异步fifo程序没看懂，求指导

1. module fifo1(rdata, wfull, rempty, wdata, winc, wclk, wrst_n,rinc, rclk, rrst_n);
   
2. parameter DSIZE = 8; parameter ASIZE = 4;
   
3. output [DSIZE-1:0] rdata;
   
4. output wfull;
   
5. output rempty;
   
6. input [DSIZE-1:0] wdata;
   
7. input winc, wclk, wrst_n;
   
8. input rinc, rclk, rrst_n;
   
9. reg wfull,rempty;
   
10. reg [ASIZE:0] wptr, rptr, wq2_rptr, rq2_wptr, wq1_rptr,rq1_wptr;
    
11. reg [ASIZE:0] rbin, wbin;
    
12. reg [DSIZE-1:0] mem[0:(1<<ASIZE)-1]; //定义存储器宽度为8，深度为16
    
13. wire [ASIZE-1:0] waddr, raddr; //读写地址
    
14. wire [ASIZE:0] rgraynext, rbinnext,wgraynext,wbinnext; //读写的格雷码与二进制码
    
15. wire rempty_val,wfull_val;
    
16. //-----------------双口RAM存储器--------------------
    
17. assign rdata=mem[raddr];
    
18. always@(posedge wclk)
    
19. if (winc && !wfull) mem[waddr] <= wdata;
    
20. //-------------同步rptr 指针-------------------------
    
21. always @(posedge wclk or negedge wrst_n)
    
22. if (!wrst_n) {wq2_rptr,wq1_rptr} <= 0;
    
23. else {wq2_rptr,wq1_rptr} <= {wq1_rptr,rptr}; //这里采用了位拼接操作符
    
24. //-------------同步wptr指针---------------------------
    
25. always @(posedge rclk or negedge rrst_n)
    
26. if (!rrst_n) {rq2_wptr,rq1_wptr} <= 0;
    
27. else {rq2_wptr,rq1_wptr} <= {rq1_wptr,wptr}; //
    
28. //-------------rempty产生与raddr产生-------------------
    
29. always @(posedge rclk or negedge rrst_n) // GRAYSTYLE2 pointer
    
30. begin
    
31. if (!rrst_n) {rbin, rptr} <= 0;
    
32. else {rbin, rptr} <= {rbinnext, rgraynext};
    
33. end
    
34. // Memory read-address pointer (okay to use binary to address memory)
    
35. assign raddr = rbin[ASIZE-1:0];
    
36. assign rbinnext = rbin + (rinc & ~rempty);
    
37. assign rgraynext = (rbinnext>>1) ^ rbinnext; //产生读格雷码
    
38. // FIFO empty when the next rptr == synchronized wptr or on reset
    
39. assign rempty_val = (rgraynext == rq2_wptr);
    
40. always @(posedge rclk or negedge rrst_n)
    
41. begin
    
42. if (!rrst_n) rempty <= 1'b1;
    
43. else rempty <= rempty_val;
    
44. end
    
45. //---------------wfull产生与waddr产生------------------------------
    
46. always @(posedge wclk or negedge wrst_n) // GRAYSTYLE2 pointer
    
47. if (!wrst_n) {wbin, wptr} <= 0;
    
48. else {wbin, wptr} <= {wbinnext, wgraynext};
    
49. // Memory write-address pointer (okay to use binary to address memory)
    
50. assign waddr = wbin[ASIZE-1:0];
    
51. assign wbinnext = wbin + (winc & ~wfull);
    
52. assign wgraynext = (wbinnext>>1) ^ wbinnext; //产生写格雷码
    
53. assign wfull_val = (wgraynext=={~wq2_rptr[ASIZE:ASIZE-1], wq2_rptr[ASIZE-2:0]}); //:ASIZE-1]
    
54. always @(posedge wclk or negedge wrst_n)
    
55. if (!wrst_n) wfull <= 1'b0;
    
56. else wfull <= wfull_val;
    
57. endmodule
    
58. 
    
59. 
    

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同步wptr、rptr是什么意思？那几行代码怎么解释？20-27行