From bc8ffd2ecbd262e001ceb2c010375446fbdc5d3b Mon Sep 17 00:00:00 2001
From: Clifford Wolf <clifford@clifford.at>
Date: Sat, 6 Jun 2015 20:40:58 +0200
Subject: [PATCH] Added memory "look-ahead" read interface

---
 dhrystone/testbench.v      | 28 ++++++++++++++++------------
 picorv32.v                 | 26 +++++++++++++-------------
 scripts/synth_vivado_soc.v | 10 +++++++---
 3 files changed, 36 insertions(+), 28 deletions(-)

diff --git a/dhrystone/testbench.v b/dhrystone/testbench.v
index 1e2de3d..216cc87 100644
--- a/dhrystone/testbench.v
+++ b/dhrystone/testbench.v
@@ -18,28 +18,32 @@ module testbench;
 	wire [31:0] mem_addr;
 	wire [31:0] mem_wdata;
 	wire [3:0] mem_wstrb;
-	wire [31:0] mem_rdata;
+	reg  [31:0] mem_rdata;
+	wire mem_la_read;
+	wire [31:0] mem_la_addr;
 
 	picorv32 uut (
-		.clk      (clk      ),
-		.resetn   (resetn   ),
-		.trap     (trap     ),
-		.mem_valid(mem_valid),
-		.mem_instr(mem_instr),
-		.mem_ready(mem_ready),
-		.mem_addr (mem_addr ),
-		.mem_wdata(mem_wdata),
-		.mem_wstrb(mem_wstrb),
-		.mem_rdata(mem_rdata)
+		.clk        (clk        ),
+		.resetn     (resetn     ),
+		.trap       (trap       ),
+		.mem_valid  (mem_valid  ),
+		.mem_instr  (mem_instr  ),
+		.mem_ready  (mem_ready  ),
+		.mem_addr   (mem_addr   ),
+		.mem_wdata  (mem_wdata  ),
+		.mem_wstrb  (mem_wstrb  ),
+		.mem_rdata  (mem_rdata  ),
+		.mem_la_read(mem_la_read),
+		.mem_la_addr(mem_la_addr)
 	);
 
 	reg [31:0] memory [0:64*1024/4-1];
 	initial $readmemh("dhry.hex", memory);
 
 	assign mem_ready = 1;
-	assign mem_rdata = memory[mem_addr >> 2];
 
 	always @(posedge clk) begin
+		mem_rdata <= memory[mem_la_addr >> 2];
 		if (mem_valid) begin
 			case (mem_addr)
 				32'h1000_0000: begin
diff --git a/picorv32.v b/picorv32.v
index f80e926..b84ce63 100644
--- a/picorv32.v
+++ b/picorv32.v
@@ -39,7 +39,11 @@ module picorv32 #(
 	output reg [31:0] mem_addr,
 	output reg [31:0] mem_wdata,
 	output reg [ 3:0] mem_wstrb,
-	input      [31:0] mem_rdata
+	input      [31:0] mem_rdata,
+
+	// look-ahead interface
+	output            mem_la_read,
+	output     [31:0] mem_la_addr
 );
 	localparam integer regfile_size = ENABLE_REGS_16_31 ? 32 : 16;
 	localparam integer regindex_bits = ENABLE_REGS_16_31 ? 5 : 4;
@@ -65,6 +69,9 @@ module picorv32 #(
 
 	wire mem_busy = |{mem_do_prefetch, mem_do_rinst, mem_do_rdata, mem_do_wdata};
 
