From 53e9d371d7aa37e22db2af861a3d00cdc6ad0499 Mon Sep 17 00:00:00 2001
From: "(Tim) Efthimis Kritikos" <efkf@firemail.cc>
Date: Tue, 16 May 2023 18:07:28 +0100
Subject: [PATCH] Fully optimised BIU. Now it can instantly deliver
 instructions back to back

---
 system/biu.v     | 50 +++++++++++++++++++++++++++++++++++++++++++-----
 system/config.v  | 18 +++++++++++++++--
 system/decoder.v |  2 +-
 3 files changed, 62 insertions(+), 8 deletions(-)

diff --git a/system/biu.v b/system/biu.v
index a50b4b3..7498680 100644
--- a/system/biu.v
+++ b/system/biu.v
@@ -129,6 +129,7 @@ always @(posedge clock) begin
 					end
 				end
 
+				`ifdef EARLY_VALID_INSTRUCTION
 				if((Isit1==1) && (FIFO_SIZE!=0) && `EARLY_VALID_INSTRUCTION_)begin
 					VALID_INSTRUCTION <= 1;
 					INSTRUCTION[31:24] <= INPUT_FIFO[FIFO_start];
@@ -148,6 +149,16 @@ always @(posedge clock) begin
 					INSTRUCTION[15: 8] <= INPUT_FIFO[FIFO_start+4'd2];
 					INSTRUCTION[ 7: 0] <= INPUT_FIFO[FIFO_start+4'd3];
 				end
+				`else
+				if(FIFO_SIZE>3)begin
+					VALID_INSTRUCTION <= 1;
+					INSTRUCTION[31:24] <= INPUT_FIFO[FIFO_start];
+					INSTRUCTION[23:16] <= INPUT_FIFO[FIFO_start+4'd1];
+					INSTRUCTION[15: 8] <= INPUT_FIFO[FIFO_start+4'd2];
+					INSTRUCTION[ 7: 0] <= INPUT_FIFO[FIFO_start+4'd3];
+				end
+				`endif
+
 
 			end
 
@@ -289,19 +300,48 @@ end
 wire [2:0] Isize;
 InstrSize InstrSize({INSTRUCTION[31:24],INSTRUCTION[21:19]},Isize);
 
+`ifdef INCLUDE_EARLY_CALC_CIRUIT
 wire [2:0] fifoIsize;
 wire Isit1;
-`ifdef EARLY_VALID_INSTRUCTION
 InstrSize fifoInstrSize({INPUT_FIFO[FIFO_start][7:0],INPUT_FIFO[FIFO_start+4'd1][5:3]},fifoIsize);
 Is1 Is1(INPUT_FIFO[FIFO_start][7:0],Isit1);
 `endif
 
+`ifdef DOUBLE_INSTRUCTION_LOAD
+wire [2:0] fifoIsize2;
+InstrSize fifoInstrSize2({INPUT_FIFO[FIFO_start+fifoIsize][7:0],INPUT_FIFO[FIFO_start+fifoIsize+4'd1][5:3]},fifoIsize2);
+`endif
+
 always @( valid_instruction_ack ) begin
-		/* verilator lint_off BLKSEQ */
-		FIFO_start = FIFO_start + {1'b0,Isize};
-		/* verilator lint_on BLKSEQ */
-		INSTRUCTION_LOCATION <= INSTRUCTION_LOCATION + {12'b0,Isize};;
+	/* verilator lint_off BLKSEQ */
+	FIFO_start = FIFO_start + {1'b0,Isize};
+	INSTRUCTION_LOCATION <= INSTRUCTION_LOCATION + {12'b0,Isize};;
+	`ifdef DOUBLE_INSTRUCTION_LOAD
+	if(FIFO_SIZE>4'd3+Isize)begin
+		if((fifoIsize2==2) && (FIFO_SIZE > 1+fifoIsize) && `EARLY_VALID_INSTRUCTION_)begin
+			VALID_INSTRUCTION <= 1;
+			INSTRUCTION[31:24] <= INPUT_FIFO[FIFO_start];
+			INSTRUCTION[23:16] <= INPUT_FIFO[FIFO_start+4'd1];
+		end else if((fifoIsize2==3) && (FIFO_SIZE > 2+fifoIsize) && `EARLY_VALID_INSTRUCTION_)begin
+			VALID_INSTRUCTION <= 1;
+			INSTRUCTION[31:24] <= INPUT_FIFO[FIFO_start];
+			INSTRUCTION[23:16] <= INPUT_FIFO[FIFO_start+4'd1];
+			INSTRUCTION[15: 8] <= INPUT_FIFO[FIFO_start+4'd2];
+		end else if(FIFO_SIZE>3+fifoIsize)begin
+			VALID_INSTRUCTION <= 1;
+			INSTRUCTION[31:24] <= INPUT_FIFO[FIFO_start];
+			INSTRUCTION[23:16] <= INPUT_FIFO[FIFO_start+4'd1];
+			INSTRUCTION[15: 8] <= INPUT_FIFO[FIFO_start+4'd2];
+			INSTRUCTION[ 7: 0] <= INPUT_FIFO[FIFO_start+4'd3];
+		end else
+			VALID_INSTRUCTION <= 0;
+	end else begin
 		VALID_INSTRUCTION <= 0;
+		/* verilator lint_on BLKSEQ */
+	end
+	`else
+		VALID_INSTRUCTION <= 0;
+	`endif
 end
 
 always @( posedge jump_req ) begin
diff --git a/system/config.v b/system/config.v
index 57c8cbd..cb31eea 100644
--- a/system/config.v
+++ b/system/config.v
@@ -34,6 +34,9 @@
  * for the maximum instruction size worth of bytes */
 `define EARLY_VALID_INSTRUCTION
 
+/* Enables the ability in BIU to pre-decode two instructions, one after the other in memory*/
+`define DOUBLE_INSTRUCTION_LOAD
+
 /* Size is in powers of two with minimal 3.
  *   3 : 8 Bytes
  *   4 : 16 Bytes
@@ -44,12 +47,23 @@
 
 
 /********** Internal **********/
+
+`ifdef OTUPUT_JSON_STATISTICS
+	`define CALCULATE_IPC
+`endif
+
+`ifdef DOUBLE_INSTRUCTION_LOAD
+	/*Needed for DOUBLE_INSTRUCTION_LOAD*/
+	`define EARLY_VALID_INSTRUCTION
+`endif
+
 `ifdef EARLY_VALID_INSTRUCTION
+	`define INCLUDE_EARLY_CALC_CIRUIT
 	`define EARLY_VALID_INSTRUCTION_ 1
 `else
 	`define EARLY_VALID_INSTRUCTION_ 0
 `endif
 
-`ifdef OTUPUT_JSON_STATISTICS
-	`define CALCULATE_IPC
+`ifdef CALCULATE_IPC
+	`define INCLUDE_EARLY_CALC_CIRUIT
 `endif
diff --git a/system/decoder.v b/system/decoder.v
index b556956..41af825 100644
--- a/system/decoder.v
+++ b/system/decoder.v
@@ -737,7 +737,7 @@ module InstrSize ( input [10:0] IN, output reg [2:0] VERDICT );
 	end
 endmodule
 
-`ifdef EARLY_VALID_INSTRUCTION
+`ifdef INCLUDE_EARLY_CALC_CIRUIT
 module Is1 ( input [7:0] IN, output reg VERDICT );
 	always @( IN ) begin
 		casez(IN)