Programming adventures in VIC-20.

How to check exits

First, I have to discover the byte with the exits of the current loc. Then, I have to discover if I have to use the first half of the byte or the second one. Then, I have to extract the byte of the exits and, finally, I check if it is a 1, there is an exit or 0, no exit.

It seems a lot of work but using bit masks it will be easy.

This is the code of the first steps, where I take the byte and the right half of the byte. I add the beginning of the north command so you can see that a command verb calls this subroutine first to get the right half of the byte.

  
MovementCommands
        ; I have in a the current loc.
        ; I store two locs in each position of the exit vecter
        ; So I divide the loc by two to obtain the index
        lsr
        tax 
        lda exits,x 
        tay 

        ; I load the current loc again and 
        ; I check the first bit.
        ; this is the way to see if loc is pair / odd
        lda loc
        and #%00000001 
        ; If loc is odd, the exists are in 4 
        ; lower bits, so their nothing else to do
        bne Go_Back
        
        ; It is pair, so it is in the 4 ihest bits
        ; I move that bits four positions right
        tya
        lsr
        lsr
        lsr
        lsr
        tay 
Go_Back 
        rts

Norte ; North
        jsr MovementCommands