atari2600land's Blog

One of the things I can't do since I have no room for it, is add a way to increase the life counter.

That's when it's time to review your code for places you can optimize it

 

 

You currently have 2 bytes free. I took a quick look and right away spotted this:

Reset
   ; Clear RAM and all TIA registers

        CLEAN_START      

        lda     INPT0       ; DGS
        sta     Controller  ; DGS paddle if D7 = 1
      
       ;------------------------------------------------
       ; Once-only initialisation...

				lda #130
				sta PlayerX+1	

				lda #4
				sta AUDC0
				sta AUDV0

				lda #0
				sta GameRunning
	
				lda #0
				sta Lives
					
				lda #$84
				sta Rand16       ; also use BOX_COLOR ($84) as seed for the LFSR

---

 

And noticed a bunch of code that's not needed:

Reset
   ; Clear RAM and all TIA registers

        CLEAN_START      

        lda     INPT0       ; DGS
        sta     Controller  ; DGS paddle if D7 = 1
      
       ;------------------------------------------------
       ; Once-only initialisation...

                lda #130
                sta PlayerX+1   

                lda #4
                sta AUDC0
                sta AUDV0
                 
                ; lda #0
                ; sta GameRunning   - already 0 because of CLEAN_START
                ; 
                ; lda #0
                ; sta Lives         - already 0 because of CLEAN_START
                ; 
                ; lda #$84       
                sta Rand16       ; - just has to be a non-zero value, so use 130

---

now there's 12 bytes free.

 

Another find just a little bit further down in TheGameStarted:

        dec Lives
        ldx Lives
        cpx #255
        bcs GameRestarted2

---

Looking at the specs for the DEC opcode you'll see this:

DEC (DECrement memory)

Affects Flags: S Z

MODE           SYNTAX       HEX LEN TIM
Zero Page     DEC $44       $C6  2   5
Zero Page,X   DEC $44,X     $D6  2   6
Absolute      DEC $4400     $CE  3   6
Absolute,X    DEC $4400,X   $DE  3   7
 

---

So DEC updates the S flag for sign, which is also known as N flag for negative(Stella uses N). This means the instruction will set the sign/negative flag if the value becomes negative because of the DECrement. In 8 bit the value of 255 is the same as -1, so we can eliminate the LDX and CPX instructions and use a BMI (Branch MInus) instead.

        dec Lives
;        ldx Lives
;        cpx #255
;        bcs GameRestarted2
        bmi GameRestarted2

---

now there's 16 bytes free.

 

When you optimize code it's best to make just a few changes at a time then test the program to make sure you didn't break anything. For instance I initially thought the lda #4/sta AUDC0/sta AUDV0 instructions in Reset could be removed, as they're updated elsewhere in the program, but then I tested it and the initial tune didn't play.