Routines
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48142: 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+0 holds the X position and IX+1 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 48142 LD C,(IX+0) C=*IX+0.
48145 LD B,(IX+1) B=*IX+1.
48148 LD A,C A=C.
48149 AND %00000111 Keep only bits 0-2.
48151 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.
48152 LD A,C A=C.
48153 RRCA Rotate right three positions to divide X by 8.
48154 RRCA
48155 RRCA
48156 AND %00011111 Keep only bits 0-4 (character column, 0-31).
48158 LD L,A L=A.
48159 LD A,B A=B.
48160 RLCA Rotate left two positions.
48161 RLCA
48162 AND %11100000 Keep only bits 5-7 (low bits of Y row).
48164 OR L Merge the column and row bits together.
48165 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 57344 (bits 5-7 set).
48166 LD A,B A=B.
48167 AND %00000111 Keep only bits 0-2 (pixel row within character cell).
48169 LD H,A H=A.
48170 LD A,B A=B.
48171 AND %11000000 Keep only bits 6-7 (screen third selector).
48173 RRCA Rotate right three positions to move bits 6-7 into bits 3-4.
48174 RRCA
48175 RRCA
48176 OR H Merge with the pixel row bits in H.
48177 OR %11100000 Set bits 5-7 for the screen buffer base address (57344).
48179 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.
48180 EX AF,AF' Store the row-within-character in shadow AF.
48181 POP AF Restore the pixel X offset from the stack.
48182 RET Return.
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