add error message if no program selected

fix error with negative numbers

njvm.c

@@ -5,10 +5,9 @@
 #include "stack.c"
 #include "program.c"
 
-
 //Comment to disable debug
 
-#define DEBUG
+//#define DEBUG
 
 // Stack
 struct stack stack;
@@ -36,7 +35,7 @@ void execute(struct program program) {
                 goto end;
                 break;
             case PUSHC:
-                  push(stack,IMMEDIATE(program.program[i]));
+                push(stack, SIGN_EXTEND(IMMEDIATE(program.program[i])));
                 break;
             case ADD:
                 push(stack, pop(stack) + pop(stack));
@@ -75,6 +74,7 @@ void execute(struct program program) {
     end:
     return;
 }
+
 #ifdef DEBUG
 
 void tests(void){
@@ -96,9 +96,11 @@ int main(int argc, char *argv[]) {
 
     // Initialize ProgrammSpeicher
     int psize = 1000;
+    int saveProgram = 0;
     unsigned int p[1000];
     program.size = &psize;
     program.program = p;
+    program.saveProgram = &saveProgram;
 
 
 #ifdef DEBUG
@@ -124,9 +126,14 @@ int main(int argc, char *argv[]) {
     } else {
         run:
 //    Started
+        if (*program.saveProgram == 1) {
             printf("Ninja Virtual Machine started\n");
             printProgram(program);
             execute(program);
+        } else {
+            printf("Error: no program selected\n");
+            return 1;
+        }
 //    Stopped
         printf("Ninja Virtual Machine stopped\n");
         return 0;

program.c

@@ -11,6 +11,7 @@
 struct program {
     int *size;
     unsigned int *program;
+    int *saveProgram;
 };
 
 void copyToProgram(const unsigned int codeToCopy[], int size, struct program program) {
@@ -18,6 +19,7 @@ void copyToProgram(const unsigned int codeToCopy[], int size, struct program pro
         program.program[i] = codeToCopy[i];
     }
     *program.size = size;
+    *program.saveProgram = 1;
 }
 
 void printProgram(struct program program) {