count up and count down

in my program i can seem to count it up! how can make it go to if(PORTB=0x01)??

i using PIC16f877a


#include<pic.h>
int decode[10]={0xC0,0xF9,0xA4,0xB0,0x99,0x92,0x82,0xF8,0x00,0x98};

void timer(void);

main()
{
TRISB=0x01;
TRISC=0x00;
PORTC=decode[0];
OPTION=0x07;
TMR0IE=1;
GIE=1;
timer();
}

void timer(void)
{
TMR0=0xD9;
while(1);
}

void interrupt blink(void)
{
static int multiply=0, x=0;
GIE=0;
multiply++;
if(TMR0IF==1)
{
TMR0IF=0;
if(multiply==10)
{

if(PORTB==0x01)
{
x++;
if(x==10)
{
x=0;
}
PORTC=decode[x];
multiply=0;
}
else
{
x--;
if(x==-1)
{
x=9;
}
PORTC=decode[x];
multiply=0;
}
}

}
GIE=1;
timer();
}