Programming adventures in VIC-20.

Movement verbs

The idea for the four verbs of movement (N, S, E, W) is the same, see if there is an exit and match LOC according to the direction. To go north I add the number of rows to loc and to go south I add the rest. To go east I add 1 to loc and to go east I add the remainder.

As I had to add or subtract and the quantity was different I made 4 subroutines.

Time later, I went through the code and discovered that I could do it with a single routine, which I'll tell you about in Movement Improvement.

This is the code for the four verbs.

  
Norte ; North
        ; Put the exit in the first
        ; four bits of y
        jsr MovementCommands
        tya
        ; Take the fourth bits
        ; the first one indicates the north
        and #%00001000
        ; If it is 0 there is no exit
        ; to north, so it is time to print 
        ; a message
        beq NoSalida 
        ; In this map, going north
        ; is adding 3
        lda loc
        clc
        adc #$3
        ; Rememeber to store the new loc
        sta loc
        jmp End

Sur ; South
        jsr MovementCommands
        tya
        and #%00000100
        beq NoSalida ; No exit
        lda loc
        ; A substraction
        ; The opposit one than north
        sec
        sbc #$3
        sta loc
        jmp End

Este ; East
        jsr MovementCommands
        tya
        and #%00000010 
        beq NoSalida ; No exit
        lda loc
        clc
        adc #$1
        sta loc
        jmp End


Oeste ; West
        jsr MovementCommands
        tya
        and #%00000001
        beq NoSalida ; No exit
        lda loc
        sec
        sbc #$1
        sta loc
        jmp End


NoSalida
        ; Print no exit
        lda #no_exit
        jsr PRINT
        rts
        
End     
        ; Jump to the subrotuine sahich perints the
        ; description of the LOC
        jsr PrintContent 
        rts