58761:
I thought I'd take a break from tearing my hair out while trying to untangle this bit of code and ask a dumb question -

What happens if you tell the CPU to access a given address and no RAM exists at that location - not even a 'shadow copy' from partial address decoding?

The code I'm looking at was apparently used in two different models - one with the main board set alone and the other with an ADC daughterboard.

This rare (and likely now extinct) ADC daughterboard had a second EPROM, along with an additional 128 bytes of RAM (either an MC6810 or 2x HM-6561).

Jeroen3:
If the bus controller doesn't know this and throw an error it will just try to read air, I think.

DiTBho:

--- Quote from: metertech58761 on March 20, 2023, 07:57:24 am ---What happens if you tell the CPU to access a given address and no RAM exists at that location - not even a 'shadow copy' from partial address decoding?

--- End quote ---

my 68hc11 EVB board has 512-byte of internal ram, 32Kbyte of ram, 16Kbyte of ROM, 2Kbyte of expandable addresses mapped to physical devices { LCD-2x16, Eth0 }

    CPU <----> 74HC138: decode(addr)->/CS <----> device=devices(/CS)

So it has (64Kb - (512 + 32K + 16K + 2K)) addresses mapped to no device.
If I try to issue a read request to those addresses, I always get 0xff.

- -

While on my atlas mips board I always get a error bus exception.

    CPU <----> bus controller: decode(addr)->/CS, wait for /ack|bus err <----> device=devices(/CS)

When the CPU issues an address on the bus, the device on the other side of the bus has to acknowledge itself within a time limit, counted in terms of bus-cycles, 10 clock edges at 33Mhz, in my case.

metertech58761:
So in other words, given that the data bus has a pull-up on it, it should just come back as FF... makes sense.

Like I said, it was a dumb question I should have known the answer to... :)

AndyBeez:
Depends on how well 'tied' to logic 1 the bus is. You might get #FF 99.9999% of the time, but random bus noise/ringing might make the outcome less than a boolean certainty.

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