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⌂ ∞ ▽ Elite on the 6502

Version analysis of LYN

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: LYN
       Type: Subroutine
   Category: Drawing the screen
    Summary: Clear most of a row of pixels

Code variation 1 of 2A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
Electron
 Set pixels 0-233 to the value in A, starting at the pixel pointed to by SC.


 Arguments:

   A                    The value to store in pixels 1-233 (the only value that
                        is actually used is A = 0, which clears those pixels)


 Returns:

   Y                    Y is set to 0


 Other entry points:

   SC5                  Contains an RTS

 Zero memory from page X to page &75 (inclusive).


 Arguments:

   X                    The page of screen memory from which to start clearing


.LYN

Code variation 2 of 2Specific to an individual platform

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
Electron
 LDY #233               \ Set up a counter in Y to count down from pixel 233

.EE2

 STA (SC),Y             \ Store A in the Y-th byte after the address pointed to
                        \ by SC

 DEY                    \ Decrement Y

 BNE EE2                \ Loop back until Y is zero

.SC5

 JSR ZES1               \ Call ZES1 to zero-fill the page in X

 INX                    \ Increment X to point to the next page in memory

 CPX #&76               \ Loop back to zero the next page until we have reached
 BNE LYN                \ page &76 (so page &75 is the last page to be zeroed)

 RTS                    \ Return from the subroutine