Alpha Programming

 

    The afterward shows changes to the 68K Anamnesis Map with a MegaCD/SegaCD attatched:

    MegaCD BIOS ROM is consistently alone arresting to the capital 68K, and decompresses a separate BIOS for the MegaCD SubCPU into Program RAM. Chat RAM admission is belted to one 68K at a time, which is bent by a ambience in the Aboideau Array. Program RAM is $80000 bytes in size, but is alone attainable by the Alpha 68K in banks of admeasurement $20000, and alone if the MegaCD 68K is in BUSREQ (stopped). The MegaCD 68K may be stopped/reset, and the accessible Program RAM coffer may be set by use of the Aboideau Array.

    The ROM attack starts at $000100 and contains advice about the cartridge. The afterward accumulation cipher shows how to acknowledge the header:

    

    1:		dc.b SEGA Alpha    

    2:		dc.b (C)SEGA 1992.SEP

    3:		dc.b YOUR Bold Actuality                                  

    4:		dc.b YOUR Bold Actuality                                  

    5:		dc.b GM XXXXXXXX-XX

    6:		dc.w $D951

    7:		dc.b J               

    8:		dc.l 0

    9:		dc.l ROM_End

    10:		dc.l $FF0000

    11:		dc.l $FFFFFF

    12:		dc.b     

    13:		dc.b     

    14:		dc.b     

    15:		dc.b             

    16:		dc.b                                         

    17:		dc.b JUE             

    

    #The name of the console. Should be either SEGA Alpha or SEGA MEGA DRIVE. Haveto be 16 characters long.

    #Firm name and body date. Haveto be 16 characters long, close name is four characters long.

    #Domestic name. Haveto be 48 characters long.

    #International name. Haveto be 48 characters long.

    #Program blazon and consecutive number. First two characters are either GM (game) or AL (educational). Afterwards the space, a alternation of eight digits gives the consecutive number. The final two digits accord the adaptation number.

    #.

    #I/O accessory support. You can apparently avoid this.

    #Start of the ROM. This should consistently be 0.

    #End of the ROM. Its best to use a characterization at the end of your antecedent book to actuate this.

    #Start of RAM. Just about every bold starts at $FF0000, but RAM is mirrored at several additional places, and you can use those if you want.

    #End of RAM. Should be $FFFF afterwards the starting point.

    #Backup RAM ID. Ample this with spaces if there is none, otherwise, use this formula:

    dc.b RA,%1x1yy000,%0010000

    where x is true if the RAM is for advancement and false if not. (Ive no abstraction what the aberration is.) yy is 11 if acclamation is odd bytes only, 10 if even only, and 00 if both.

    #Start abode of advancement RAM. Ample with spaces if there is no advancement RAM.

    #End abode of advancement RAM.

    #Modem support. Ample with spaces if there is no modem, contrarily chase this formula:

    MOxxxxyy.zzz

    where xxxx is the close name, yy is the modem number, and zzz is the adaptation number.

    #Notes. Put whatever you wish here, and ample the abandoned spaces with zeroes.

    #Country codes. J is for Japan, U is for the Affiliated States, and E is for Europe. You charge to ample in the blow of this acreage with spaces.

    Back in 1990, Approval appear a few actionable amateur for the Genesis. Sega was not admiring about this, so they invented TMSS and put it on all new consoles they manufactured. TMSS armament the bold to address the ASCII cord SEGA to $A14000 aural a abbreviate aeon of time, or the animate will not work. On adaptation 0 of the console, this area is reserved, and autograph actuality may couldcause a crash, so the bold has to first create abiding that its active on a animate that has TMSS. Sega uses the afterward accepted for that:

    

             move.b  $A10001,d0

             andi.b  #$0F,d0

             beq.b   version_0

             move.l  $SEGA,$A14000

    version_0:

    

    The checksum is a 16-bit amount (word) amid in the attack which is present in all accountant games.It was acclimated to analysis the candor of the data by autumn a pre-calculated amount (the checksum) using the aboriginal data and comparing it with the checksum of the absolute data in the cart, apparently to anticipate cipher changes after approval (in-house).

    The checksum is, basically, the sum of the accomplished cart, all its bytes, starting at abode $200.As you can guess, if alone a byte is afflicted the checksum changes too.Somewhat like the CRC/CRC32 algorithms, admitting those are abundant added authentic than this method.

    You dont charge to apparatus the checksum if you program your own homebrew rom, because the checksum is alone arrested by software.But, anyway, this is how a archetypal checksum accepted from a accountant bold would look.

    

    		movea.l	#$200,a0

    		movea.l	#ROM_End,a1 

    		move.l	(a1),d0

    		moveq	#0,d1

    loop:

    		add.w	(a0)+,d1

    		cmp.l	a0,d0

    		bcc.s	loop

    		movea.l	#$18E,a1	; Checksum

    		cmp.w	(a1),d1

    		bne.w	WrongChecksum

    

    To attenuate it, animadversion out that endure branch. If you adopt to fix the checksum, grab the Fix Checksum account from [http://www.hacking-cult.org Hacking CulTs] download section.