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[/] [zipcpu/] [trunk/] [rtl/] [core/] [prefetch.v] - Rev 82
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//////////////////////////////////////////////////////////////////////////////// // // Filename: prefetch.v // // Project: Zip CPU -- a small, lightweight, RISC CPU soft core // // Purpose: This is a very simple instruction fetch approach. It gets // one instruction at a time. Future versions should pipeline // fetches and perhaps even cache results--this doesn't do that. // It should, however, be simple enough to get things running. // // The interface is fascinating. The 'i_pc' input wire is just // a suggestion of what to load. Other wires may be loaded // instead. i_pc is what must be output, not necessarily input. // // 20150919 -- Added support for the WB error signal. When reading an // instruction results in this signal being raised, the pipefetch // module will set an illegal instruction flag to be returned to // the CPU together with the instruction. Hence, the ZipCPU // can trap on it if necessary. // // Creator: Dan Gisselquist, Ph.D. // Gisselquist Technology, LLC // //////////////////////////////////////////////////////////////////////////////// // // Copyright (C) 2015, Gisselquist Technology, LLC // // This program is free software (firmware): you can redistribute it and/or // modify it under the terms of the GNU General Public License as published // by the Free Software Foundation, either version 3 of the License, or (at // your option) any later version. // // This program is distributed in the hope that it will be useful, but WITHOUT // ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or // FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License // for more details. // // License: GPL, v3, as defined and found on www.gnu.org, // http://www.gnu.org/licenses/gpl.html // // //////////////////////////////////////////////////////////////////////////////// // // Flash requires a minimum of 4 clocks per byte to read, so that would be // 4*(4bytes/32bit word) = 16 clocks per word read---and that's in pipeline // mode which this prefetch does not support. In non--pipelined mode, the // flash will require (16+6+6)*2 = 56 clocks plus 16 clocks per word read, // or 72 clocks to fetch one instruction. module prefetch(i_clk, i_rst, i_ce, i_stalled_n, i_pc, i_aux, o_i, o_pc, o_aux, o_valid, o_illegal, o_wb_cyc, o_wb_stb, o_wb_we, o_wb_addr, o_wb_data, i_wb_ack, i_wb_stall, i_wb_err, i_wb_data); parameter ADDRESS_WIDTH=32, AUX_WIDTH = 1, AW=ADDRESS_WIDTH; input i_clk, i_rst, i_ce, i_stalled_n; input [(AW-1):0] i_pc; input [(AUX_WIDTH-1):0] i_aux; output reg [31:0] o_i; output reg [(AW-1):0] o_pc; output reg [(AUX_WIDTH-1):0] o_aux; output reg o_valid, o_illegal; // Wishbone outputs output reg o_wb_cyc, o_wb_stb; output wire o_wb_we; output reg [(AW-1):0] o_wb_addr; output wire [31:0] o_wb_data; // And return inputs input i_wb_ack, i_wb_stall, i_wb_err; input [31:0] i_wb_data; assign o_wb_we = 1'b0; assign o_wb_data = 32'h0000; // Let's build it simple and upgrade later: For each instruction // we do one bus cycle to get the instruction. Later we should // pipeline this, but for now let's just do one at a time. initial o_wb_cyc = 1'b0; initial o_wb_stb = 1'b0; initial o_wb_addr= 0; always @(posedge i_clk) if ((i_rst)||(i_wb_ack)) begin o_wb_cyc <= 1'b0; o_wb_stb <= 1'b0; end else if ((i_ce)&&(~o_wb_cyc)) // Initiate a bus cycle begin o_wb_cyc <= 1'b1; o_wb_stb <= 1'b1; end else if (o_wb_cyc) // Independent of ce begin if ((o_wb_cyc)&&(o_wb_stb)&&(~i_wb_stall)) o_wb_stb <= 1'b0; if (i_wb_ack) o_wb_cyc <= 1'b0; end always @(posedge i_clk) if (i_rst) // Set the address to guarantee the result is invalid o_wb_addr <= {(AW){1'b1}}; else if ((i_ce)&&(~o_wb_cyc)) o_wb_addr <= i_pc; always @(posedge i_clk) if ((o_wb_cyc)&&(i_wb_ack)) o_aux <= i_aux; always @(posedge i_clk) if ((o_wb_cyc)&&(i_wb_ack)) o_i <= i_wb_data; always @(posedge i_clk) if ((o_wb_cyc)&&(i_wb_ack)) o_pc <= o_wb_addr; initial o_valid = 1'b0; initial o_illegal = 1'b0; always @(posedge i_clk) if ((o_wb_cyc)&&(i_wb_ack)) begin o_valid <= (i_pc == o_wb_addr)&&(~i_wb_err); o_illegal <= i_wb_err; end else if (i_stalled_n) begin o_valid <= 1'b0; o_illegal <= 1'b0; end endmodule
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