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Implemented a basic Instruction Fetch stage and added some examples in the 8086 doc

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(Tim) Efthimis Kritikos 2023-02-08 23:59:06 +00:00
parent 08bf5d3031
commit 5371caa3bb
6 changed files with 37 additions and 22 deletions
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1
.gitignore vendored
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@ -2,3 +2,4 @@
*.vpi
*.lx2
*.o
*.swp

12
8086_documentation.md
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@ -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 |

6
cpu/boot_code.txt
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@ -1,7 +1,7 @@
// 0x00000000
55AA
0000
0000
81C0
AA55
FEC0
0000
0000
0000

14
cpu/gtkwave_savefile.gtkw
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@ -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

22
cpu/processor.v
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@ -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

4
cpu/testbench.v
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@ -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