diff --git a/assembly.c b/assembly.c
index 212bd98..0d45011 100644
--- a/assembly.c
+++ b/assembly.c
@@ -47,17 +47,36 @@
 //  +-------+----------------------------------+--------------+
 //  | NUM   | DESCRIPTION                      | AFFECT FLAGS |
 //  +-------+----------------------------------+--------------+
-//  | 5'd0  | NOP                              |      NO      |
+//  | 5'h0  | NOP                              |      NO      |
 //  +-------+----------------------------------+--------------+
-//  | 5'd1  | Unconditional Jump               |      NO      |
+//  | 5'h1  | Unconditional Jump               |      NO      |
 //  +-------+----------------------------------+--------------+
-//  | 5'd2  | Jump if zero flag is set         |      NO      |
+//  | 5'h2  | Jump if zero flag is set         |      NO      |
 //  +-------+----------------------------------+--------------+
-//  | 5'd3  | Jump if zero flag is not set     |      NO      |
+//  | 5'h3  | Jump if zero flag is not set     |      NO      |
 //  +-------+----------------------------------+--------------+
-//  | 5'd4  | Jump if carry flag is set        |      NO      |
+//  | 5'h4  | Jump if carry flag is set        |      NO      |
 //  +-------+----------------------------------+--------------+
-//  | 5'd5  | Jump if carry flag is not set    |      NO      |
+//  | 5'h5  | Jump if carry flag is not set    |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h10 | Unconditional ret                |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h11 | Unconditional call               |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h12 | Call if zero flag is set         |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h13 | Call if zero flag is not set     |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h14 | Call if carry flag is set        |      NO      |
+//  +-------+----------------------------------+--------------+
+//  | 5'h15 | Call if carry flag is not set    |      NO      |
+//
+//  |       |                                  |              |
+//  +-------+----------------------------------+--------------+
+//  | 5'h0F | MOV immediate to SP              |              |
+//  +-------+----------------------------------+--------------+
+//  |       |                                  |              |
+//
 //
 //
 //  INSTRUCTION FORMAT 1 OPCODE NUM:
@@ -111,16 +130,27 @@ char *disassemble(uint32_t opcode_be){
 			case 0x00:
 				snprintf(ret,MAX_INSTRUCTION_LENGTH,"NOP");
 				break;
+			case 0x10:
+				snprintf(ret,MAX_INSTRUCTION_LENGTH,"RET");
+				break;
 			case 0x01:
-				snprintf(ret,MAX_INSTRUCTION_LENGTH,"JMP $%06X",val);
+			case 0x11:
+				snprintf(ret,MAX_INSTRUCTION_LENGTH,"%s $%06X",(opcode_num&0x10)?"CALL":"JMP",val);
 				break;
 			case 0x02:
 			case 0x03:
-				snprintf(ret,MAX_INSTRUCTION_LENGTH,"JMP,%s $%06X",opcode_num&1?"NZ":"Z",val);
+			case 0x12:
+			case 0x13:
+				snprintf(ret,MAX_INSTRUCTION_LENGTH,"%s,%s $%06X",(opcode_num&0x10)?"CALL":"JMP",opcode_num&1?"NZ":"Z",val);
 				break;
 			case 0x04:
 			case 0x05:
-				snprintf(ret,MAX_INSTRUCTION_LENGTH,"JMP,%s $%06X",opcode_num&1?"NC":"C",val);
+			case 0x14:
+			case 0x15:
+				snprintf(ret,MAX_INSTRUCTION_LENGTH,"%s,%s $%06X",(opcode_num&0x10)?"CALL":"JMP",opcode_num&1?"NC":"C",val);
+				break;
+			case 0x0F:
+				snprintf(ret,MAX_INSTRUCTION_LENGTH,"MOV $%06X,%%SP",val);
 				break;
 			default:
 				snprintf(ret,MAX_INSTRUCTION_LENGTH,"UNRECOGNISED INSTR");
@@ -182,25 +212,32 @@ char *disassemble(uint32_t opcode_be){
 uint32_t assemble_line(char *line, __attribute__((unused)) struct assembler_context_t *assembler_context){
 	uint16_t opcode;
 	int x;
-	if(strncmp(line,"JMP",3)==0){
-		x=3;
+	int call;
+	if( ((call=strncmp(line,"JMP",3))==0) || (strncmp(line,"CALL",4)==0) ){
