Newbie trying to figure out horizontal movement. Like a space invaders base/ship.

[dgrams2000]

So, I am doing some asm, up to the point of putting my sprite/ship on the screen at the bottom, like in space invaders.

 

I can get it to move left, right... but I can't for the life of me figure out how to figure out the screen limits for the ship.. and per my code, my ship is stopping at weird points on the screen.

 

The more I tweak it, I get things like fluttering sprites... then I adjust things like the height of my sprite through the drawing of my bitmap loop.  (Its 8, with a 9th row of blank)

but sometimes I make the drawing loop 8, sometimes 9, to get it to move smooth.  Not sure what is affecting it.  

 

I get that coding on the 2600 is a challenge.. but what is the best way to simplify it for us Newbies that are trying to understand, but need a little more explanation with things like horizontal movement...

Are other retro consoles this challenging?  C64/Atari400/800/nes, etc..

[+splendidnut]

8 hours ago, dgrams2000 said:

Are other retro consoles this challenging?

Not that I know of.  The 2600 method of horizontal positioning is very unique.

 

Without seeing any code, it's hard to make suggestions.

[+bent_pin]

As you really don't have enough of the game to give away the farm yet, consider posting your code for better help.

[dgrams2000]

                processor 6502              ; Default stuff

                include  "vcs.h"            ; Default stuff

                include  "macro.h"          ; Default stuff

 

;###################Cleanup tasks & variables################

    seg.u Variables

    org $80

 

Player0XPos                      byte   ; sprite X coordinate

 

    seg Code

    org $F000

reset

; Clear RAM and all TIA registers

    ldx #0

    lda #0

Clear          

    sta 0,x

    inx

    bne Clear

 

BACKCOLOR            = $94             ; define a symbol to represent a TIA color value (NTSC) $82 is Drk blue

                lda #$C2                ; greenw playfield color

                sta COLUPF              ; sets color value to PF

 

                lda #$62                ;color purple

                sta COLUP0              ;set color to player 0

                lda #39

                sta Player0XPos     ; initialize player X coordinate        

;################END Cleanup Tasks and variables#################

 

;##################PLAYFIELD REFLECT OPTION##############

reflect:

    ldx #%00000001 ; CTRLPF register (D0 is the reflect flag)

    stx CTRLPF

;##################### End PLAYFIELD REFLECT OPTION #####

 

;###########SET BACKGROUND####################            

background:

    lda #BACKCOLOR                    ; load the value from the symbol 'blue' into (a)

    sta COLUBK                          ; store (a) into the TIA background color register

;############END BACKGROUND###################

 

;#########DRAW THAT SCREEN!###################

 

StartFrame:                             ; start of new frame @ top of screen (Runs WYSYNC 3 times)

        ldx #%00000000

        stx PF0

        stx PF1

        stx PF2

 

    lda #2

    sta VBLANK     ; turn VBLANK on

    sta VSYNC      ; turn VSYNC on

    REPEAT 3

        sta WSYNC  ; three VSYNC scanlines

    REPEND

    lda #0

    sta VSYNC      ; turn VSYNC off                                       ; start of vertical blank processing

 

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Set player horizontal position while we are in the VBLANK

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

    lda Player0XPos     ; load register A with desired X position

    and #$7F       ; same as AND 01111111, forces bit 7 to zero

                   ; keeping the value inside A always positive

 

    sec            ; set carry flag before subtraction

 

    sta WSYNC      ; wait for next scanline

    sta HMCLR      ; clear old horizontal position values

 

DivideLoop:

    sbc #15        ; Subtract 15 from A

    bcs DivideLoop ; loop while carry flag is still set

    eor #7         ; adjust the remainder in A between -8 and 7

    asl            ; shift left by 4, as HMP0 uses only 4 bits

    asl

    asl

    asl

    sta HMP0       ; set fine position value

    sta RESP0      ; reset rough position

    sta WSYNC      ; wait for next scanline

    sta HMOVE      ; apply the fine position offset

 

