Routines
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A149: 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 BF is the bottom row.
Input
H Vertical position
L Horizontal position
Output
HL Screen address
GetScreenAddress A149 LD A,L Fetch the horizontal position.
A14A RRC A Divide it by eight.
A14C RRC A
A14E RRC A
A150 AND %00011111 Keep the five bits of the column.
A152 LD L,A Store it in L.
A153 LD A,H Fetch the vertical position.
A154 RLC A Multiply it by four.
A156 RLC A
A158 AND %11100000 Keep the bits that give the row of the cell.
A15A OR L Combine them with the column.
A15B LD L,A Store the low byte of the address.
A15C LD A,H Fetch the vertical position.
A15D AND %00000111 Keep the bits that give the pixel row within the cell.
A15F EX AF,AF' Save them.
A160 LD A,H Fetch the vertical position.
A161 RRC A Divide it by eight.
A163 RRC A
A165 RRC A
A167 AND %00011000 Keep the bits that give the third of the screen.
A169 OR %01000000 Add the address of the display file.
A16B LD H,A Store the high byte of the address.
A16C EX AF,AF' Restore the pixel row.
A16D OR H Combine it with the high byte.
A16E LD H,A Store the high byte of the address.
A16F RET Return.
View the equivalent code in;
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