+	assign mem_la_read = resetn && !mem_state && (mem_do_rinst || mem_do_prefetch || mem_do_rdata);
+	assign mem_la_addr = mem_do_prefetch ? reg_pc + 4 : mem_do_rinst ? reg_pc : {reg_op1[31:2], 2'b00};
+
 	always @(posedge clk) begin
 		mem_done <= 0;
 		if (!resetn) begin
@@ -72,17 +79,10 @@ module picorv32 #(
 			mem_valid <= 0;
 		end else case (mem_state)
 			0: begin
-				if (mem_do_rinst || mem_do_prefetch) begin
+				mem_addr <= mem_la_addr;
+				if (mem_do_rinst || mem_do_prefetch || mem_do_rdata) begin
 					mem_valid <= 1;
-					mem_addr <= mem_do_prefetch ? reg_pc + 4 : reg_pc;
-					mem_instr <= 1;
-					mem_wstrb <= 0;
-					mem_state <= 1;
-				end
-				if (mem_do_rdata) begin
-					mem_valid <= 1;
-					mem_addr <= {reg_op1[31:2], 2'b00};
-					mem_instr <= 0;
+					mem_instr <= mem_do_rinst || mem_do_rdata;
 					mem_wstrb <= 0;
 					mem_state <= 1;
 				end
@@ -195,7 +195,7 @@ module picorv32 #(
 		is_lbu_lhu_lw <= |{instr_lbu, instr_lhu, instr_lw};
 	end
 
-	assign instr_trap = ~|{instr_lui, instr_auipc, instr_jal, instr_jalr,
+	assign instr_trap = !{instr_lui, instr_auipc, instr_jal, instr_jalr,
 			instr_beq, instr_bne, instr_blt, instr_bge, instr_bltu, instr_bgeu,
 			instr_lb, instr_lh, instr_lw, instr_lbu, instr_lhu, instr_sb, instr_sh, instr_sw,
 			instr_addi, instr_slti, instr_sltiu, instr_xori, instr_ori, instr_andi, instr_slli, instr_srli, instr_srai,
@@ -810,7 +810,7 @@ module picorv32_axi_adapter (
 	assign mem_axi_awaddr = mem_addr;
 	assign mem_axi_awprot = 0;
 
-	assign mem_axi_arvalid = mem_valid && ~|mem_wstrb && !ack_arvalid;
+	assign mem_axi_arvalid = mem_valid && !mem_wstrb && !ack_arvalid;
 	assign mem_axi_araddr = mem_addr;
 	assign mem_axi_arprot = mem_instr ? 3'b100 : 3'b000;
 
diff --git a/scripts/synth_vivado_soc.v b/scripts/synth_vivado_soc.v
index 64d70ae..b63c524 100644
--- a/scripts/synth_vivado_soc.v
+++ b/scripts/synth_vivado_soc.v
@@ -19,7 +19,9 @@ module test_soc (
 	wire [31:0] mem_addr;
 	wire [31:0] mem_wdata;
 	wire [3:0] mem_wstrb;
-	wire [31:0] mem_rdata;
+	reg [31:0] mem_rdata;
+	wire mem_la_read;
+	wire [31:0] mem_la_addr;
 
 	picorv32 uut (
 		.clk      (clk      ),
@@ -31,7 +33,9 @@ module test_soc (
 		.mem_addr (mem_addr ),
 		.mem_wdata(mem_wdata),
 		.mem_wstrb(mem_wstrb),
-		.mem_rdata(mem_rdata)
+		.mem_rdata(mem_rdata),
+		.mem_la_read(mem_la_read),
+		.mem_la_addr(mem_la_addr)
 	);
 
 	assign monitor_valid = mem_valid;
@@ -42,12 +46,12 @@ module test_soc (
 	initial $readmemh("../firmware/firmware.hex", memory);
 
 	assign mem_ready = 1;
-	assign mem_rdata = memory[mem_addr >> 2];
 
 	assign out_byte = mem_wdata[7:0];
 	assign out_byte_en = mem_addr == 32'h1000_0000;
 
 	always @(posedge clk) begin
+		mem_rdata <= memory[mem_la_addr >> 2];
 		if (mem_valid && (mem_addr >> 2) < MEM_SIZE) begin
 			if (mem_wstrb[0]) memory[mem_addr >> 2][ 7: 0] <= mem_wdata[ 7: 0];
 			if (mem_wstrb[1]) memory[mem_addr >> 2][15: 8] <= mem_wdata[15: 8];