Routines
Prev: A149 Up: Map Next: A18D
A170: Find Attribute Address
Used by the routines at PrintText, UpdateStarLayer, DrawShipTop, FillAttributes, CopyAttributes and ColourSegment.
Finds the address in the attribute file of a pixel position. The vertical position is the number of the row of pixels, counted from the top of the screen.
Input
H Vertical position
L Horizontal position
Output
HL Attribute address
AttributeAddress A170 PUSH BC Stash BC on the stack.
A171 LD A,L Fetch the horizontal position.
A172 RRC A Divide it by eight.
A174 RRC A
A176 RRC A
A178 AND %00011111 Keep the five bits of the column.
A17A LD L,A Store it in L.
A17B LD A,H Fetch the vertical position.
A17C RLC A Multiply it by four.
A17E RLC A
A180 LD C,A Keep a copy.
A181 AND %11100000 Keep the bits that give the row of the cell.
A183 OR L Combine them with the column.
A184 LD L,A Store the low byte of the address.
A185 LD A,C Fetch the multiplied vertical position.
A186 AND %00000011 Keep the bits that give the third of the screen.
A188 OR %01011000 Add the address of the attribute file.
A18A LD H,A Store the high byte of the address.
A18B POP BC Bring BC back off the stack.
A18C RET Return.
Prev: A149 Up: Map Next: A18D