Routines
Prev: 6563 Up: Map Next: 659B
6564: Prepare Scroll Data
Used by the routines at ScrollPlayarea_Left and ScrollPlayarea_Right.
Input
A Scroll phase
PrepareScrollData 6564 ADD A,A C=A*04.
6565 ADD A,A
6566 LD C,A
6567 EXX Switch to the shadow registers.
6568 LD BC,$05FF Set a counter in B of 05 (the FF in C isn't used).
656B LD DE,$7800 DE=7800.
PrepareScrollData_Loop 656E EXX Switch back to the normal registers.
656F LD A,(DE) Read the index byte.
6570 INC D Move to the next byte.
6571 RRCA Rotate the index byte right three times to extract the address components.
6572 RRCA
6573 RRCA
6574 LD L,A Copy the result to L.
6575 AND %00011111 Keep only bits 0-4.
6577 ADD A,$80 Add 80 to form the high byte of the graphics address in HL.
6579 LD H,A
657A LD A,L Restore the rotated value.
657B AND %11100000 Keep only bits 5-7.
657D ADD A,C Add the scroll offset from C.
657E LD L,A Complete the graphics address in HL.
657F PUSH HL Stash the graphics address on the stack.
6580 EXX Switch to the shadow registers.
6581 POP HL Restore the graphics address from the stack.
6582 LDI Copy 04 bytes from the graphics line to the destination.
6584 LDI
6586 LDI
6588 LDI
658A DJNZ PrepareScrollData_Loop Decrease the row counter by one and loop back to PrepareScrollData_Loop until all the rows have been copied.
658C LD A,(DE) A=*DE.
658D INC A Increment A by one.
658E LD (DE),A Write A to *DE.
658F INC E Increment E by one.
6590 LD A,(DE) A=*DE.
6591 EXX Switch to the shadow registers.
6592 ADD A,B A+=B.
6593 EXX Switch to the shadow registers.
6594 LD (DE),A Write A to *DE.
6595 LD E,$02 E=02.
6597 LD B,$10 B=10.
6599 EXX Switch to the shadow registers.
659A RET Return.
Prev: 6563 Up: Map Next: 659B