Set up the Goggle Monitor to work with it

(Original 3 sections of the Hivebrain guide by Selbi, the End of Level Air Reapply code by theocas, the Title Screen Air Reapply code by Bendabest19, final section of Hivebrain guide and the Github guide by Inferno Gear, Github guide based off oringial Hivebrain guide by Selbi, grammar fixes by makotoyuki.)

Did you know that there are 2 unused monitors in Sonic 1? The 'S' and 'Goggle' monitors. You can place and also destroy them, but nothing will happen. In the split, the code for the 'S' monitor is under "Obj2E_ChkS:" (all it has is just a check for the monitor subtype and a 'rts'), but there is no code for the Goggle monitor, which just branches to Obj2E_ChkS! If you want to have that monitor to do something, you need to set it up first. It's not too hard, so I will explain it to you step-by-step.

Hivebrain disassembly

Quick guide

I know there are many people who just want to rip off the code without learning because they think they have better things to do. For those people I took the time to make a simple copy/paste guide:

Search for Obj2E_ChkS: and replace everything from there with this:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne	Obj2E_ChkEnd		; if not, branch to ChkEnd
		nop

That was everything. If you are too dumb to add/understand this, then you weren't reading the real guide.

Full guide

This version tells you how to do this in detail, and very well explained. It's not more than what you can see in the quick version, but with comments and explanations you can learn something, so this one is recommended.

First off, open sonic1.asm and go to the Routine for the monitor contents (Obj2E). Scroll down until you see this:

	...
Obj2E_RingSound:
		move.w	#$B5,d0
		jmp	(PlaySound).l	; play ring sound
; ===========================================================================

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkEnd
		nop	

Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing
; ===========================================================================

Obj2E_Delete:				; XREF: Obj2E_Index
		subq.w	#1,$1E(a0)
		bmi.w	DeleteObject
		rts

As you can see, there is no Goggle Code. But there is code that checks if the monitor contains 'S'. However, you can open SonED2 and add a new monitor using the object editor. Scroll through the different monitors, and you will see, in Obj2E are the monitors in the same order as in SonED2. The Goggle monitor is the last monitor in this list. This means that you will have to add your new code under the code for the S.

Before Obj2E_ChkEnd add a new label. The most fitting one is Obj2E_ChkGoggles: in my opinion:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkEnd
		nop

Obj2E_ChkGoggles:	

Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

In ChkS is a code to branch to ChkEnd. Scroll up and you will see, there is in every monitor a code like this, but instead of branching to ChkEnd, they are branching to the next monitor code. So do the same with the S code. Change bne Obj2E_ChkEnd to bne Obj2E_ChkGoggles:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	

Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

Okay, we've set a few things up, but we are not done yet. Maybe you've noticed with the last step, those 2 lines in each monitor at the beginning (a 'cmpi.b' code and a 'bne' code). They are there to check if that monitor was destroyed and if so, do the code under it, otherwise check for the next monitor. We need that for the goggle monitor as well, so copy any of these 2 lines to your Obj2E_ChkGoggles: label. I'm using the lines of ChkS:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code

		cmpi.b	#7,d0		; does monitor contain 'S'
Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

As you can see, this can't work, because they are checking for the same things! Look up again. Every monitor has a higher number in the cmpi.b command. The number in S was 7. So what is the next number? 8 of course! So change the 7 to 8 in your Goggle code:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code

Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

7: We are nearly done. The last thing we have to do is to change the bne command in your Goggle code to ChkEnd. Like this:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain 'S'
		bne	Obj2E_ChkEnd		; if not, branch to Goggle code

Obj2E_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

Now we're done, and you can insert code to run when you destroy a Goggle monitor. But if you don't wanna add code for now, you have to add a nop under the bne command, otherwise you will get an Illegal zero length branch error when building. Also, you should change the comments, because they are really misleading. Here is my finished code:

Obj2E_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne	Obj2E_ChkGoggles		; if not, branch to Goggle code
		nop

Obj2E_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne	Obj2E_ChkEnd		; if not, branch to ChkEnd
		nop

Obj2E_ChkEnd:
		rts

Example Code

Although that code is perfect now, it still does nothing. Unless you already have plans to do something with it, you can use this very simple code, that gives infinite air in LZ.

Go to the Goggle code and add a line, which is moving $1 to an unused RAM adress:

		move.b	#1,($FFFFFFA0).w ; move 1 to the goggle check

It should look like this:

		...
		nop

Obj2E_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne	Obj2E_ChkEnd		; if not, branch to ChkEnd
		move.b	#1,($FFFFFFA0).w ; move 1 to the goggle check

Obj2E_ChkEnd:
		...

Next go to to Obj0A_ReduceAir: and add these 2 lines right after the label:

		tst.b	($FFFFFFA0).w	; was a goggle monitor broken?
		bne	Obj0A_GoMakeItem	; if yes, branch

It should look like this:

Obj0A_ReduceAir:
		tst.b	($FFFFFFA0).w	; was a goggle monitor broken?
		bne	Obj0A_GoMakeItem	; if yes, branch
		...

Fixing the infinite air glitch

The code now works perfectly, but now you have infinite air forever. There are many ways you could go about fixing this, but theocas on the SSRG version of this tutorial proposed moving $0 the ram that signifies that a goggle monitor has been hit. This works for our purposes, so let's use that. Place the following code at the end of Obj0D_Touch, but before locret_EBBA:

		...
		move.b	#0,($FFFFFFA0).w ; move 0 to the goggle check
		...

Now, after touching the signpost of any level, $0 is moved to $FFFFFFA0, making the game go through the normal code for reducing Sonic's amount of air underwater, which means Sonic no longer has an infinite amount of air forever.

