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JAPDATA.ASM
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JAPDATA.ASM
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*
* ROBOTRON 3-10-82 RELEASE 1
*
ORG $26B7
FCB $65
ORG $5E77
FCB $68
ORG $DE54
FCB $1B,$1B,$1B,$1B,$1B,$1B,$1B,$1B
*
* CHECKSUMS
*
*
* 0-$4D
* 1-$19
* 2-$D9
* 3-$1B
* 4-$7E
* 5-$DD
* 6-$F1
* 7-$3E
* 8-$6D
*
* D-$4D
* E-$48
* SUPER CHECKERS - 98F3 = $37
* BE72 = $4A
**************************************************************************
* *
* Here is a detailed explaination of the Robotron 2084 *
* "JAP ZAPPERS". This was a primitive system setup to *
* try and make it more difficult for game copiers to *
* Modify the game. There were scrambled sections of the *
* code that would try and monitor "random" checksums *
* of regions of the code. If they detected that the code *
* had been modified, they would cause random program *
* events to occur at a low frequency (the random and *
* low frequency combination would make it harder to track *
* down the culprits). We always assumed that developers *
* with good tools would find these quickly, but Christian *
* Gingras, removed this small nuisance while trying to *
* fix the other bugs he had found in the program (with *
* the aid of his Radio Shack Color Computer). *
* *
* There are 2 regions for which checksums are kept. The *
* checksums are not the sum of all bytes, but the sum *
* of every 8 and 16 bytes respectively. The first checksum *
* is the sum of every 16th byte from $0007 to $8927 inclusive *
* and is stored at $98F3 (HCK1). This sum should come out to *
* $37. The second checksum is the sum of every 8th byte *
* from $D015 to $EAAD and is stored at $BE72 (HCK2). This sum *
* should come out to $4A. *
* *
* There are 6 places in the software that are concerned *
* with the checksums. *
* *
* In RRTEXT, at label WORDZ (the call for message handling) *
* there is a check of HCK1 for its $37 value, and handling *
* to zap if incorrect. *
* *
* In RRS22, the code to set the HCK1 value (by performing the *
* required addition) is found within the PINIT routine. *
* PINIT is used to initialize the process structure at the *
* start of each turn. *
* *
* In RRS22 there is a check of HCK1 for its $37 value in the *
* ASCAN routine. ASCAN is called at the start of each wave *
* by each object to make sure the space it wants is un-occupied *
* *
* In RRG23 the code to setup the HCK2 value (by performing *
* the required addition) is found within the game start *
* routine (START1 and START2). *
* *
* In RRX7 there is a check of HCK2 for its $4A value in *
* the explosion starter (EXSTV). *
* *
* In RRB10 there is a routine that computes the HCK2 value *
* (on the fly in case the copiers goose the values into *
* ram) and zaps if it does not come up with the $4A value. *
* This is found in the brain starter (BRNSTV) and is run *
* in any waves that contain BRAIN robots. *
* *
* The checksums are computed with the equivilents of the *
* following code: *
* *
* HCK1 ($98F3) = $37 *
* *
* clra *
* ldx #$0007 *
* loop: adda 0,x *
* leax $10,x *
* cmpx #$8935 *
* blo loop *
* sta hck1 *
* *
* *
* HCK2 ($BE72) = $4A *
* *
* clra *
* ldx #$d015 *
* loop: adda 0,x *
* leax $8,x *
* cmpx #$EAB1 *
* blo loop *
* sta hck2 *
* *
* *
**************************************************************************
END