+		if(call){
+			x=4;
+			opcode=0x10;
+		}else{
+			x=3;
+			opcode=0x00;
+		}
 		if(line[x]==' '){
-			opcode=0x01;
+			opcode|=0x01;
 		}else if(line[x]==','){
 			x++;
 			if(line[x]=='Z'){
-				opcode=0x02;
+				opcode|=0x02;
 				x++;
 			}else if(line[x]=='C'){
-				opcode=0x04;
+				opcode|=0x04;
 				x++;
 			}else if(line[x]=='N'){
 				x++;
 				if(line[x]=='Z'){
-					opcode=0x03;
+					opcode|=0x03;
 					x++;
 				}else if(line[x]=='C'){
-					opcode=0x05;
+					opcode|=0x09;
 					x++;
 				}else
 					return 0xFFFFFFFF;
@@ -285,9 +322,13 @@ uint32_t assemble_line(char *line, __attribute__((unused)) struct assembler_cont
 			while(line[x]==' ')x++;
 			if(line[x]=='$'){
 				x++;
-				if(sscanf(line+x,"%04X",&data)==1){
+				if(sscanf(line+x,"%06X",&data)==1){
 					x+=4;
 					while(line[x]==' ')x++;
+					if(line[x]!=','){ //24 bit TODO: not the best way to parse this
+						x+=2;
+						while(line[x]==' ')x++;
+					}
 					if(line[x]==','){
 						x++;
 						while(line[x]==' ')x++;
@@ -299,13 +340,19 @@ uint32_t assemble_line(char *line, __attribute__((unused)) struct assembler_cont
 									r1=line[x]-'0';
 									x++;
 									if(line[x]=='l')
-										return 0x40000000|r1<<16|data;
+										return 0x40000000|r1<<16|(data&0xFFFF);
 									else if(line[x]=='h')
-										return 0x41000000|r1<<16|data;
+										return 0x41000000|r1<<16|(data&0xFFFF);
 									else
 										return 0xFFFFFFFF;
 								}else
 									return 0xFFFFFFFF;
+							}else if(line[x]=='S'){
+								x++;
+								if(line[x]=='P'){
+									return 0x0F000000|(data&0x00FFFFFF);
+								}else
+									return 0xFFFFFFFF;
 							}else
 								return 0xFFFFFFFF;
 						}else
@@ -323,8 +370,10 @@ uint32_t assemble_line(char *line, __attribute__((unused)) struct assembler_cont
 				return 0x20050000;
 			else
 				return 0xFFFFFFFF;
-		}else
-			return 0xFFFFFFFF;
+		}else if(strncmp(line,"RET",3)==0){
+			return 0x10000000;
+		}
+		return 0xFFFFFFFF;
 	}
 }
 
diff --git a/cpu.c b/cpu.c
index db3dcdd..3af8e53 100644
--- a/cpu.c
+++ b/cpu.c
@@ -34,24 +34,40 @@ int decode(struct simdata_t *simdata){
 		case 0:
 			opcode=(simdata->decode_data->in_bytecode&0x1F000000)>>24;
 			switch(opcode){
-				case 0:
+				case 0x00:
 					simdata->exec_data->EXEC_ACTION=NOP;
 					break;
-				case 1:
-				case 2:
-				case 3:
-				case 4:
-				case 5:
+				case 0x01:
+				case 0x02:
+				case 0x03:
+				case 0x04:
+				case 0x05:
+				case 0x11:
+				case 0x12:
+				case 0x13:
+				case 0x14:
+				case 0x15:
 					switch(opcode){
-						case 1: simdata->exec_data->COND=NONE; break;
-						case 2: simdata->exec_data->COND=ZERO; break;
-						case 3: simdata->exec_data->COND=NZERO; break;
-						case 4: simdata->exec_data->COND=CARRY; break;
-						case 5: simdata->exec_data->COND=NCARRY; break;
+						case 0x01: case 0x11: simdata->exec_data->COND=NONE; break;
+						case 0x02: case 0x12: simdata->exec_data->COND=ZERO; break;
+						case 0x03: case 0x13: simdata->exec_data->COND=NZERO; break;
+						case 0x04: case 0x14: simdata->exec_data->COND=CARRY; break;
+						case 0x05: case 0x15: simdata->exec_data->COND=NCARRY; break;
+						default: return 1;
 					}
 					simdata->exec_data->out_op->OP_ADDR=IMMEDIATE;
 					simdata->exec_data->out_op->data=simdata->decode_data->in_bytecode&0x00FFFFFF;
-					simdata->exec_data->EXEC_ACTION=JUMP;
+					simdata->exec_data->EXEC_ACTION=(opcode&0x10)?CALL:JUMP;
+					break;
+				case 0x0F:
+					simdata->exec_data->EXEC_ACTION=MOVE;
+					simdata->exec_data->in_op1->OP_ADDR=IMMEDIATE;
+					simdata->exec_data->in_op1->data=simdata->decode_data->in_bytecode&0x00FFFFFF;
+					simdata->exec_data->out_op->OP_ADDR=REGISTER;
+					simdata->exec_data->out_op->data=0;
+					break;
+				case 0x10:
