Let’s Make a Game!

Step 6: Spec Change

By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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Original Blog Entry

As anybody involved in writing software can tell you, project specifications will often change when new information becomes available.

When I started working on Collect, my plan was to use it for my presentation at Classic Game Fest. As I've progressed I've come to the realization that a full blown game is going to be just too much information for a one hour presentation. I decided that I'm going to leave the existing example in place and just add a few slides about Collect with a link to these blog entries for anybody who is interested.

Since I'm no longer planning to fit this project into a presentation, I've decided on a few changes:

Make Collect a 1-2 player game
Add the Ball object to draw an additional box, especially useful for 2 player games
Change Timer display to the right player's score
Add a Timer Bar to indicate time remaining

Mockup of two player game with new timer bar:

Timer has decreased:

One player variation will hide right player's score and use player1 as an additional box:

The Ball Object has a vertical delay feature. When used, the ball should be updated on the same scanline as player0. Due to this, I've revised the 2LK to be like this:

update player1, update playfield, precalc player0 data for next scanline
update player0, update playfield, precalc player1 data for next scanline

This was done to plan ahead for when the playfield is no longer updated on every scanline. Updating the playfield and ball will make the 2LK look something like this:

update player1, update playfield, precalc player0 and ball data for next scanline
update player0, update ball, precalc player 1 data for next scanline

I'll also need to add in updates for the missiles. Ideally we want to update them on every scanline like this:

update player1, update missile0, update missile1, update playfield, precalc player0 and ball and missile data for next scanline
update player0, update missile0, update missile1, update ball, precalc player 1 and missile data for next scanline

It's possible the timing won't work out for that. If it doesn't, then a change like this should work:

update player1, update missile1, update playfield, precalc player0 and ball and missile0 data for next scanline
update player0, update missile0, update ball, precalc player1 and missile1 data for next scanline

That would make it so that the missile objects can only start on every-other-scanline, but that's an OK compromise for our game.

In this build I've revised the Arena to be a little bit shorter to make room for the new timer display. The timer currently "ticks" once every 64 frames. Whenever it ticks, a bunch of byte rotations are done to shorten the length of the timer bar.

DecrementTimer:
        lsr TimerPF+5   ; PF2 right side, reversed bits so shift right
        rol TimerPF+4   ; PF1 right side, normal bits so shift left
        ror TimerPF+3   ; PF0 right side, reversed bits so shift right
        lda TimerPF+3   ; only upper nybble used, so we need to put bit 3 into C
        lsr
        lsr
        lsr
        lsr
        ror TimerPF+2   ; PF2 left side, reversed bits so shift right
        rol TimerPF+1   ; PF1 left side, normal bits so shift left
        ror TimerPF     ; PF0 left side, reversed bits so shift right
        rts

Since there are 40 playfield pixels, the total playtime would be 40*64/60 = 42.7 seconds. We might decide that's too short of a play time. If so, we'll just change the tick to occur every 128 frames for 40*128/60 = 85.3 seconds of game time, or maybe even once very 256 frames for 40*256/60 = 170.7 seconds.

SetObjectColors has been modified to add a color for the timer bar. The Timer Bar and the Arena are both drawn using the playfield, so to make the Arena a different color than the Timer Bar I store the current Arena color in a RAM location.

SetObjectColors:        
        ldx #4          ; we're going to set 5 colors (0-4)
        ldy #4          ; default to the color entries in the table (0-4)
        lda SWCHB       ; read the state of the console switches
        and #%00001000  ; test state of D3, the TV Type switch
        bne SOCloop     ; if D3=1 then use color
        ldy #9          ; else use the b&w entries in the table (5-9)
SOCloop:        
        lda Colors,y    ; get the color or b&w value
        sta COLUP0-1,x  ; and set it
        dey             ; decrease Y
        dex             ; decrease X 
        bne SOCloop     ; Branch Not Equal to Zero
        lda Colors,y    ; get the Arena color
        sta ArenaColor  ; save in RAM for Kernal Usage
        rts             ; ReTurn from Subroutine
        
Colors:   
        .byte $46   ; red        - goes into COLUPF, color for Arena (after Timer is drawn)
        .byte $86   ; blue       - goes into COLUP0, color for player0 and missile0
        .byte $C6   ; green      - goes into COLUP1, color for player1 and missile1
        .byte $64   ; purple     - goes into COLUPF, color for Timer
        .byte $00   ; black      - goes into COLUBK, color for background
        .byte $0A   ; light grey - goes into COLUPF, color for Arena (after Timer is drawn)
        .byte $0E   ; white      - goes into COLUP0, color for player0 and missile0
        .byte $06   ; dark grey  - goes into COLUP1, color for player1 and missile1
        .byte $04   ; dark grey  - goes into COLUPF, color for Timer
        .byte $00   ; black      - goes into COLUBK, color for background

For testing, I've set it up so the Right Difficulty switch is used to determine if the game is a one or two player game for which graphics to use for player1:

ldx #0
        bit SWCHB
        bpl TwoPlayer
        ldx #1
TwoPlayer:        
    ; Player1Ptr = BoxGfx + HUMAN_HEIGHT - 1 - Y position
        lda ShapePtrLow,x
        sec
        sbc Temp
        sta Player1Ptr
        lda ShapePtrHi,x
        sbc #0
        sta Player1Ptr+1
        
        rts
        
ShapePtrLow:
        .byte <(HumanGfx + HUMAN_HEIGHT - 1)
        .byte <(BoxGfx + HUMAN_HEIGHT - 1)
        
ShapePtrHi:
        .byte >(HumanGfx + HUMAN_HEIGHT - 1)
        .byte >(BoxGfx + HUMAN_HEIGHT - 1)

Right Difficulty = B:

Right Difficulty = A:

The ROM and the source are at the bottom of my blog entry.