IN PBP we will write something like this...
Code:
TRISB=0 ' set PORTB as output 
TRISA=%11111101 ' set PORTA as input except PORTA.1

a VAR BYTE
a=1


Loop:
     Toggle PORTA.1
     IF PORTA.0=0 then Gosub DoSomething ' testing PORTA.0 status
     goto Loop

DoSomething:
     PORTB=a       ' send to PORTB
     if a<255 then
          a=a+1
     else
          a=1
     endif
     return
IN PBC, IMO it will look something like.. please correct me someone if i'm wrong

Code:
POKE $86,0 ' set PORTB as output
POKE $85,253 ' set PORTA.1 as output other as input
b0=0

loop:
     toggle 0
     PEEK $05,b1 ' read PORTA status
     b1=b1 & 1 ' keep only bit 0 of PORTA
               ' well i don't know if you can use something like
               ' if b1.0=0 then gosub DoSomething... worth to give a try
     if b1=0 then gosub DoSomething
     goto loop

DoSomething:
     POKE $06,b0       ' send to PORTB
     if b0<255 then
          b0=b0+1
     else
          b0=1
     endif
     return
this will toggle the PORTA.1 pin untill PORTA.0 go low, once it go to low, it will send the value of b0 var to PORTB.... well it's suppose to