diff --git a/system/decoder.v b/system/decoder.v
index e66b3c2..4659a81 100644
--- a/system/decoder.v
+++ b/system/decoder.v
@@ -445,6 +445,18 @@ always @( CIR or SIMPLE_MICRO or seq_addr_input ) begin
 				PARAM2=2;
 				seq_addr_entry=`UCODE_RET_ENTRY;
 			end
+			11'b1010101x_xxx:begin
+				/*  STOS - Write byte/word to [DI] and increment accordingly */
+				/*                     | 1 0 1 0  1 0 1 W |                  */
+				`start_unaligning_instruction
+				opcode_size=0;
+				has_operands=0;
+				Wbit=CIR[8:8];
+				Sbit=0;
+				RM=101;
+				seq_addr_entry=`UCODE_STOS_ENTRY;
+				PARAM2=(Wbit==1)?2:1;
+			end
 			default:begin
 				`invalid_instruction
 			end
@@ -477,7 +489,7 @@ always @( CIR or SIMPLE_MICRO or seq_addr_input ) begin
 		endcase
 		reg_read_port1_addr=ucode_data[25:22];
 		IN_MOD             =ucode_data[28:26];
-		reg_read_port1_addr=ucode_data[32:29];
+		reg_read_port2_addr=ucode_data[32:29];
 	end
 end
 
diff --git a/system/processor.v b/system/processor.v
index d44a880..3c17cb0 100644
--- a/system/processor.v
+++ b/system/processor.v
@@ -112,18 +112,18 @@ reg [2:0] IN_MOD;
 reg [2:0] OUT_MOD;
 
 mux4 #(.WIDTH(16)) MUX16_1A(
-	PARAM1,
-	reg_read_port1_data,
-	{ProgCount[14:0],unaligned_access^unaligning_instruction},
-	16'b0000000000000000, /*0 Constant*/
+/*0*/	PARAM1,
+/*1*/	reg_read_port1_data,
+/*2*/	{ProgCount[14:0],unaligned_access^unaligning_instruction},
+/*3*/	16'b0000000000000000, /*0 Constant*/
 	in_alu1_sel1,
 	ALU_1A);
 
 mux4 #(.WIDTH(16)) MUX16_1B(
-	PARAM2,
-	reg_read_port2_data,
-	{ProgCount[14:0],unaligned_access^unaligning_instruction},
-	16'b0000000000000000, /*0 Constant*/
+/*0*/	PARAM2,
+/*1*/	reg_read_port2_data,
+/*2*/	{ProgCount[14:0],unaligned_access^unaligning_instruction},
+/*3*/	16'b0000000000000000, /*0 Constant*/
 	in_alu1_sel2,
 	ALU_1B);
 
diff --git a/system/ucode.txt b/system/ucode.txt
index 1bf4766..9bb43da 100644
--- a/system/ucode.txt
+++ b/system/ucode.txt
@@ -17,6 +17,8 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+//rr2: reg_read_port2_addr
+//
 //imd: IN_MOD
 //
 //rr1: reg_read_port1_addr
@@ -48,12 +50,19 @@
 
 @000 0000_000_000__00__00_0000__00_000000
 
-//   32   28  25  21  18  15  13  11   7   5     0
+//   32   28  25   21  18  15  13  11   7   5    0
+// CALL
 //   rr2 |imd|rr1 |a1f|a1o|a12|a11|rwa |nxs|Nxt M |
 @001 0000_011_1100_001_011__00__01_1100__01_000010 // ALU_1: SP           ALU_2: PARAM2 (2) ALU_OP:SUB  ALU_out: SP   (also fetch the opcode argument to PARAM1)
 @002 0000_011_zzzz_000_110__10__11_zzzz__00_000011 // ALU_1: 0            ALU_2: PC         ALU_OP:ADD  ALU_out: [SP]
 @003 0000_011_zzzz_000_101__10__00_zzzz__00_000000 // ALU_1: PARAM1 (arg) ALU_2: PC         ALU_OP:ADD  ALU_out: PC
 
-//   rr2|imd|rr1 |a1f|a1o|a12|a11|rwa |nxs|Nxt M |
+// RET
+//   rr2 |imd|rr1 |a1f|a1o|a12|a11|rwa |nxs|Nxt M |
 @004 0000_110_zzzz_000_101__00__11_zzzz__11_000101 // ALU_1: 0           ALU_2: PARAM2 ([SP) ALU_OP:ADD  ALU_out: PC (also read [SP] to PARAM2)
 @005 0000_011_1100_000_011__01__00_1100__00_000000 // ALU_1: PARAM1 (2)  ALU_2: SP           ALU_OP:ADD  ALU_out: SP
+
+// STOS
+//   rr2 |imd|rr1 |a1f|a1o|a12|a11|rwa |nxs|Nxt M |
+@006 1000_011_zzzz_000_000__01__11_zzzz__00_000111 // ALU_1: 0   ALU_2: AX           ALU_OP:ADD  ALU_out: [DI]
+@007 zzzz_011_1111_000_011__00__01_1111__00_000000 // ALU_1: DI  ALU_2: PARAM2 (2)   ALU_OP:ADD  ALU_OUT: DI
diff --git a/system/ucode_header.v b/system/ucode_header.v
index 22b841c..5fe32d8 100644
--- a/system/ucode_header.v
+++ b/system/ucode_header.v
@@ -19,9 +19,10 @@
 
 `define UCODE_ADDR_BITS 5
 `define UCODE_DATA_BITS 33
-`define UCODE_SIZE 6
+`define UCODE_SIZE 8
 
 /* DEFINE ADDRESSES IN THE MICROCODE */
 `define UCODE_NO_INSTRUCTION	5'b00000
 `define UCODE_CALL_ENTRY	5'b00001
 `define UCODE_RET_ENTRY		5'b00100
+`define UCODE_STOS_ENTRY	5'b00110