[Tutorial] How to create a Simple Scrolling HUD in Sonic 1

[Mildanner]

Original Guide by Mildanner (ft. RetroKoH)

In game development, the developers didn't know how to make a simple scrolling HUD and decided to make the HUD pop in the screen without any doubts. In this guide, you will learn how to make a simple scrolling HUD.

Hivebrain 2005 (ASM68K)​

In the Hivebrain 2005, the object for HUD was still in sonic1.asm file. You should find the Obj21_Main label and replace this entire routine with this:

Obj21_Main:                ; XREF: Obj21_Main
        addq.b    #4,$24(a0)    ; Advance to Routine 4 (Obj21_Move) on the next frame
        move.w    #0,8(a0)    ; Start offscreen (X = 0)
        move.w    #$108,$a(a0)    ; Set Y coordinate (obScreenY)
        move.l    #Map_obj21,4(a0); Set mappings pointer (obMap)
        move.w    #$6ca,2(a0)    ; Set VRAM art tile (obGfx)
        move.b    #0,1(a0)    ; Set render settings (obRender)
        move.b    #0,$18(a0)    ; Set display priority (obPriority)
        rts

; ===========================================================================

Obj21_Move:                ; Routine 4
        move.w    8(a0),d0    ; Load current X position
        cmpi.w    #$90,d0        ; Has it reached position $90?
        bge.s    @reached    ; If greater or equal, finalize movement

        addq.w    #5,d0        ; Move 5 pixels to the right
        move.w    d0,8(a0)    ; Update X coordinate directly
        jmp    DisplaySprite

@reached:
        move.b    #2,$24(a0)    ; Set routine status to Routine 2
        jmp    DisplaySprite

If the code doesn't work, you should go above Obj21_Index and after the line. Insert this (After the Obj21_Move label)

        dc.w Obj21_Move-Obj21_Index

Done! Now your hack has a fellow HUD scrolling.

(OPTIONAL) Adding sound effect if HUD is in the position​

Hivebrain 2005 has no macros, so you should modify it simply and paste this in the label you pasted below @reached

        move.b    #$C5,d0    ;    set sound to cash
        jsr    (PlaySound_Special).l ;    play it

Okay, now it will play a sound once the HUD scrolling is completed.

Hivebrain 2022​

Also, Hivebrain 2022 is a upgraded version of Hivebrain 2005 but better version and more documented by following the current version of Sonic 1 disassembly from Sonic Retro.

In your disassembly, go to the Objects/HUD.asm and find the label HUD_Main:

Replace the entire routine with this

HUD_Main:    ; Routine 0
        addq.b    #4,ost_routine(a0)        ; Advance to Routine 4 (HUD_Move)
        move.w    #screen_left-80,ost_x_pos(a0)    ; Start HUD far off-screen to the left
        move.w    #screen_top+136,ost_y_screen(a0) ; Set Y position coordinate
        move.l    #Map_HUD,ost_mappings(a0)    ; Set mappings pointer
        move.w    #tile_Nem_Hud,ost_tile(a0)    ; Set art tile pattern index
        move.b    #render_abs,ost_render(a0)    ; Use absolute rendering screen space
        move.b    #0,ost_priority(a0)        ; Set sprite layout priority
        rts

HUD_Move:    ; Routine 4
        move.w    ost_x_pos(a0),d0        ; Get current HUD X position
        cmpi.w    #screen_left+16,d0        ; Has it reached the classic target position ($10)?
        bge.s    .reached            ; If yes, branch to lock movement

        addq.w    #5,d0                ; Shift 5 pixels to the right
        move.w    d0,ost_x_pos(a0)        ; Update object X position
        jmp    (DisplaySprite).l

.reached:
        move.b    #2,ost_routine(a0)        ; Set to static state routine (Routine 2 / HUD_Flash)
        jmp    (DisplaySprite).l

If there's a issue and it is not working, you should go below the pointer of HUD_Flash and insert this below the line.

        ptr HUD_Move

Done! Now your fellow HUD scrolling is now functional, enjoy your new HUD scrolling.
(OPTIONAL) Adding sound effect if HUD is in the position
So, if you want a sound effect that simply plays when the HUD is moved at the correct position, just go to the ".reached" label and add this below the label:

        play.w    1, jsr, sfx_Register            ; play "ker-ching" sound

Done! Now your HUD has a sound every time it finishes scrolling, happy hacking!

GitHub version​

For getting started, you want to go in the _incObj/21 HUD.asm and go at the routine HUD_Main and replace it entirely with this new routine:

HUD_Main:    ; Routine 0
        addq.b    #4,obRoutine(a0)  ; Advances to Routine 4 (HUD_Move) on the next frame
        move.w    #0,obX(a0)        ; Starts the HUD offscreen (X = 0)
        move.w    #$108,obScreenY(a0)
        move.l    #Map_HUD,obMap(a0)
        move.w    #ArtTile_HUD,obGfx(a0)
        move.b    #0,obRender(a0)
        move.b    #0,obPriority(a0)
        rts

HUD_Move:    ; Routine 4
        move.w    obX(a0),d0        ; Loads the current HUD X position
        cmpi.w    #$90,d0           ; Has it reached position $90?
        bge.s    .reached          ; If greater or equal, finalize the movement
 
        addq.w    #5,d0             ; Move 5 pixels to the right
        move.w    d0,obX(a0)        ; Update the object's X coordinate directly
        jmp       DisplaySprite

.reached:
        move.b    #2,obRoutine(a0)
        jmp       DisplaySprite

Optional: You can change the scrolling speed to what you want, just simply replace the 5 with the number you think it's better.

If the code mentioned in particular doesn't work, you did some mistake! Just go ahead in HUD_Index: label and insert this line below the HUD_Flash-HUD_Index

        dc.w HUD_Move-HUD_Index

Did you like it? Now your hack's HUD is scrolling like how it would be.
(OPTIONAL) Adding sound effect if HUD is in the position
So, if you want a sound effect that simply plays when the HUD is moved at the correct position, just go to the ".reached" label and add this below the label:

        move.b    #sfx_Cash,d0            ; set cash sound to be played
        jsr    QueueSound2            ; play it

I hope you enjoyed this tutorial, now your hack has a HUD scrolling once the game starts.