What is the difference between these two carts?

[Mr Robot]

I'm finding differences like this in a few 5200 cart roms, where D0 becomes F0 and DC becomes FC in a few places in the file.

Any explanation as to what this means and why it happens would be greatly received.

 

Here's a screenshot of Hex fiend showing the diffs

 

Here are the two roms

Buck Rogers - Planet of Zoom (1983) (Sega).binBuck Rogers - Planet of Zoom (1983) (Sega) [a1].bin

[+-^CrossBow^-]

I wonder if the newer revisions are to account for differences in the 5200 BIOS? Do the physical carts look any different label printing...etc?

 

[Mr Robot]

I don't have the carts, just the rom files

 

[Tempest]

Bankswitching hot spot?  Happens all the time in 2600 games.

[+Mitch]

5200 games don't use bankswitching generally.

 

Mitch

[Mr Robot]

The D to F thing seems quite common, I found one earlier this a single byte difference, a change from DF to FF. I wish I knew what the change was, having [a] in the filename isn't good enough, I want to know why its an alt.

 

 

[Wrathchild]

there's no code there so maybe these are just some graphic changes?

D = 11 01 or colours 3 & 1

F = 11 11 or colours 3 & 3

 

But (just speculating) could also be a value written to h/w register where the value of the changed bit (here = bit 5) actually makes no difference or perhaps makes a 4 port behave the same as a 2 port

 

[Edit] also wouldn't be hard t use Altirra to stick a breakpoint on read of one of the addresses and then step through to see how it is used.

Edited May 26, 2020 by Wrathchild

[Mr Robot]

I think I may have to do the Altirra thing. It'll be a learning experience at least!

 

[+playsoft]

9 hours ago, Wrathchild said:

there's no code there so maybe these are just some graphic changes?

D = 11 01 or colours 3 & 1

F = 11 11 or colours 3 & 3

 

But (just speculating) could also be a value written to h/w register where the value of the changed bit (here = bit 5) actually makes no difference or perhaps makes a 4 port behave the same as a 2 port

 

[Edit] also wouldn't be hard t use Altirra to stick a breakpoint on read of one of the addresses and then step through to see how it is used.

With a read breakpoint on the first byte which is different, using the first image posted it stopped here:

(2409: 26, 88) A=00 X=59 Y=00 S=FB P=36 (   IZ )  67EE: 31 72             AND ($72),Y  [$9581] = $DC
(2409: 26, 89) A=00 X=59 Y=00 S=FB P=36 (   IZ )  67F0: 11 76             ORA ($76),Y  [$89DB] = $03
(2409: 26, 94) A=03 X=59 Y=00 S=FB P=34 (   I  )  67F2: 91 74             STA ($74),Y  [$2EFF]

The byte is a mask. It looks like it is trying to set the two bottom bits (i.e. one pixel) leaving the rest of the byte unchanged, but the $DC mask results in a bit being cleared in the upper nibble. The mask value should be $FC, which it is in the second image.

 

So it looks like someone altered it to correct the masks. I think it will only make a difference when things overlap on-screen and even then you may need a young man's eyesight to notice it.