Let’s Make a Game!

It's common for Atari games to have a number of game variations. To simplify the logic, the variations are usually driven by individual bits and/or groups of bits within a single byte that holds the game variation. A good example of that would be Space Invaders—check out the game matrix from the manual:

 

 

A byte is comprised of 8 bits, usually numbered 0-7 where 7 is the leftmost bit as in 76543210.

 

For Space Invaders, the bits in the game variation are used in this fashion:

• 7 - not used
• 654 - selects 1 of 7 player variations
• Invisible Invaders
• Fast Bombs
• ZigZagging Bombs
• Moving Shields

 

Humans start counting at 1, but computers start at 0, so if you select game variation 12, internally it's really 11. 11 in binary is %00001011, which means bits 0, 1 and 3 are all turned on so the game variation has Invisible Invaders, ZigZagging Bombs and Moving Shields. You can confirm that by looking at column 12 of the Game Matrix above.

 

With this update to Collect, we're using bits 1 and 0 to give us 4 game variations:

• 765432 - not used
• 1 - Arena 1 or 2
• 0 - # of players

 

If we have space at the end of the project, I plan to add some additional arenas. If we can add 2 more we'd just start using bit 2 and let the game Variation go from 1-8:

• 76543 - not used
• 21 - Arena 1, 2, 3 or 4
• 0 - # of players

 

If we can add 6 more we'd add bit 3 and let the game variation go from 1-16

• 76543 - not used
• 321 - Arena 1, 2, 3, 4, 5, 6, 7 or 8
• 0 - # of players

 

The ProcessSwitches routine has been modified so that hitting Select will increment the new variable Variation. It will also limit Variation to only the values 0-3. After changing Variation, the left score will be set to show Variation+1 as Humans prefer to see 1-4 instead of 0-3. The Right Score will be used to show the # of players, either 1 or 2.

NotReset:
        lsr             ; D1 is now in C
        bcs NotSelect   ; if D1 was on, the SELECT switch was not held
        lda #0
        sta GameState   ; clear D7 to signify Game Over        
        ldx Variation   ; Get the Game Variation
        inx             ; and increase it
        txa             ; transfer it to A
        and #%00000011  ; limit Variation to 0-3
        sta Variation   ; save it
        tax             ; transfer it to X
        inx             ; and increase it by 1 for the human readable varation 1-4
        stx Score       ; save in Score so it shows on left side
        ldy #1          ; default to showing 1 player variation 
        lsr             ; D0 of Variation, # of players, now in Carry flag
        bcc Not2        ; if Carry is clear, then show 1 player 
        iny             ; else set Y to 2 to show 2 players
Not2:
        ror Players     ; put Carry into D7 for BIT testing of # of players
        sty Score+1     ; show the human readable # of players on right side
NotSelect:
        rts

The routine works, but when Select is pressed the game variation will rapidly change making it difficult to select a specific game variation. You can see that in the build in my blog entry.

 

To fix that, we'll add a SelectDelay variable so that holding down SELECT will only result in Variation changing at the rate of once per second. However, if the user rapidly presses/releases SELECT then Variation will also rapidly change.

ProcessSwitches:
        lda SWCHB       ; load in the state of the switches
        lsr             ; D0 is now in C
        bcs NotReset    ; if D0 was on, the RESET switch was not held
        jsr InitPos     ; Prep for new game
        lda #%10000000  
        sta GameState   ; set D7 on to signify Game Active  
        bne NotSelect   ; clear SelectDelay
        
NotReset:
        lsr             ; D1 is now in C
        bcs NotSelect   ; if D1 was on, the SELECT switch was not held
        lda #0
        sta GameState   ; clear D7 to signify Game Over        
        lda SelectDelay ; do we need to delay the Select switch?
        beq SelectOK    ; if delay is 0 then no
        dec SelectDelay ; else decrement the delay
        rts             ; and exit the subroutine
        