;-----------END OF 3 VSYNC LINES Now draw VERTICAL BLANK (37-2 Lines)

    REPEAT 35

        sta WSYNC

    REPEND

 

    lda #0

    sta VBLANK     ; turn VBLANK off

;----------------------------End Of Top 'Blank' Areas------------------------------

;---------------------DRAW SCREEN ROUTINE - draw 192 lines-----------------------

 

;------------------- Initialize X for 192 scanlines------------------------------

 

     REPEAT 184

        sta WSYNC  ; wait for 60 empty scanlines

    REPEND

 

        ldy 9      ; counter to draw 8 rows of bitmap

DrawBitmap:

    lda Player0Bitmap,Y ; load player bitmap slice of data

    sta GRP0       ; set graphics for player 0 slice

 

    lda P0Color,Y  ; load player color from lookup table

    sta COLUP0     ; set color for player 0 slice

 

    sta WSYNC      ; wait for next scanline

 

   dey

    bne DrawBitmap ; repeat next scanline until finished

 

    lda #0

    sta GRP0       ; disable P0 bitmap graphics

 

;----------------------End Player0 Bitmap-------------

 

    ldx #%00000000

    stx PF0

    stx PF1

    stx PF2

 

; -------------------------------------------END OF PLAYFIELD---------------

 

;------------------------- generate 30 scanlines of overscan bottom of screen (no changes needed here)----

 

;   ldx #0                              ; uses loop of 30 times to complete

overscan:    

    lda #2  

    sta VBLANK                          ; turn on vblank for overscann

    REPEAT 30

        sta WSYNC

    REPEND

 

CheckP0Left:

    lda #%01000000

    bit SWCHA

    bne CheckP0Right

    dec Player0XPos

            cmp Player0XPos,5

            bne CheckP0Right

             inc Player0XPos

               

 

CheckP0Right:

    lda #%10000000

    bit SWCHA

    bne NoInput

    inc Player0XPos

        cmp Player0XPos,50

        bne NoInput

        dec Player0XPos

 

skipincrease:

 

skipdecrease:

 

NoInput:

    ; fallback when no input was performed

 

    ldx #%00000000

    stx PF0

    stx PF1

    stx PF2

 

    jmp StartFrame                    ; frame is completed, branch back up to the 'startFrame' label

;############################## end of screen draw  LOOPS BACK TO StartFrame ##################

 

;########### BITMAPS ##########################################################################

;--------------Player 0 bitmap----------------

;   org $FFE8

Player0Bitmap:

    .byte %00000000 ;

    .byte %10000001 ;

    .byte %01011010 ;

    .byte %10100101 ;

    .byte %00011000 ;

    .byte %00011000 ;

    .byte %00011000 ;

    .byte %00011000 ;

    .byte %00011000 ;

 

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Lookup table for the player colors

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

P0Color:

    byte #$00

    byte #$62

    byte #$56

    byte #$44

    byte #$44

    byte #$44

    byte #$44

    byte #$44

    byte #$44

 

;############# END OF BITMAPS #################################################################

 

; ---------------------------DEFAULT END OF PROGRAM JUNK

;   org $fffa  

    org $FFFC                         ; set origin to last 6 bytes of 4k rom

 

irq:

                .word reset                 ; NMI  ($FFFA) (Default stuff)

                .word reset                 ; RESET ($FFFC) (Default stuff)

                .word reset                 ; IRQ ($FFFE) (Default stuff)

 

[dgrams2000]

10 hours ago, bent_pin said:

As you really don't have enough of the game to give away the farm yet, consider posting your code for better help.

done. thx

[+bent_pin]

You have an extra compare in your left and right positioning adjustments, can you explain the purpose?

 

In the future, please post code as text in code tags:

 

Makes it much easier for those trying to help.

[dgrams2000]

On 1/14/2024 at 11:10 AM, bent_pin said:

You have an extra compare in your left and right positioning adjustments, can you explain the purpose?

 

In the future, please post code as text in code tags:

 

Makes it much easier for those trying to help.

