Fix a few scattered bugs, and get fetch to work.

[firearm.git] / ICache.v

diff --git a/ICache.v b/ICache.v
new file mode 100644 (file)
index 0000000..3a9f144
--- /dev/null
+++ b/ICache.v
@@ -0,0 +1,74 @@
+/* 16 cache entries, 64-byte long cache lines */
+
+module ICache(
+       input clk,
+       
+       /* ARM core interface */
+       input [31:0] rd_addr,
+       input rd_req,
+       output reg rd_wait,
+       output reg [31:0] rd_data,
+       
+       /* bus interface */
+       output reg bus_req,
+       input bus_ack,
+       output reg [31:0] bus_addr,
+       input [31:0] bus_rdata,
+       output wire [31:0] bus_wdata,
+       output reg bus_rd,
+       output wire bus_wr,
+       input bus_ready);
+       
+       assign bus_wr = 0;
+       assign bus_wdata = 0;
+       
+       /* [31 tag 10] [9 cache index 6] [5 data index 0]
+        * so the data index is 6 bits long
+        * so the cache index is 4 bits long
+        * so the tag is 22 bits long. c.c
+        */
+       
+       reg cache_valid [15:0];
+       reg [21:0] cache_tags [15:0];
+       reg [31:0] cache_data [15:0 /* line */] [15:0 /* word */];
+       
+       reg [4:0] i;
+       initial
+               for (i = 0; i < 16; i = i + 1)
+                       cache_valid[i[3:0]] = 0;
+       
+       wire [5:0] rd_didx = rd_addr[5:0];
+       wire [3:0] rd_didx_word = rd_didx[5:2];
+       wire [3:0] rd_idx = rd_addr[9:6];
+       wire [21:0] rd_tag = rd_addr[31:10];
+       
+       wire cache_hit = cache_valid[rd_idx] && (cache_tags[rd_idx] == rd_tag);
+       
+       always @(*) begin       /* XXX does this work nowadays? */
+               rd_wait = rd_req && !cache_hit;
+               rd_data = cache_data[rd_idx][rd_didx_word];
+       end
+       
+       reg [3:0] cache_fill_pos = 0;
+       assign bus_req = rd_req && !cache_hit; /* xxx, needed for Verilator */
+       always @(*)
+               if (rd_req && !cache_hit && bus_ack) begin
+                       bus_addr = {rd_addr[31:6], cache_fill_pos[3:0], 2'b00 /* reads are 32-bits */};
+                       bus_rd = 1;
+               end else begin
+                       bus_addr = 0;
+                       bus_rd = 0;
+               end
+       
+       always @(posedge clk)
+               if (rd_req && !cache_hit) begin
+                       if (bus_ready) begin    /* Started the fill, and we have data. */
+                               cache_data[rd_idx][cache_fill_pos] <= bus_rdata;
+                               cache_fill_pos <= cache_fill_pos + 1;
+                               if (cache_fill_pos == 15) begin /* Done? */
+                                       cache_tags[rd_idx] <= rd_tag;
+                                       cache_valid[rd_idx] <= 1;
+                               end
+                       end
+               end
+endmodule