Lpc2000

I'm doing a simple port timing test using an IAR dev board and having
trouble understanding what I'm seeing. My simple routine is:

#define T 0x10

int main (void)  {                        

IODIR = 0xff0010;                             Outputs */
VPBDIV = T;
/*
VPBDIV = T;
VPBDIV = T;
VPBDIV = T;
VPBDIV = T;
VPBDIV = T;
  */

while(1)
{
	IOSET = 0x0f0010;
	IOCLR = 0x0f0010;
}

and I'm watching the port pin with a scope. The fastest I can get the
pin to toggle is ~2.5Mhz with VPBDIV=0x01. With it equal to 0x00 or
0x10 the toggle frequncy is the same, about 1.7Mhz. I'm running the
clock at~60Mhz,
and the peripheral buss at 60Mhz with VPDIV=0x01. Single stepping
using the Keil simulator shows that I'm executing 9 instuctions for
the loop. This should give me a toggle frequency of 60/9= 6.7Mhz.

Also, if I set VPBDIV multiple times, as shown in the commented out
code above, the toggle frequency decreases, all other things held the
same.

Other relavent register settings are:

MAMCR = 0x01  MAM fully enabled
MAMTIM = 0x03  3 flash fetch cycles

PLLSTAT = 0x0744  PLL Enabled M=4 D=2

The register values were read from the Keil simulator.

Can anyone set me staight?

Thanks,

Dave Sidlauskas