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Routines |
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Used by the routines at 28994, 29160, 29518 and 29746. Updates the screen refresh buffer (SRB) so that the character squares at (E,D) and (E+1,D) are marked dirty. Two character squares in a row are marked dirty to ensure that blackboard contents are properly displayed (characters written on a blackboard may cross character square boundaries).
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| 28807 | LD A,(32512) | A=X: leftmost column of the skool on screen (0-64) | ||||||||
| 28810 | SUB 2 | Return unless X-1<=E<=X+31 (i.e. unless the coordinates in DE correspond to a skool location that is currently on-screen or just off to the left) | ||||||||
| 28812 | JR C,28816 | |||||||||
| 28814 | CP E | |||||||||
| 28815 | RET NC | |||||||||
| 28816 | ADD A,33 | |||||||||
| 28818 | CP E | |||||||||
| 28819 | RET C | |||||||||
| 28820 | PUSH DE | Store the skool coordinates | ||||||||
| 28821 | SUB 32 | A=E-X (-1, 0-31) | ||||||||
| 28823 | CPL | |||||||||
| 28824 | ADD A,E | |||||||||
| 28825 | BIT 7,A | Are the coordinates in DE on-screen? | ||||||||
| 28827 | JR Z,28830 | Jump if so | ||||||||
| 28829 | INC A | A=0 | ||||||||
| 28830 | RLCA | A=8x (x=0-31, the screen x-coordinate) | ||||||||
| 28831 | RLCA | |||||||||
| 28832 | RLCA | |||||||||
| 28833 | LD E,A | Save this value in E briefly | ||||||||
| 28834 | AND 56 | Modify the SET m,(HL) instruction at 28857 below | ||||||||
| 28836 | ADD A,198 | |||||||||
| 28838 | LD (28858),A | |||||||||
| 28841 | LD A,E | Point HL at the appropriate byte in the screen refresh buffer | ||||||||
| 28842 | RLCA | |||||||||
| 28843 | RLCA | |||||||||
| 28844 | AND 3 | |||||||||
| 28846 | LD E,A | |||||||||
| 28847 | LD A,D | |||||||||
| 28848 | SUB 149 | |||||||||
| 28850 | ADD A,A | |||||||||
| 28851 | ADD A,A | |||||||||
| 28852 | ADD A,E | |||||||||
| 28853 | LD E,A | |||||||||
| 28854 | LD D,127 | |||||||||
| 28856 | EX DE,HL | |||||||||
| 28857 | SET 0,(HL) | Set the appropriate bit in the SRB byte; this instruction is modified earlier in this routine to set the required bit | ||||||||
| 28859 | LD A,(28858) | |||||||||
| 28862 | ADD A,8 | Was m=7? | ||||||||
| 28864 | JR NC,28874 | Jump if not | ||||||||
| 28866 | INC L | Move to the next SRB byte | ||||||||
| 28867 | LD A,L | A will be 0 if we wrapped round to the next row of the screen | ||||||||
| 28868 | AND 3 | |||||||||
| 28870 | LD A,198 | 198 will set n=0 | ||||||||
| 28872 | JR Z,28879 | Jump if we wrapped round to the next row | ||||||||
| 28874 | LD (28878),A | Modify the SET n,(HL) instruction below so that n=m+1 mod 8 | ||||||||
| 28877 | SET 0,(HL) | Set the appropriate bit in the SRB byte; this instruction is modified immediately before execution to set the required bit | ||||||||
| 28879 | EX DE,HL | |||||||||
| 28880 | POP DE | Restore the skool coordinates to DE | ||||||||
| 28881 | RET | |||||||||
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