Initial v0.8 ALPHA commit. Features multi-tasking run-time command execution (feed hold, cycle start, reset, status query). Extensive re-structuring of code for future features.

- ALPHA status. - Multitasking ability with run-time command executions
for real-time control and feedback. - Decelerating feed hold and resume
during operation. - System abort/reset, which immediately kills all
movement and re-initializes grbl. - Re-structured grbl to easily allow
for new features: Status reporting, jogging, backlash compensation. (To
be completed in the following releases.) - Resized TX/RX serial buffers
(32/128 bytes) - Increased planner buffer size to 20 blocks. - Updated
documentation.

main.c

@@ -3,7 +3,9 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-
+  Copyright (c) 2011 Sungeun K. Jeon
+  Copyright (c) 2011 Jens Geisler
+  
   Grbl is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
@@ -21,9 +23,11 @@
 #include <avr/io.h>
 #include <avr/sleep.h>
 #include <avr/interrupt.h>
+#include <avr/pgmspace.h>
 #include <util/delay.h>
 #include "config.h"
 #include "planner.h"
+#include "nuts_bolts.h"
 #include "stepper.h"
 #include "spindle_control.h"
 #include "motion_control.h"
@@ -34,22 +38,56 @@
 #include "settings.h"
 #include "serial.h"
 
+#include "print.h"
+
+// Declare system global variables
+uint8_t sys_abort; // Global system abort flag
+volatile uint8_t sys_state; // Global system state variable
+
 int main(void)
 {
-  sei();
-  
-  serial_init(BAUD_RATE);
-  protocol_init();        
-  settings_init();  
-  plan_init();      
-  st_init();        
-  spindle_init();   
-  gc_init();
-  limits_init();
+  // Initialize system
+  sei(); // Enable interrupts
+  serial_init(BAUD_RATE); // Setup serial baud rate and interrupts
+  st_init(); // Setup stepper pins and interrupt timers
+  sys_abort = true;   // Set abort to complete initialization
                     
-  for(;;){
-    sleep_mode(); // Wait for it ...
+  while(1) {
+  
+    // Upon a system abort, the main program will return to this loop. Once here, it is safe to 
+    // re-initialize the system. Upon startup, the system will automatically reset to finish the 
+    // initialization process.
+    if (sys_abort) {
+      // Execute system reset
+      sys_state = 0; // Reset system state
+      sys_abort = false; // Release system abort
+        
+      // Reset system.
+      serial_reset_read_buffer(); // Clear serial read buffer
+      settings_init(); // Load grbl settings from EEPROM
+      protocol_init(); // Clear incoming line data
+      plan_init(); // Clear block buffer and planner variables
+      gc_init(); // Set g-code parser to default state
+      spindle_init();   
+      limits_init();
+
+      // TODO: For now, the stepper subsystem tracks the absolute stepper position from the point
+      // of power up or hard reset. This reset is a soft reset, where the information of the current
+      // position is not lost after a system abort. This is not guaranteed to be correct, since 
+      // during an abort, the steppers can lose steps in the immediate stop. However, if a feed
+      // hold is performed before a system abort, this position should be correct. In the next few
+      // updates, this soft reset feature will be fleshed out along with the status reporting and
+      // jogging features.
+      st_reset(); // Clear stepper subsystem variables. Machine position variable is not reset.
+
+      // Print grbl initialization message
+      printPgmString(PSTR("\r\nGrbl " GRBL_VERSION));
+      printPgmString(PSTR("\r\n'$' to dump current settings\r\n"));
+    }
+    
+    protocol_execute_runtime();
     protocol_process(); // ... process the serial protocol
+    
   }
   return 0;   /* never reached */
 }