From d347475b645328cb6ba7fe00c7e93804430b4ab0 Mon Sep 17 00:00:00 2001
From: Aadi Desai <21363892+supleed2@users.noreply.github.com>
Date: Sun, 6 Dec 2020 17:42:23 +0000
Subject: [PATCH] Update mips_cpu_regfile.v

lb, lbu, lh, lhu now  select data according to address alignment
$0 is assigned to 0, may cause an error when written to, unknown.
---
 rtl/mips_cpu_regfile.v | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/rtl/mips_cpu_regfile.v b/rtl/mips_cpu_regfile.v
index bc0f7d5..859bb6a 100644
--- a/rtl/mips_cpu_regfile.v
+++ b/rtl/mips_cpu_regfile.v
@@ -15,11 +15,12 @@ reg[31:0] memory [31:0]; //32 register slots, 32-bits wide
 
 initial begin
 	integer i; //Initialise to zero by default
-    for (i = 0; i < 32; i++) begin
+    for (i = 1; i < 32; i++) begin
         memory[i] = 0;
     end
 end
 
+assign memory[0] = 32'h00000000;
 assign regv0 = memory[2]; //assigning debug $v0 line to $2 of memory
 
 always_comb begin
@@ -31,16 +32,32 @@ always_ff @(negedge clk) begin
 	if (regwrite) begin
 		case (opcode)
 			6'b100000: begin //lb, load byte
-				memory[writereg] <= {{24{writedata[7]}}, writedata[7:0]};
+				case (readdata1[1:0])
+					2'b00: memory[writereg] <= {{24{writedata[7]}}, writedata[7:0]};
+					2'b01: memory[writereg] <= {{24{writedata[15]}}, writedata[15:8]};
+					2'b10: memory[writereg] <= {{24{writedata[23]}}, writedata[23:16]};
+					2'b11: memory[writereg] <= {{24{writedata[31]}}, writedata[31:24]};
+				endcase // readdata1[1:0]
 			end
 			6'b100100: begin //lbu, load byte unsigned
-				memory[writereg] <= {{24{1'b0}}, writedata[7:0]};
+				case (readdata1[1:0])
+					2'b00: memory[writereg] <= {{24{1'b0}}, writedata[7:0]};
+					2'b01: memory[writereg] <= {{24{1'b0}}, writedata[15:8]};
+					2'b10: memory[writereg] <= {{24{1'b0}}, writedata[23:16]};
+					2'b11: memory[writereg] <= {{24{1'b0}}, writedata[31:24]};
+				endcase // readdata1[1:0]
 			end
 			6'b100001: begin //lh, load half-word
-				memory[writereg] <= {{16{writedata[15]}}, writedata[15:0]};
+				case (readdata1[1:0]) // must be half-word aligned, readdata1[0] = 0
+					2'b00: memory[writereg] <= {{16{writedata[15]}}, writedata[15:0]};
+					2'b10: memory[writereg] <= {{16{writedata[31]}}, writedata[31:16]};
+				endcase // readdata1[1:0]
 			end
 			6'b100101: begin //lhu, load half-word unsigned
-				memory[writereg] <= {{16{1'b0}}, writedata[15:0]};
+				case (readdata1[1:0]) // must be half-word aligned, readdata1[0] = 0
+					2'b00: memory[writereg] <= {{16{1'b0}}, writedata[15:0]};
+					2'b10: memory[writereg] <= {{16{1'b0}}, writedata[31:16]};
+				endcase // readdata1[1:0]
 			end
 			6'b100010: begin //lwl, load word left
 				case (readdata1[1:0])