| Routines

F4D2: Get the attribute file address of a pellet's potential target or the safe

Used by the routines at 6DEF (to check whether a shield has been hit) and F726 (to make the safe flash). If the pellet's potential target or the safe is on-screen, this routine returns with the carry flag reset and HL holding the attribute file address.
Input
D | Target y-coordinate
E | 128 + target x-coordinate
F4D2 | LD HL,$7F00 | 7F00 holds the leftmost column of the skool on screen (0-64)
F4D5 | LD A,E | Set A to the true x-coordinate of the potential target
F4D6 | AND $7F
F4D8 | SUB (HL) | Return with the carry flag set if the pellet is off-screen to the left
F4D9 | RET C
F4DA | CP $20 | The screen is 32 columns wide
F4DC | CCF | Return with carry flag set if the pellet is off-screen to the right
F4DD | RET C
The potential target is on-screen.
F4DE | LD L,A | L=screen x-coordinate (0-31) of the potential target
F4DF | LD A,D | A=screen y-coordinate (2, 8 or 14) of the potential target
F4E0 | SUB $98
F4E2 | RRCA | A=0x40 (top floor), 0x01 (middle floor), or 0xC1 (bottom floor
F4E3 | RRCA
F4E4 | RRCA
F4E5 | LD H,A | Save this floor indicator in H for now
F4E6 | AND $E0 | A=0x40 (top floor), 0x00 (middle) or 0xC0 (bottom)
F4E8 | ADD A,L | Add the screen x-coordinate to get the LSB of the attribute file address of the potential target
F4E9 | LD L,A | Copy the LSB to L
F4EA | LD A,H | A=0x00 (top floor) or 0x01 (middle floor or bottom floor)
F4EB | AND $03
F4ED | ADD A,$58 | Add 0x58 to get the MSB of the attribute file address of the potential target
F4EF | LD H,A | Copy the MSB to H
F4F0 | RET | Return with the carry flag reset