GitHub disassembly

Quick guide

I know there are many people who just want to rip off the code without learning because they think they have better things to do. For those people I took the time to make a simple copy/paste guide:

In _incObj\2E Monitor Content Power-Up.asm, search for Pow_ChkS: and replace everything from there with this:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne.s	Pow_ChkEnd		; if not, branch to Pow_ChkEnd
		nop

That was everything. If you are too dumb to add/understand this, then you weren't reading the real guide.

Full guide

This version tells you how to do this in detail, and very well explained. It's not more than what you can see in the quick version, but with comments and explanations you can learn something, so this one is recommended.

First off, open _incObj\2E Monitor Content Power-Up.asm' and go to the Routine for the S monitor contents (Pow_ChkS). You should see this.

	...
Pow_RingSound:
		music	sfx_Ring,1,0,0	; play ring sound
; ===========================================================================

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkEnd
		nop	

Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing
; ===========================================================================

Pow_Delete:				; XREF: Obj2E_Index
		subq.w	#1,obTimeFrame(a0)
		bmi.w	DeleteObject	; delete after half a second
		rts

As you can see, there is no Goggle Code. But there is code that checks if the monitor contains 'S'. However, you can open SonED2 and add a new monitor using the object editor. Scroll through the different monitors, and you will see, in _incObj\2E Monitor Content Power-Up.asm' are the monitors in the same order as in SonED2. The Goggle monitor is the last monitor in this list. This means that you will have to add your new code under the code for the S.

Before Pow_ChkEnd add a new label. The most fitting one is Pow_ChkGoggles: in my opinion:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkEnd
		nop

Pow_ChkGoggles:	

Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

In ChkS is a code to branch to ChkEnd. Scroll up and you will see, there is in every monitor a code like this, but instead of branching to ChkEnd, they are branching to the next monitor code. So do the same with the S code. Change bne.s Pow_ChkEnd to bne.s Pow_ChkGoggles:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	

Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

Okay, we've set a few things up, but we are not done yet. Maybe you've noticed with the last step, those 2 lines in each monitor at the beginning (a 'cmpi.b' code and a 'bne.s' code). They are there to check if that monitor was destroyed and if so, do the code under it, otherwise check for the next monitor. We need that for the goggle monitor as well, so copy any of these 2 lines to your Pow_ChkGoggles: label. I'm using the lines of ChkS:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code

		cmpi.b	#7,d0		; does monitor contain 'S'
Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

As you can see, this can't work, because they are checking for the same things! Look up again. Every monitor has a higher number in the cmpi.b command. The number in S was 7. So what is the next number? 8 of course! So change the 7 to 8 in your Goggle code:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code

Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

7: We are nearly done. The last thing we have to do is to change the bne command in your Goggle code to ChkEnd. Like this:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain 'S'
		bne.s	Pow_ChkEnd		; if not, branch to Goggle code

Pow_ChkEnd:
		rts			; 'S' and Goggles monitors do nothing

Now we're done, and you can insert code to run when you destroy a Goggle monitor. But if you don't wanna add code for now, you have to add a nop under the bne command, otherwise you will get an Illegal zero length branch error when building. Also, you should change the comments, because they are really misleading. Here is my finished code:

Pow_ChkS:
		cmpi.b	#7,d0		; does monitor contain 'S'
		bne.s	Pow_ChkGoggles		; if not, branch to Goggle code
		nop

Pow_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne.s	Pow_ChkEnd		; if not, branch to ChkEnd
		nop

Pow_ChkEnd:
		rts

Example Code

Although that code is perfect now, it still does nothing. Unless you already have plans to do something with it, you can use this very simple code, that gives infinite air in LZ.

Preparation

In Variables.asm, at the end of the file in a good spot, put the following piece of code.

f_gogglecheck	= ramaddr ( $FFFFFFA0 )	; goggle flag

Adding the code

Go to the Goggle code and add a line, which is moving $1 to an unused RAM adress:

		move.b	#1,(f_gogglecheck).w ; move 1 to the goggle check

It should look like this:

		...
		nop

Pow_ChkGoggles:	
		cmpi.b	#8,d0		; does monitor contain Goggles?
		bne	Pow_ChkEnd		; if not, branch to ChkEnd
		move.b	#1,(f_gogglecheck).w ; move 1 to the goggle check

Pow_ChkEnd:
		...

Next find _incObj\0A Drowning Countdown and find .reduceair / @reduceair and add these 2 lines right after the label:

		tst.b	(f_gogglecheck).w	; was a goggle monitor broken?
		bne	@gotomakenum	; if yes, branch

It should look like this:

@reduceair
		tst.b	(f_gogglecheck).w	; was a goggle monitor broken?
		bne	@gotomakenum	; if yes, branch
		...

Fixing the infinite air glitch

The code now works perfectly, but now you have infinite air forever. There are many ways you could go about fixing this, but theocas on the SSRG version of this tutorial proposed moving $0 the ram that signifies that a goggle monitor has been hit. This works for our purposes, so let's use that. Place the following code in _incObj\0D Signpost.asm and place it at the end of Sign_Touch, but before @notouch:

		...
		move.b	#0,(f_gogglecheck).w ; move 0 to the goggle check
		...

Now, after touching the signpost of any level, $0 is moved to f_gogglecheck/$FFFFFFA0, making the game go through the normal code for reducing Sonic's amount of air underwater, which means Sonic no longer has an infinite amount of air forever. Also in sonic.asm place the previous code into the beginning of "Tit_MainLoop" This makes it so that if you get a game over or soft reset the game Sonic will not have an infinite amount of air.

|Set up the Goggle Monitor to work with it]]