verilog AXI slave with outstanding

An AXI slave with outstanding transactions can be implemented in Verilog by using a state machine to manage the response to incoming transactions. The state machine can keep track of the number of outstanding transactions and respond accordingly. Here is an example implementation:

module axi_slave_outstanding(
  input          aclk,
  input          aresetn,
  input          awvalid,
  input  [31:0]  awaddr,
  input  [2:0]   awprot,
  input          awcache,
  input          awburst,
  input  [3:0]   awlen,
  input          arvalid,
  input  [31:0]  araddr,
  input  [2:0]   arprot,
  input          arcache,
  input          arburst,
  input  [3:0]   arlen,
  input          wvalid,
  input  [31:0]  wdata,
  input  [3:0]   wstrb,
  input          wlast,
  output         bvalid,
  output         rvalid,
  output  [1:0]  rresp,
  output  [31:0] rdata,
  output         rlast
);

  reg [1:0] state;
  reg [31:0] mem[0:1023];
  reg [3:0] arcount;
  reg [3:0] awcount;
  
  // Reset state machine and counters on reset
  always @(posedge aclk) begin
    if (!aresetn) begin
      state <= 2'b00;
      arcount <= 4'b0000;
      awcount <= 4'b0000;
    end
  end
  
  // State machine
  always @(posedge aclk) begin
    case (state)
      // Idle state
      2'b00: begin
        if (awvalid) begin
          state <= 2'b01;
          awcount <= awlen;
        end else if (arvalid) begin
          state <= 2'b10;
          arcount <= arlen;
        end
      end
      // Write data state
      2'b01: begin
        if (wvalid) begin
          mem[awaddr] <= wdata;
          awaddr <= awaddr + 1;
          awcount <= awcount - 1;
          if (wlast) begin
            bvalid <= 1'b1;
            state <= 2'b00;
          end
        end
      end
      // Read data state
      2'b10: begin
        if (rcount == 4'b0000) begin
          rvalid <= 1'b1;
          rdata <= mem[araddr];
        end
        araddr <= araddr + 1;
        arcount <= arcount - 1;
        if (arcount == 4'b0000) begin
          rlast <= 1'b1;
          rvalid <= 1'b0;
          state <= 2'b00;
        end
      end
      // Error state
      default: begin
        state <= 2'b00;
      end
    endcase
  end
endmodule

In this implementation, the state machine has three states: idle, write data, and read data. When an AXI write transaction is received, the state machine transitions to the write data state and writes the incoming data to memory. It also keeps track of the number of outstanding write transactions using the `awcount` counter. When an AXI read transaction is received, the state machine transitions to the read data state and reads the requested data from memory. It also keeps track of the number of outstanding read transactions using the `arcount` counter.

The `bvalid`, `rvalid`, `rdata`, and `rlast` outputs are used to signal the completion of the AXI transactions to the master. The `bvalid` output is set to 1 when a write response is ready, and the `rvalid` output is set to 1 when a read response is ready. The `rdata` output contains the read data, and the `rlast` output signals the end of a read transaction.

This implementation assumes a single master and a single slave, and does not support burst transactions or interleaved transactions. It also does not support any AXI extensions beyond the basic read and write transactions.