From 5371caa3bb018e8f4c6d65fbaa83c179c2e86126 Mon Sep 17 00:00:00 2001
From: "(Tim) Efthimis Kritikos" <efkf@firemail.cc>
Date: Wed, 8 Feb 2023 23:59:06 +0000
Subject: [PATCH] Implemented a basic Instruction Fetch stage and added some
 examples in the 8086 doc

---
 .gitignore                |  1 +
 8086_documentation.md     | 12 +++++++++++-
 cpu/boot_code.txt         |  6 +++---
 cpu/gtkwave_savefile.gtkw | 14 ++++++++------
 cpu/processor.v           | 22 +++++++++++-----------
 cpu/testbench.v           |  4 +++-
 6 files changed, 37 insertions(+), 22 deletions(-)

diff --git a/.gitignore b/.gitignore
index c1a8af5..6ac34cc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 *.vpi
 *.lx2
 *.o
+*.swp
diff --git a/8086_documentation.md b/8086_documentation.md
index 5666c0d..50108e1 100644
--- a/8086_documentation.md
+++ b/8086_documentation.md
@@ -11,7 +11,7 @@ Instructions vary from 1 to 6 bytes.
 
 On some instructions:
 
-* **S**-bit : An 8-bit 2’s complement number. It can be extended to a 16-bit 2’s complement number depending on the W-bit by making all of the bits in the higher-order byte equal the most significant bit in the low order byte. This is known as sign extension.
+* **S**-bit : An 8-bit 2's complement number. It can be extended to a 16-bit 2’s complement number depending on the W-bit by making all of the bits in the higher-order byte equal the most significant bit in the low order byte. This is known as sign extension.
 
 | S   | W   |    Operation    |
 | --- | --- |  -------------- |
@@ -46,3 +46,13 @@ The second byte of the instruction usually identifies the instruction's operands
 |101  | [DI]                                     | [DI] + d8                                   | [DI] + d16                                   |    CH                       | BP                          |
 |110  | d16 (direct)                             | [BP] + d8                                   | [BP] + d16                                   |    DH                       | SI                          |
 |111  | [BX]                                     | [BX] + d8                                   | [BX] + d16                                   |    BH                       | DI                          |
+
+Example instructions:
+
+|  Bytecode  |   AT&T Syntax   |                         meaning                            |
+| ---------- | --------------- | ---------------------------------------------------------- |
+|81 c0 aa 55 | add $0x55aa,%ax | write 0x55aa to register ax                                |
+|03 06 aa 55 | add 0x55aa,%ax  | write the contents of memory locaton 0x55aa to register ax |
+|fe c0       | inc %al         | increment register al                                      |
+|ff c0       | inc %ax         | increment register ax                                      |
+|40          | inc %ax         | increment register ax                                      |
diff --git a/cpu/boot_code.txt b/cpu/boot_code.txt
index 7c34b60..d3a945c 100644
--- a/cpu/boot_code.txt
+++ b/cpu/boot_code.txt
@@ -1,7 +1,7 @@
 // 0x00000000
-55AA
-0000
-0000
+81C0
+AA55
+FEC0
 0000
 0000
 0000
diff --git a/cpu/gtkwave_savefile.gtkw b/cpu/gtkwave_savefile.gtkw
index e78cae2..9b2c765 100644
--- a/cpu/gtkwave_savefile.gtkw
+++ b/cpu/gtkwave_savefile.gtkw
@@ -1,13 +1,13 @@
 [*]
 [*] GTKWave Analyzer v3.3.104 (w)1999-2020 BSI
-[*] Wed Feb  8 11:44:52 2023
+[*] Wed Feb  8 23:43:14 2023
 [*]
 [dumpfile] "/home/user/UNI_DATA/COMS30046_2022_TB-2/projects/9086/cpu/test.lx2"
-[dumpfile_mtime] "Wed Feb  8 11:44:20 2023"
-[dumpfile_size] 430
+[dumpfile_mtime] "Wed Feb  8 23:42:51 2023"
+[dumpfile_size] 470
 [savefile] "/home/user/UNI_DATA/COMS30046_2022_TB-2/projects/9086/cpu/gtkwave_savefile.gtkw"
 [timestart] 0
-[size] 1342 1059
+[size] 1534 1059
 [pos] -1 -1
 *-20.795050 2883000 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 [treeopen] tb.
@@ -19,12 +19,14 @@
 tb.p.clock[0]
 tb.p.reset[0]
 tb.p.start[0]
-tb.p.state[1:0]
+@29
+tb.p.state[2:0]
+@28
 tb.p.instruction_finished[0]
 @22
 tb.p.external_address_bus[19:0]
 tb.p.external_data_bus[15:0]
-@29
+@28
 tb.p.read[0]
 [pattern_trace] 1
 [pattern_trace] 0
diff --git a/cpu/processor.v b/cpu/processor.v
index b270d38..f6ef2dd 100644
--- a/cpu/processor.v
+++ b/cpu/processor.v
@@ -54,13 +54,13 @@ endmodule
 
 module processor ( input clock, input reset , output reg [19:0] external_address_bus, inout [15:0] external_data_bus,output reg read, output reg write);
 /* State */
-reg [1:0] state;
+reg [2:0] state;
 reg start=0;
 reg instruction_finished;
 
 /* Registers */
 reg [19:0] ProgCount;
-
+reg [14:0] CIR;
 
 /* RESET LOGIC */
 always @(negedge reset) begin
@@ -75,29 +75,29 @@ end
 
 /* CLOCK LOGIC */
 always @(posedge clock) begin
-	if(instruction_finished) begin 
+	if(instruction_finished)
 		state =0;
-	end else begin
-		if (clock && start==1) begin
+	else
+		if (clock && start==1)
 			state=state+1;
-		end
-	end
 end
 
 always @(state) begin
-	if (state==2) begin
+	if (state==5)
 		instruction_finished=1;
-	end else begin
+	else
 		instruction_finished=0;
-	end
 end
 
 /* Processor stages */
 always @(state) begin
-	if (state==0) begin
+	if (state=='b000) begin
 		external_address_bus <= ProgCount;
 		read <= 0;
 		write <= 1;
+	end else if ( state=='b001 ) begin
+		CIR <= external_data_bus;
+		ProgCount=ProgCount+1;
 	end
 end
 
diff --git a/cpu/testbench.v b/cpu/testbench.v
index 3a8681d..c680966 100644
--- a/cpu/testbench.v
+++ b/cpu/testbench.v
@@ -9,6 +9,8 @@ wire rd,wr,romcs;
 processor p(clock,reset,address_bus,data_bus,rd,wr);
 rom bootrom(address_bus,data_bus,rd,romcs);
 
+`define CPU_SPEED 1000
+
 clock_gen #(.FREQ(1000)) u1(clk_enable, clock);
 
 assign romcs=0;
@@ -22,7 +24,7 @@ initial begin
 	reset = 0;
 	#(100) 
 	reset = 1;
-	#(10000)
+	#(`CPU_SPEED*30)
 
 	#50 $finish;
 end