$ cat memory_controller.v
module memory_controller(clk,
             devices_mem_en,
             device_1_mem_addr,
             device_2_mem_addr,
             device_3_mem_addr,
             device_1_mem_di,
             device_2_mem_di,
             device_3_mem_di,
             devices_mem_we,
             devices_do_ack,
             mem_do);

input   clk;
input   [2:0] devices_mem_en;
input   [7:0] device_1_mem_addr;
input   [7:0] device_2_mem_addr;
input   [7:0] device_3_mem_addr;
input   [7:0] device_1_mem_di;
input   [7:0] device_2_mem_di;
input   [7:0] device_3_mem_di;
input   [2:0] devices_mem_we;
output  [2:0] devices_do_ack;
output  [7:0] mem_do;

parameter DEVICE_1 = 3'd0;
parameter DEVICE_2 = 3'd1;
parameter DEVICE_3 = 3'd2;
parameter NO_ONE = 3'b111;

reg mem_enable = 0;
reg [7:0] mem_addr = 7'd0;
reg [7:0] mem_di = 7'd0;
reg [2:0] current_slave = NO_ONE;
reg mem_we = 0;
reg [2:0] serving_slave = 2'd0;
reg [2:0] devices_do_ack = 3'd0;

always @(posedge clk)
begin
    if (current_slave == NO_ONE)
        mem_enable <= 0;
    else
        mem_enable <= devices_mem_en[0] | devices_mem_en[1] | devices_mem_en[2];
end

always @(posedge clk)
begin
    case (current_slave)

    DEVICE_1:
    begin
        mem_addr <= device_1_mem_addr;
        mem_di <= device_1_mem_di;
        mem_we <= devices_mem_we[0];
    end

    DEVICE_2:
    begin
        mem_addr <= device_2_mem_addr;
        mem_di <= device_2_mem_di;
        mem_we <= devices_mem_we[1];
    end

    DEVICE_3:
    begin
        mem_addr <= device_3_mem_addr;
        mem_di <= device_3_mem_di;
        mem_we <= devices_mem_we[2];
    end

    NO_ONE:
    begin
        mem_addr <= 7'd0;
        mem_di <= 7'd0;
        mem_we <= 0;
    end

    endcase
end

always @(posedge clk)
begin

    case(current_slave)

    NO_ONE:
    begin
        devices_do_ack = 3'b000;
        if (devices_mem_en[0])
            current_slave <= DEVICE_1;
        else if (devices_mem_en[1])
            current_slave <= DEVICE_2;
        else if (devices_mem_en[2])
            current_slave <= DEVICE_3;
        else
            current_slave <= NO_ONE;
    end

    DEVICE_1:
    begin
        devices_do_ack = 3'b001;
        if (devices_mem_en[1])
            current_slave <= DEVICE_2;
        else if (devices_mem_en[2])
            current_slave <= DEVICE_3;
    end

    DEVICE_2:
    begin
        devices_do_ack = 3'b010;
        if (devices_mem_en[2])
            current_slave <= DEVICE_3;
        else if (devices_mem_en[0])
            current_slave <= DEVICE_1;
    end

    DEVICE_3:
    begin
        devices_do_ack = 3'b100;
        if (devices_mem_en[0])
            current_slave <= DEVICE_1;
        else if (devices_mem_en[1])
            current_slave <= DEVICE_2;
    end

    endcase
end

ram mem(clk, mem_enable, mem_addr, mem_di, mem_do, mem_we);

endmodule