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Message: 7051

Author: ian.rushforth

Date: 13/02/2017

Subject: Re: wraparound / mm

 

Sorry, that latter part should have read:

(the exact address to be over-written with a graphic byte from the left half of Willy's sprite is determined by taking the address picked up from the look-up table and OR'ing it with Willy's x-coordinate; and then the next byte along is over-written with a graphic byte from Willy's right half).

The outcome being that if Willy [his left half] has an even x-coordinate, then addresses #9CFE and #9CFD are overwritten (harmlessly - the base of the stack is defined as #9CFE but this means that the first Return address or PUSH command fills up #9CFC-FD, then #9CFA-FB, etc). Whereas if he has an odd x-coordinate, then addresses #9CFF and #9D00 are overwritten (the latter address corresponding to the first character of the scrolling message).

N.B. This overwriting is also done via an OR operation, so if the right-hand graphic byte at this particular y-coordinate happens to be empty, then no corruption of the first scrolly character occurs.

 

 

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