How to Add a Second Digit to the Lifecounter in Sonic 2 SMS

The 8-bit games need more love. :V Anyway copy-paste from SR:


Alright I always hated that you couldn't have more than 9 lives in Sonic 2, so I decided to change that. I didn't use the updated version on


the SVN, this is from the wiki version of the disassembly. First look for CapLifeCounterValue in S2.asm. It should look like this:

Code:

LABEL_25AC:

	ld	a, ($D292)

	or	a

	ret	nz

	ld	a, (LifeCounter)

	cp	$09 ;cap the life display at 9

	jr	c, +

	ld	a, $09

+:	rlca

	and	$1E

	add	a, $10

	ld	($DBA9), a

	ret

Now, delete every line after ret nz and add this:

Code:

	ld	a, (LifeCounter)

	and	$0F

	rlca

	and	$1E

	add	a, $10

	ld	($DBA9), a

	ret

This sets up the last nibble of the life counter. This is what your code should look like at this:

Code:

	

	ld	a, ($D292)

	or	a

	ret	nz

	ld	a, ($D292)

	or	a

	ret	nz

	ld	a, (LifeCounter)

	and	$0F

	rlca

	and	$1E

	add	a, $10

	ld	($DBA9), a

Alright time for a brief explination, the rlca operand allows the display to be updated, while the other parts of the code updates the display.

We need to change it to where it can set another nibble for the counter. Right after "ld ($DBA9), a" add this:

Code:

	ld	a, (LifeCounter)

	and	$F0

	rrca

	rrca

	rrca

	and	$1E

	add	a, $10

	ld	($DBAF), a

	ret

This will allow the "x" on the counter to become the other nibble. That does allow the counter to display the proper numbers, but when Sonic gains

a life after 9, it still displays the wrong value in the wrong nibble. So we need to change that.

Find Collision_Monitor_Life in the source code, it should look like this:

Code:

	

	res	1, (hl)

	ld	a, ($D298)

	inc	a

	ld	(LifeCounter), a

	ld	a, SFX_ExtraLife

	ld	($DD04), a

	jp	LABEL_25AC

First, bellow res 1,(hl) add these lines:

Code:

	

	ld	a, (LifeCounter)

	cp	$99

	ret	z

This will allow the game to cap the life counter after 99 lives. Next we need to change these lines:

Code:

		

	ld	a, ($D298)

	inc	a

	ld	(LifeCounter), a

To this:

Code:

	

	ld	a, (LifeCounter)

	add	a, $01

	daa	(LifeCounter),a

The "daa" operand is used for counters, which is why the "inc" operand wouldn't work here. After that, save build and enjoy.

This is what the routine looks like after everything is added:

Code:

	

CapLifeCounterValue:

LABEL_25AC:

	ld	a, ($D292)

	or	a

	ret	nz

	ld	a, (LifeCounter)

	and	$0F

	rlca

	and	$1E

	add	a, $10

	ld	($DBA9), a

	ld	a, (LifeCounter)

	and	$F0

	rrca

	rrca

	rrca

	and	$1E

	add	a, $10

	ld	($DBAF), a

	ret



Collision_Monitor_Life:

	res	1, (hl)

	ld	a, (LifeCounter)

	cp	$99

	ret	z

	add	a, $01

	daa

	ld	(LifeCounter), a

	ld	a, SFX_ExtraLife

	ld	($DD04), a

	jp	LABEL_25AC

 

Please use code boxes in the future. Added them for you.


Apart from that, awesome work.

 

Oops sorry Selbi, i'll remember that in the future. ^^' And thank you.