The MIPS Register File

• We observed from figure 4.15 page 263, the register file has the followin configuration
  • 
  • Notice they prvide slightly different labels in their diagrams,
  • RS1 is Read Register 1, RS2 is Read Register 2, ...
• How does the register file operate?
• To answer this, let us take a look at a few instructions (green sheet)
  • add
    • RTN: R[rd] ← R[rs] + R[rt]
    • This is detailed on page A-51
    • add rs, rt, rd
    • Notice that this is an R type instruction
      • Green Sheet, bottom of page
      • 
      • The opcode is bits 26-31, 6 bits
      • The rs is bits 21-25, 5 bits
      • The rt is bits 16-20, 5 bits
      • The rd is bits 11-15, 5 bits
      • shamt is bits 6-10, 5 bits
      • funct is bits 0 - 5, 6 bits.
    • Uses of these fields
      • opcode, funct are opcode bits.
        • Look at figure 4.19 page 266
        • Trace the path for instruction[31-26]
        • Trace the path for instruction[5-0]
        • Look at the green sheet for add
          • OPCODE/FUNCT 0/20
          • As opposed to and 0/21
      • rs, rt are source registers for this instruction
        • Back to figure 4.15
        • These are the values for RS1 and RS2
        • So they select what is produced on RO1 and RO2
      • rd is the destination register
        • Again of figure 4.15 it is the address for the destination or results of the computation.
      • We will postpone shamt for the moment.
  • We have sufficient information to build the register file.
    • There are 32 registers, 0 through 31. (Green sheet, lower left)
    • Register 0 is special
      • Writes to register 0 do not occur
      • It contains 0
    • All others can be written to
    • Some have special purposes
    • But more on that later.
• I will build a 4 register register file
  • Because 32 registers just take too much space
  • And you get the same idea.
  • RS1 and RS2 use a mux to select from the registers for the value of r RO1 and RO2
  • Rdest provides the address for a decoder to light the clock for a selected register
    • RegWrite enables the decoder
    • But we could do this with an and as well
    • Output 0 of the decoder is discarded
      • I hooked it up to an led to keep tkgate from crashing
    • The outputs of the decoder go to the clocks of the registers
    • Except register 0, which the clock is hard wired to ground
  • Data is just the input to the registers.
  • I added a reset input to reset the entire register file.
    • This is wired to not(clear) through an inverter.
  • 
  • 
  • the tkgate file