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Elite on the BBC Micro

Drawing ships: LL9 (Part 4 of 12) (6502SP version)

Name: LL9 (Part 4 of 12) [View in context] Type: Subroutine Category: Drawing ships Summary: Draw ship: Set visibility for exploding ship (all faces visible) Deep dive: Drawing ships
This part sets up the visibility block in XX2 for a ship that is exploding. The XX2 block consists of one byte for each face in the ship's blueprint, which holds the visibility of that face. Because the ship is exploding, we want to set all the faces to be visible. A value of 255 in the visibility table means the face is visible, so the following code sets each face to 255 and then skips over the face visibility calculations that we would apply to a non-exploding ship.
LDA (XX0),Y \ Fetch byte #12 of the ship's blueprint, which contains \ the number of faces * 4 LSR A \ Set X = A / 4 LSR A \ = the number of faces TAX LDA #255 \ Set A = 255 .EE30 STA XX2,X \ Set the X-th byte of XX2 to 255 DEX \ Decrement the loop counter BPL EE30 \ Loop back for the next byte until there is one byte \ set to 255 for each face INX \ Set XX4 = 0 for the distance value we use to test STX XX4 \ for visibility, so we always shows everything .LL41 JMP LL42 \ Jump to LL42 to skip the face visibility calculations \ as we don't need to do them now we've set up the XX2 \ block for the explosion