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Reservation Station & ALU

The reservation station & ALU handles the calculation part of instructions. It supports 13 types of instructions, including 5 arithmetic operations (addition, subtraction, XOR, OR and AND), 3 shift operations (left shift, logical right shift and arithmetic right shift), 4 comparison operations (equal operation, unequal operation, less than operation, unsigned less than operation).

Reservation Station Operators

num binary operator abbr
0 0000 add ADD
1 0001 sub SUB
2 0010 xor XOR
3 0011 or OR
4 0100 and AND
5 0101 shift left logical SLL
6 0110 shift right logical SRL
7 0111 shift right arithmetic SRA
8 1000 equal EQ
9 1001 not equal NE
10 1010 less than LT
11 1011 less than unsigned LTU
12 1100 greater than and equal GE
13 1101 greater than and equal unsigned GEU
14 1110 reserved -
15 1111 reserved -

Reservation Station Module

The Reservation Station module is located at src/reservation_station.v.

The reservation station have 16 lines of data. The number cannot be modified easily because there is some hard-coded logic in the reservation station.

The ALU section is also included in the Reservation Station Module in order to reduce the number of wires and the latency caused by the distance of wires.

The interfaces are listed below:

module ReservationStation #(
  parameter RS_OP_WIDTH = 4,
  parameter RS_WIDTH = 4,
  parameter ROB_WIDTH = 4
) (
  input wire resetIn,   // reset signal
  input wire clockIn,   // clock signal
  input wire readyIn,   // ready signal

  // Instruction Unit part
  input  wire                   addValid,    // add valid signal
  input  wire [RS_OP_WIDTH-1:0] addOp,       // add op
  input  wire [ROB_WIDTH-1:0]   addRobIndex, // add rob index
  input  wire [31:0]            addVal1,     // add value1
  input  wire                   addHasDep1,  // add value1 dependency
  input  wire [ROB_WIDTH-1:0]   addConstrt1, // add value1 constraint
  input  wire [31:0]            addVal2,     // add value2
  input  wire                   addHasDep2,  // add value2 dependency
  input  wire [ROB_WIDTH-1:0]   addConstrt2, // add value2 constraint
  output wire                   full,        // full signal
  output wire                   update,      // update signal
  output wire [ROB_WIDTH-1:0]   updateRobId, // rob index
  output wire [31:0]            updateVal,   // value

  // Load & Store Buffer part
  input  wire                 lsbUpdate,    // load & store buffer update signal
  input  wire [ROB_WIDTH-1:0] lsbRobIndex,  // load & store buffer rob index
  input  wire [31:0]          lsbUpdateVal  // load & store buffer value
);
endmodule

On the positive edge of the clock signal, the module will do several things

  • adds new line if the addValid is high,
  • pushes a line that both values are ready to the ALU section,
  • updates the value that constrained by the update data from Load & Store Buffer,
  • updates the value that constrained by the data calculated last clock,
  • sends the updated data out.