uVision simulation of external pullup resistor

Hi all,

For an LPC2132 application I required a third I2C bus and implemented this using
bit banging of GPIO pins. A logical 0 is implemented by driving the GPIO pin to
low, a logical 1 is implemented by setting the GPIO to input and having an
external resistor pulling the signal high.
This works perfectly in real hardware.

As I generally simulate my software with Keil uVision, I have the problem of
simulating the external pullup resistors. The simulation function below, shows
errors when I try to raise the PORT0 to high.
The error message is something like "Trying to write to a GPIO that is
configured as output".

Does anybody have a different solution to simulate pullups?
What is wrong with my pullup implementation?

Thanks.

----

define unsigned long io0dir_old[32];

func void
pullup( int port, int bit_num )
{
    unsigned long io0dir;
    unsigned long mask;

    mask = (1<<bit_num);
    io0dir = _RDWORD( &IO0DIR );
    printf( "IO0DIR event: IO0DIR = %08x\n", io0dir );
    if( (io0dir & mask) != (io0dir_old[bit_num] & mask) ) {
        printf( "io0dir(%d) changed to %d\n", bit_num, (io0dir&mask)>>bit_num );
        if( (io0dir & mask) == 0 ) {        // if direction==input
            //swatch( 0.0000001 );
            if( (PORT0 & mask) == 0 ) {
                PORT0 |= mask;              // ...scl=1
                printf( "Pullup activated on PORT%d.%d\n", port, bit_num );
            }
        }
    }
    io0dir_old[bit_num] = io0dir;
}


signal void
pullup_port0_18(void)
{
    int bit_num;

    bit_num = 18;
    io0dir_old[bit_num] = _RDWORD( &IO0DIR );
    while (1) {
        wwatch( &IO0DIR );
        pullup( 0, bit_num );
    }
}

pullup_port0_18();