add platform files

Add files defining platform constants

In particular, break out platform.c and do not optimize so that side-effects are honored.

platform.c

@@ -0,0 +1,35 @@
+#include "platform.h"
+
+#define KBD_STATUS_OR_KEY ((volatile unsigned char *)0xC000)
+#define KBD_CLEAR_CURRENT_KEY ((volatile unsigned char *)0xC010)
+
+#define KEY_READY_MASK 0x80
+#define KEY_VALUE_MASK 0x7F
+
+// Returns non-zero if a key is ready to be read.
+int keyboard_test(void)
+{
+    return *KBD_STATUS_OR_KEY & KEY_READY_MASK;
+}
+
+// Clears current key, sets up next one for test or get
+void keyboard_clear(void)
+{
+    unsigned char clear_key;
+
+    // On 6502, a write cycle READS AND THEN WRITES, so just read the
+    // "latch next key" strobe.
+    clear_key = *KBD_CLEAR_CURRENT_KEY;
+}
+
+// Wait until a key is ready and then return it without high bit set
+unsigned char keyboard_get(void)
+{
+    unsigned char key;
+
+    while(!((key = *KBD_STATUS_OR_KEY) & KEY_READY_MASK));
+    
+    keyboard_clear();
+
+    return key & KEY_VALUE_MASK;
+}

platform.h

@@ -0,0 +1,11 @@
+#define TEXT_PAGE1_BASE ((volatile unsigned char *)0x400)
+#define TEXT_PAGE2_BASE ((volatile unsigned char *)0x800)
+
+// Returns non-zero if a key is ready to be read.
+extern int keyboard_test(void);
+
+// Clears current key, sets up next one for test or get
+extern void keyboard_next(void);
+
+// Wait until a key is ready and then return it without high bit set
+extern unsigned char keyboard_get(void);