SelectOK:
        lda #60         ; Set the Select Delay to 1 second
        sta SelectDelay ;
        ldx Variation   ; Get the Game Variation
        inx             ; and increase it
        txa             ; transfer it to A
        and #%00000011  ; limit Variation to 0-3
        sta Variation   ; save it
        tax             ; transfer it to X
        inx             ; and increase it by 1 for the human readable varation 1-4
        stx Score       ; save in Score so it shows on left side
        ldy #1          ; default to showing 1 player variation 
        lsr             ; D0 of Variation, # of players, now in Carry flag
        bcc Not2        ; if Carry is clear, then show 1 player 
        iny             ; else set Y to 2 to show 2 players
Not2:
        ror Players     ; put Carry into D7 for BIT testing of # of players
        sty Score+1     ; show the human readable # of players on right side
        rts
        
NotSelect:
        lda #0          ; clears SelectDelay if SELECT not held
        sta SelectDelay
        rts

The routine PositionObjects has been modified to use a Box Graphic for player1 if a 1 player game has been selected:

PositionObjects:
...
        lda Variation       ; get the game variation
        and #1              ; and find out if we're 1 or 2 player
        tax
    ; 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 <(BoxGfx + HUMAN_HEIGHT - 1)
        .byte <(HumanGfx + HUMAN_HEIGHT - 1)
        
ShapePtrHi:
        .byte >(BoxGfx + HUMAN_HEIGHT - 1)
        .byte >(HumanGfx + HUMAN_HEIGHT - 1)

The Kernel has also been modified so that the correct Arena will be drawn. A little bit after TimerBar: you'll find this:

TimerBar:
...
        lda Variation       ; 3 20
        lsr                 ; 2 22 - which Arena to show
        tay                 ; 2 24 - set for index
        ldx ArenaOffset,y   ; 4 28 - set X for which arena to draw
        lda ArenaPF0,x      ; 4 32 - reflect and priority for playfield
        and #%00000111      ; 2 34 - get the lower 3 bits for CTRLPF
        ora #%00110000      ; 2 36 - set ball to display as 8x pixel 
        sta CTRLPF          ; 3 39
...
ArenaOffset:
        .byte 0         ; Arena 1
        .byte 22        ; Arena 2

The lsr command shifts bit 1 down to bit 0 so that we end up with 0 or 1 for the Arena number. That's used to set X to either 0 or 22 via the command ldx ArenaOffset,y.

 

I also added code to update CTRLPF based on the first PF0 data byte for the selected Arena. CTRLPF uses its bits like this:

• 76 - not used
• 54 - set width of BALL object
• 3 - not used
• 2 - Playfield Priority
• 1 - Score Mode
• 0 - Reflected Playfield

 

Since PF0 only uses bits 7654, also known as the upper nybble of the byte, we can use the lower nybble to hold extra information to specify whether or not the selected Arena uses Playfield Priority (as opposed to Player Priority) or has a Reflected Playfield(as opposed to Repeated Playfield). We could even specify Score Mode which would just color the two sides of the playfield to match the colors of the players (like in the score display).

ArenaPF0:   ; PF0 is drawn in reverse order, and only the upper nybble
        .byte %11110001 ; Arena 1   lower nybble controls playfield, set for REFLECT
        .byte %00010000
        .byte %00010000
        .byte %00010000
...        
        .byte %11110100 ; Arena 2 - lower nybble controls playfield, set for PLAYFIELD PRIORITY
        .byte %00010000
        .byte %00010000
        .byte %00010000

Game Variation 2, Arena 1 with 2 players. Arena 1 features Reflected Playfield and Player Priority:

Game Variation 3, Arena 2 with 1 player. Arena 2 features Repeated Playfield and Playfield Priority:

Look at the left Humanoid's head in each screenshot to see the difference that setting Playfield Priority makes. You might remember this being used in some games like Combat where the planes go behind the clouds.

Just for fun, here's Arena 2 with SCORE mode set (I've moved the players to the side of the screen they didn't start on):

The code change for that is:

.byte %11110010 ; Arena 2 - lower nybble controls playfield, set for SCORE

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

 

 

 

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Table of Contents for Let’s Make a Game!

Introduction

Step 1: Generate a Stable Display

Step 2: Timers

Step 3: Score and Timer Display

Step 4: 2 Line Kernel

Step 5: Automate Vertical Delay

Step 6: Spec Change

Step 7: Draw the Playfield

Step 8: Select and Reset Support

Step 9: Game Variations

Step 10: “Random Numbers”

Step 11: Add the Ball Object

Step 12: Add the Missile Objects

Step 13: Add Sound Effects

Step 14: Add Animation