basic types and memory operations

common.c

@@ -0,0 +1,64 @@
+#include "common.h"
+
+void putchar(char ch);
+
+void printf(const char *fmt, ...) {
+  va_list vargs;
+  va_start(vargs, fmt);
+
+  while (*fmt) {
+    if (*fmt == '%') {
+      fmt++; // Skip '%'
+      switch (*fmt) { // Read the next character
+        case '\0': // '%' at the end of the format string
+          putchar('%');
+          goto end;
+        case '%': // Print '%'
+          putchar('%');
+          break;
+        case 's': { // Print a NULL-termintated string
+          const char *s = va_arg(vargs, const char *);
+          while (*s) {
+            putchar(*s);
+            s++;
+          }
+          break;
+        }
+        case 'd': { // Print an integer in decimal
+          int value = va_arg(vargs, int);
+          unsigned magnitude = value;
+          if (value < 0) {
+            putchar('-');
+            magnitude = -magnitude;
+          }
+
+          unsigned divisor = 1;
+          while (magnitude / divisor > 9)
+            divisor *= 10;
+
+          while (divisor > 0) {
+            putchar('0' + magnitude / divisor);
+            magnitude %= divisor;
+            divisor /= 10;
+          }
+
+          break;
+        }
+        case 'x': { // Print an integer in hexademical
+          unsigned value = va_arg(vargs, unsigned);
+          for (int i = 7; i >= 0; i--) {
+            unsigned nibble = (value >> (i * 4)) & 0xf;
+            putchar("0123456789abcdef"[nibble]);
+          }
+        }
+      }
+    } else {
+      putchar(*fmt);
+    }
+
+    fmt++;
+  }
+
+end:
+  va_end(vargs);
+}