Introduction to Memory

• This is B.8, you should read it.
• This is called sequential logic
  • The output depends on the state of the circuit and the input.
  • This is different from the combinational logic we have been studying.
  • There the input completely determines the output.
• The basic unit is the flip-flop.
  • A flip-flop is a memory element where the output is equal to the value of the stored state.
  • This state is only changed on a clock edge.
• The basic flip-flop is the RS flip-flop
  • Two inputs,
    • R - reset
    • S - set.
    • These are probably just push buttons, not long term data lines.
  • Two outputs Q and $\overline{Q}$
  • Built from two nor gates.
  • 
  • Note, this is not a circuit you can "reason out".
• The D flip-fop is more common.
  • Two inputs
    • D - data
    • C - clock
  • Same outputs.
  • When C changes from low to high, the value of D is stored in the circuit.
  • You can buy D Flip-Flops here for example.
• Registers
  • The most basic register is a collection of flip-flops.
  • They have input like
    • An n bit data line.
    • A clock line
    • An enable line
  • The output is a n-bit data line.
  • The operation is
    • Place the data on the data line and allow it to stabilize.
    • Raise the enable line
    • Raise the clock line
    • Keep all raised until the store completes.
  • 
  • You can purchase registers.
  • And they can be much more complex, with more tasks.
• The register file
• Look at Any CPU diagram
  • Input
    • Read Register 1
    • Read Register 2
    • Write Register
    • Data Write
  • Output:
    • ReadData 1
    • ReadData 2
  • Control: RegWrite - actually write data to the register.
• Let's come up with an instruction that will use each of these.
  • add $r1, $r2, $r3?
• Read Register1 and ReadRegister2 select data to put on the corresponding output lines.
  • Since there are 32 registers, we need 5 bits. But we knew this.
• ReadData 1 and 2 are 32 bit lines as they hold data.
• Write Register is again a 5 bit line as it is a register address.
• WriteData is a 32 bit line as it holds data.
• RegWrite is a single bit, which is lit when all is ready to write to a register.
• A digital file of these circuits.