Routines
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41289: Get Screen Address
Used by the routines at FindBeamExtent, FadeOutPlayfield, ClearPlayfield, PrintText, SetSecondRowGroup, FindObjectScreenAddress, DrawSprites, AddToStack, BuildCarriedItemGraphic, DrawPiece, RunFrame, DrawShipTop, DrawSideFlames, DrawMainFlame, DrawBitmap, UpdateFuelGauge, FindSegmentAddress, DrawVerticalEdge and LookForGround.
Finds the address in the display file of a pixel position. The vertical position is the number of the row of pixels, counted from the top of the screen, so 191 is the bottom row.
Input
H Vertical position
L Horizontal position
Output
HL Screen address
GetScreenAddress 41289 LD A,L Fetch the horizontal position.
41290 RRC A Divide it by eight.
41292 RRC A
41294 RRC A
41296 AND %00011111 Keep the five bits of the column.
41298 LD L,A Store it in L.
41299 LD A,H Fetch the vertical position.
41300 RLC A Multiply it by four.
41302 RLC A
41304 AND %11100000 Keep the bits that give the row of the cell.
41306 OR L Combine them with the column.
41307 LD L,A Store the low byte of the address.
41308 LD A,H Fetch the vertical position.
41309 AND %00000111 Keep the bits that give the pixel row within the cell.
41311 EX AF,AF' Save them.
41312 LD A,H Fetch the vertical position.
41313 RRC A Divide it by eight.
41315 RRC A
41317 RRC A
41319 AND %00011000 Keep the bits that give the third of the screen.
41321 OR %01000000 Add the address of the display file.
41323 LD H,A Store the high byte of the address.
41324 EX AF,AF' Restore the pixel row.
41325 OR H Combine it with the high byte.
41326 LD H,A Store the high byte of the address.
41327 RET Return.
View the equivalent code in;
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