Routines
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BC0E: Calculate Screen Buffer Address
Used by the routine at Draw_Sprites.
Converts a screen coordinate stored at IX into a screen buffer address and pixel offset. The coordinate is stored as a two-byte value where IX+00 holds the X position and IX+01 holds the Y position. Returns with HL pointing to the screen buffer address, A (shadow) holding the row within the character cell (bits 0-2 of Y), and A holding the pixel offset within the byte (bits 0-2 of X).
Input
IX Pointer to a two-byte screen coordinate (X, Y)
Output
HL Screen buffer address
A Pixel X offset within the byte (bits 0-2)
AF' Row within the character cell (bits 0-2 of Y)
CalculateScreenBufferAddress BC0E LD C,(IX+$00) C=*IX+00.
BC11 LD B,(IX+$01) B=*IX+01.
BC14 LD A,C A=C.
BC15 AND %00000111 Keep only bits 0-2.
BC17 PUSH AF Stash the pixel X offset on the stack.
Calculate the low byte of the screen buffer address. This combines the character column (X / 8) with the Y pixel row bits.
BC18 LD A,C A=C.
BC19 RRCA Rotate right three positions to divide X by 08.
BC1A RRCA
BC1B RRCA
BC1C AND %00011111 Keep only bits 0-4 (character column, 00-1F).
BC1E LD L,A L=A.
BC1F LD A,B A=B.
BC20 RLCA Rotate left two positions.
BC21 RLCA
BC22 AND %11100000 Keep only bits 5-7 (low bits of Y row).
BC24 OR L Merge the column and row bits together.
BC25 LD L,A L=A.
Calculate the high byte of the screen buffer address. This encodes the character row and third-of-screen into the high byte, with the base address of E000 (bits 5-7 set).
BC26 LD A,B A=B.
BC27 AND %00000111 Keep only bits 0-2 (pixel row within character cell).
BC29 LD H,A H=A.
BC2A LD A,B A=B.
BC2B AND %11000000 Keep only bits 6-7 (screen third selector).
BC2D RRCA Rotate right three positions to move bits 6-7 into bits 3-4.
BC2E RRCA
BC2F RRCA
BC30 OR H Merge with the pixel row bits in H.
BC31 OR %11100000 Set bits 5-7 for the screen buffer base address (E000).
BC33 LD H,A H=A.
Store the Y pixel row (character cell row) in shadow AF and restore the pixel X offset as the return value in A.
BC34 EX AF,AF' Store the row-within-character in shadow AF.
BC35 POP AF Restore the pixel X offset from the stack.
BC36 RET Return.
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