After the joystick position check, the first compare just lowers the value for moving the sprite...  The second compare says that if it crosses a certain point, to increase by one, so that it cant keep moving left.  (Stops at left side of screen for example.)

 

[+bent_pin]

33 minutes ago, dgrams2000 said:

The second compare says that if it crosses a certain point, to increase by one, so that it cant keep moving left.

I thought that is what you were trying to do. That is not what you've done, though.

cmp Player0XPos,50
; cmp followed by a memory ZP memory address,offset
; compares what is in the accumulator to what is in the memory address + offset
; same flags as sbc with the same values
; Player0XPos address = $80
; $80 + 50 = 178 or $B2
; This instruction compares value in ZP$B2 to value in accumulator
; Value currently in accumulator is bitmask #%10000000 or -128

 

To compare the Player0XPos to 50, you need to load that value into the accumulator first, then use compare with the immediate value of 50.

lda Player0Pos
cmp #50

[dgrams2000]

47 minutes ago, bent_pin said:

I thought that is what you were trying to do. That is not what you've done, though.

cmp Player0XPos,50
; cmp followed by a memory ZP memory address,offset
; compares what is in the accumulator to what is in the memory address + offset
; same flags as sbc with the same values
; Player0XPos address = $80
; $80 + 50 = 178 or $B2
; This instruction compares value in ZP$B2 to value in accumulator
; Value currently in accumulator is bitmask #%10000000 or -128

 

To compare the Player0XPos to 50, you need to load that value into the accumulator first, then use compare with the immediate value of 50.

lda Player0Pos
cmp #50

CheckP0Left:
    lda #%01000000
    bit SWCHA
    bne CheckP0Right
    dec Player0XPos
    lda Player0XPos
    cmp #13
    bne CheckP0Right
    inc Player0XPos
               
CheckP0Right:
    lda #%10000000
    bit SWCHA
    bne NoInput
    inc Player0XPos
    lda Player0XPos
    cmp #124
    bne NoInput
    dec Player0XPos

Interesting.  That sound like a crucial thing that I did not know about.  Looks like that worked.  I can now move my ship accurately across the bottom, stopping at certain points.

 

Seems like you just cant plug in any old positions to compare against..  Like in my example above, If I change the left side to CMP #10, My sprite jumps all over moving left and right.  If it is CMP #13.. works fine.   I am guessing this has something to do with the fine tuning the object in a certain area (-8 to +7 pixels)  and that your numbers have to match up somewhere within that.   But anyways.. yes, it is working.  Thank you for the tip!  I will have about a bazillion more questions soon, I am sure.  haha

Edited January 18 by dgrams2000

[+bent_pin]

17 minutes ago, dgrams2000 said:

After the compare is done, should the accumulator get blanked out then?

Typically, I begin each operation by loading values into registers. As such, I see no reason in particular to clear values from registers. I usually assume that they've got garbage in them unless I'm intentionally carrying values through as part of an algorithm.

[dgrams2000]

2 minutes ago, bent_pin said:

Typically, I begin each operation by loading values into registers. As such, I see no reason in particular to clear values from registers. I usually assume that they've got garbage in them unless I'm intentionally carrying values through as part of an algorithm.

Makes sense.   

[+bent_pin]

28 minutes ago, dgrams2000 said:

Seems like you just cant plug in any old positions to compare against..  Like in my example above, If I change the left side to CMP #10, My sprite jumps all over moving left and right.  If it is CMP #13.. works fine.   I am guessing this has something to do with the fine tuning the object in a certain area (-8 to +7 pixels)  and that your numbers have to match up somewhere within that.   But anyways.. yes, it is working.  Thank you for the tip!  I will have about a bazillion more questions soon, I am sure.  haha

That's because of the type of branch that you are using. You are only checking for equality when you should be checking for a range. Try this one on your own. Think of what types of flags can branches that there are. Try just the left edge to begin with. You can use the carry flag and if you subtract a border value from the position value and that subtraction crosses the zero barrier, the carry flag will be cleared.

 

If you get the left side working, check back for a tip on the right side.