More software that may or may not be helpful or correct - ksjp 11/16/12

PF_done_ISR:  // frame++; that's it.

UART_ISR:
   case ' ': // Pause or unpause motion on the screen
	Pause = !Pause; break;
   case 'a': // add an object 
	if (Objects < MaxObj-1) Objects++; break;
   case 'h': //decrement X velocity
	Ship.vx--; break;
   case 'j': //increment Y velocity
	Ship.vy++ ; break;

Application.c
// cmds live at tail end of buffers, where rows 768--1023 would be
#define buf0 0x10400000
#define buf1 0x10800000
#define cmd0 0x10700000
#define cmd1 0x10b00000

while(1) {
    if (frame%2) {
	PF_frame = buf0; // PF pulls from buffer0
	WriteGPinst(cmd1,frame);  // cmds to cmd1
	GP_frame = buf1;  // GP writes to buffer1...
	GP_code = cmd1;  // reading cmds from cmd1
    } else {
 	PF_frame = buf1; // PF pulls from buffer1
	WriteGPinst(cmd0,frame);  // cmds to cmd0
	GP_frame = buf0;  // GP writes to buffer0...
	GP_code = cmd0;  // reading cmds from cmd0.
   }
   old = frame;
   while (old == frame) ; // spin waiting for PF_done_ISR
}

// my objects are single lines.  You can be fancier.
#define MaxObj 3
typedef struct {int x0,y0,x1,y1,color;} line;
line obj[MaxObj] = { 	{5,5,10,10,0x00FF00FF},       //short purple line
			{100,200,300,400,0x0000FF00},
			{700,500,30,20,0x00FFFFFF} }  // long white line

WriteGPinst(int * dst, int fr)
	*dst++ = 0x0100_0000; // fill frame with black
// Draw the border
	*dst++ = 0x02FF_0000; // red line from
	*dst++ = 0x0000_0000; //     (0,0) to
	*dst++ = 0x031F_0000; //      (799,0)
	*dst++ = 0x0200_FF00; // green line from
	*dst++ = 0x031F_0000; //      (799,0) to
	*dst++ = 0x031F_0257; //       (799, 599)
	*dst++ = ...
//update the ship position if we're not paused, and draw either way
	if (!Pause) {
		Ship.x += Ship.vx;
		Ship.y += Ship.vy;
	}
	dst = drawShip(dst);	// returns ptr to next cmd
//draw "objects".  Total number controlled by keyboard input to Objects
	for (i=0; i<Objects; i++) dst = drawObject(dst,i);

drawObject(int*dst, max)
   for (i=0; i<max; i++) {
	*dst++ = (0x02 << 24) | obj[i].color ;
	*dst++ = (obj[i].x0 << 16) | obj[i].y0;
	*dst++ = (obj[i].x1 << 16) | obj[i].y1;
   }