pic - dsPIC33 and the upper half of PORTB

I am trying to use some of the IO pins of the dsPIC33FJ128GP802. The pins in question are RB8 thru RB11.

I have written a little program that reads the input value of one of those pins and outputs it to pin RA4.

All I get is straight 0, which ever of those pins I use.

If I use any PORTB pin from 7 down (i.e., the lower byte of the register), it functions perfectly.

I have scraped through the errata, and found nothing that has any bearing on this problem.

What am I missing?

Test program:

#include 

_FWDT(FWDTEN_OFF)
_FOSCSEL(FNOSC_PRIPLL)
_FOSC(POSCMD_HS)

int main(void)
{
    unsigned int x=0;
    // Configure clock and PLL for 40 MIPS from 4MHz crystal

    CLKDIVbits.DOZE = 0;
    CLKDIVbits.DOZEN = 0;
    CLKDIVbits.FRCDIV = 0;
    CLKDIVbits.PLLPOST = 0;
    CLKDIVbits.PLLPRE = 0;
    PLLFBDbits.PLLDIV = 78;

    // Disable the CCP output pins as per a bug mentioned in the errata.
    // Should not apply to this case, but just to be sure...
    CMCONbits.C1OUTEN=0;

    CMCONbits.C2OUTEN=0;

    // Disable all ADC pins (make them digital)
    AD1PCFGL = 0xFFFF;

    // Configure IO pins
    TRISBbits.TRISB10=1;    // RB10 Input
    TRISAbits.TRISA4=0;     // RA4 Output

    while(1)

    {
        // Send input state to output pin
        LATAbits.LATA4 = PORTBbits.RB10;
    }
}

Other useful info:

Microchip Language Tool Shell Version v3_30 (Build date: Jun 21 2011).

The inputs are being driven by an Arduino, but using that, or hard-wiring to \$V_{LH}\$ or \$V_{LL}\$ makes no difference.

_{P.S. I am no newbie to PICs, having spent many hours on PIC16s and PIC18s, so don't bother with the "obvious" answers.}

Answer

Ok, I have had a response on the Microchip forums...

Apparently the pins associated with the upper half of the PORTB register are shared with the JTAG system. JTAG defaults to on, and can be disabled in the configuration fuses. In MPLAB C30 it is:

_FICD(JTAGEN_OFF)

to disable JTAG.

Once this is done those pins all work perfectly.