+					simdata->exec_data->EXEC_ACTION=RET;
 					break;
 				default:
 					return 1;
@@ -172,6 +188,7 @@ void free_exec_data(struct exec_data_t *tofree){
 int exec(struct simdata_t *simdata){
 	free_instr_list(&simdata->cpu_gui_hints->executing_list);
 	switch(simdata->exec_data->EXEC_ACTION){
+		case CALL:
 		case JUMP:
 			int condition=0;
 			switch(simdata->exec_data->COND){
@@ -182,12 +199,20 @@ int exec(struct simdata_t *simdata){
 				case NCARRY: condition=!(simdata->registers->FLAGS&2); break;
 			}
 			if(condition){
+				if(simdata->exec_data->EXEC_ACTION==CALL){
+					simdata->RAM[simdata->SP]=simdata->PC;
+					simdata->SP+=6;
+				}
 				if(simdata->exec_data->out_op->OP_ADDR==IMMEDIATE)
 					simdata->PC=simdata->exec_data->out_op->data;
 				else
 					return 1;
 			}
 			break;
+		case RET:
+			simdata->SP-=6;
+			simdata->PC=simdata->RAM[simdata->SP];
+			break;
 		case EXEC_ALU:
 			if(	simdata->exec_data->in_op1->OP_ADDR==REGISTER &&
 					simdata->exec_data->in_op2->OP_ADDR==REGISTER &&
@@ -245,6 +270,19 @@ int exec(struct simdata_t *simdata){
 						simdata->registers->GPR[simdata->exec_data->out_op->data]=(simdata->registers->GPR[simdata->exec_data->out_op->data]&0x0000FFFF)|((0x0000FFFF&
 							simdata->registers->GPR[simdata->exec_data->in_op1->data])<<16);
 					break;
+				case REGISTER: /* This is for special registers like the SP which is 24bits long */
+					if(simdata->exec_data->in_op1->OP_ADDR==IMMEDIATE){
+						if( (simdata->exec_data->in_op1->data&0xFF000000) == 0 )
+							simdata->SP=simdata->exec_data->in_op1->data;
+						else
+							return 2;
+					}else{
+						if( (simdata->registers->GPR[simdata->exec_data->in_op1->data]&0xFF000000) == 0 )
+							simdata->SP=simdata->registers->GPR[simdata->exec_data->in_op1->data];
+						else
+							return 2;
+					}
+					break;
 				default:
 					return 1;
 			}
@@ -264,14 +302,39 @@ int exec(struct simdata_t *simdata){
 
 int state=0;
 
+
+/*
+ * RETURN CODES:
+ * 0 : success
+ * 1 : error in execution stage
+ * 2 : error in decode stage
+ * 3 : error in fetch stage
+ * 4 : internal error
+ */
 int cpu_cycle_clock(struct simdata_t *simdata){
 	free_instr_list(&simdata->cpu_gui_hints->fetching_list);
 	free_instr_list(&simdata->cpu_gui_hints->decoding_list);
 	free_instr_list(&simdata->cpu_gui_hints->executing_list);
 	switch(state){
-		case 0: if( fetch(simdata)  ) return 3; break;
-		case 1: if( decode(simdata) ) return 2; break;
-		case 2: if( exec(simdata)   ) return 1; break;
+		case 0:
+			switch(fetch(simdata)){
+				case 0:  break;
+				default: return 3;
+			}
+			break;
+		case 1:
+			switch(decode(simdata)){
+				case 0:  break;
+				default: return 2;
+			}
+			break;
+		case 2:
+			switch(exec(simdata)){
+				case 0:  break;
+				case 2:  return 4;
+				default: return 1;
+			}
+			break;
 	}
 	if(state==2)
 		state=0;
diff --git a/cpu.h b/cpu.h
index 223fd6a..e231db2 100644
--- a/cpu.h
+++ b/cpu.h
@@ -17,8 +17,10 @@ enum EXEC_ACTION_t {
 	EXEC_ALU,
 	MOVE,
 	JUMP,
+	CALL,
 	NOP,
-	HALT
+	HALT,
+	RET
 };
 
 enum ALU_OP_t {
diff --git a/main.c b/main.c
index 7955f76..f41e76d 100644
--- a/main.c
+++ b/main.c
@@ -198,6 +198,9 @@ int main(int argc, char* argd[] ){
 						case 3:
 							printf("Failed to fetch instruction\n");
 							break;
+						case 4:
+							printf("Internal simulator error\n");
+							break;
 						default:
 							printf("Unkown CPU failure\n");
 					}
diff --git a/test.asm b/test.asm
index 11dd054..0154e2a 100644
--- a/test.asm
+++ b/test.asm
@@ -1,10 +1,11 @@
-MOV $0001,%R0l
-MOV $0000,%R0h
-MOV $0001,%R1l
-MOV $0000,%R1h
-MOV $0010,%R2l
-MOV $0000,%R2h
-ADD %R0,%R1
-CMP %R1,%R2
-JMP,NZ $000018
+MOV $123456,%SP
+MOV $0x0001,%R0l
+MOV $0x0000,%R0h
+MOV $0x0000,%R1l
+MOV $0x0000,%R1h
+CALL $0x0024
+CALL $0x0024
+CALL $0x0024
 HALT
+ADD %R0,%R1
+RET