Electronic – How many external devices can be linked to a CPU with n address buses and m data buses

addressingbuscpu

(Hypothetical)
Let's say there is a CPU with 8 address lines and 8 data lines.
If each external device has 128 accessible registers, how many external devices can be linked?

For 8 address lines, I assumed 2^8 = 256 addressable locations.A nd then 256*8 (dl) = 2048.
2048 / 128 (R) = 16 external devices . Is this correct thinking?

Edit : question is: if each external device has 128 accessible registers of control and data. No mentions on bits

Best Answer

You have correctly calculated that with 8 address lines and 8 data lines you can uniquely address 2048 bits.

However, your question is vague in that it uses the word register without any explanation of how many bits are in a register. You must know that to answer the question. As DaveTweed pointed out, your conclusion is only valid if each register is just one bit. If the registers have more bits than one then your answer would be different.

Related Solutions

Electronic – How do registers connect to CPU buses

In some device technologies, registers are connected to a bus using three-state outputs. Such an approach does have some advantages, but it generally either requires that either there be some "dead time" between the moment one register releases the bus and the moment another register starts driving it, or else runs the risk that a device might start driving the bus before the previous device has fully released it.

In other technologies, this approach is avoided in favor of using nested multiplexers. If there are 64 registers that can output to a bus bit, the device might have eight 8-way multiplexers each of which accepts input from one register, and one more 8-way multiplexer which accepts input from one of the first eight. While this may use slightly more circuitry than the bus-based approach, it has the advantage that every signal throughout the system will be driven by exactly one device at all times.

Electronic – Implementing multiplication

At minimum, you need a branching instruction which can determine whether the previous addition generated a carry. Also, performance will be improved enormously if you expand your register set to three or four registers.

Start with one operand in R0 and the other in R1. Start with R2 zero.

Repeat the following sequence eight times (first ADD may be skipped on first pass)

  Add R2,R2
  Add R0,R0
  Bnc Skip
  Add R2,R1
Skip:

Note that if the instruction set includes an add-with-carry instruction, using it for the second instruction would cause this instruction sequence to yield a 16-bit result in R0:R2. Further, if R2 starts non-zero, its value times 256 will be added to the outgoing result.

Best Answer

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Electronic – How do registers connect to CPU buses

Electronic – Implementing multiplication

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