Memory Elements

• Since we are moving quickly towards an implementation, I would like to look at memory elements.
• You should read B.1 - B.3.
  • We should be mostly done with this. We will not cover PLAs, but you should give this section a read.
• I will skip B.4 Verilog
• We will come back to B.5 and B.6 as needed.
• We have Discussed B.7
• This is B.8
• The basic unit of CPU memory is a flip-flop
  • A flip-flop stores a single bit.
  • Flip-flops are sometimes referred to as latches as well, but we will use that term differently later.
  • There different types of flip-flops, but we will look at two.
  • Flip-flops are built from transistors.
• RS Flip-Flop
  • Two nor gates
  • Two inputs
  • Two outputs
  • If the S input is raised, the value of the flip-flop is set.
  • If the R input is raised, the value of the flip-flop is reset.
  • Undefined if both are raised at the same time.
  • The output is the value and not the value. (Q and Q )
  • 
• More complex flip-flops provide further functionality.
• The D flip-flop
  • Has two inputs
    • A data line D
    • A clock line C
  • has two outputs
    • Q
    • Q
  • When the clock changes from low to high, the value on D will be stored in the flip-flop
  • 
• In TKgate the D flip-flop has two additional ports
  • A clear-not, clears the flip-flop when the data is 0
  • Enable-not, enables the flip-flop when the data is 0
  • 
• All are here
• A register, then is just a collection of flip-flops. A
  • it should have a Clock and Data input line
  • And possibly an output line
  • Also clear and enable.
  • 
• How should the MIPS register file work?
• Build one