Routines
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680F: Make a teacher wipe a blackboard
The address of this interruptible subcommand routine is placed into bytes 0x09 and 0x0A of the teacher's buffer by the routine at F100.
Input
H Teacher's character number (0xC9-0xCC)
680F CALL \$6FF2 Collect information about the board
6812 LD L,\$0B 32 distinct actions are needed to wipe the board; store this counter in byte 0x0B of the teacher's buffer
6814 LD (HL),\$20
6816 INC L L=0x0C
6817 LD A,E A=x-coordinate of the left edge of the board
6818 ADD A,\$07 A=x-coordinate of the right edge of the board
681A LD (HL),A Store this in byte 0x0C of the teacher's buffer
681B INC L Place the y-coordinate of the top line of the board into byte 0x0D of the teacher's buffer
681C LD (HL),D
681D INC L And the board identifier (LSB of 7F54, 7F5A, 7F60, 7F66 or 7F6C) into byte 0x0E
681E LD (HL),B
681F LD L,\$09 Change the routine address in bytes 0x09 and 0x0A of the teacher's buffer to 6823 (below)
6821 LD (HL),\$23
6823 LD L,\$0B Byte 0x0B holds the board-wiping action counter
6825 DEC (HL) Decrement the counter
6826 JP P,\$6836 Jump if the board has not yet been fully wiped
The teacher has finished wiping the board. Mark the board as clean, and move the teacher on to his next task.
6829 LD L,\$0E Collect the board identifier in L
682B LD L,(HL)
682C LD H,\$7F HL now points to the board's buffer
682E LD (HL),\$00 Signal that the board has been wiped by zeroing the first two bytes of the blackboard's buffer
6830 INC L
6831 LD (HL),\$00
6833 JP \$6390 Now the job's been done, terminate this interruptible subcommand
The board's not clean yet. Work out what the teacher has to do next: wipe, or move down the board to the next spot.
6836 CALL \$61B4 Update the SRB for the teacher's current animatory state and location
6839 AND \$78 A=teacher's base animatory state
683B LD L,\$0B Point HL at the board-wiping action counter
683D BIT 0,(HL) Is the teacher midstride or wiping (arm up)?
683F JR Z,\$6876 Jump if so
6841 BIT 1,(HL) Are we ready for a wipe action?
6843 JR Z,\$6849 Jump if so
6845 INC A A=animatory state of the teacher midstride
6846 JP \$6130 Update the teacher's animatory state and update the SRB
The teacher's next action will be a wipe, so raise his arm.
6849 ADD A,\$07 A=animatory state of the teacher with his arm up
684B CALL \$6130 Update the teacher's animatory state and update the SRB
684E LD L,\$0C Collect the x-coordinate of the part of the board to be wiped into E
6850 LD E,(HL)
6851 DEC (HL) Decrease this x-coordinate ready for the next wipe
6852 INC L Collect the y-coordinate of the top line of the blackboard into D
6853 LD D,(HL)
6854 CALL \$670C Update the SRB for the top line of the blackboard
6857 INC D D=y-coordinate of the bottom line of the blackboard
6858 CALL \$670C Update the SRB for the bottom line of the blackboard
685B LD A,D Prepare H for picking up skool UDG base address LSBs later
685E LD H,A
685F LD D,\$B5 Pick up the Q value (0x00<=Q<=0x8F) for the lower blackboard tile being wiped from page 0xB5 in L
6861 LD A,(DE)
6862 LD L,A
6863 LD A,(HL) Collect the LSB of the base address of the skool UDG for this blackboard tile in L
6864 LD L,A
6865 SUB \$08 Set E to the LSB of the base address of the skool UDG for the upper blackboard tile
6867 LD E,A
6868 LD H,\$80 The base page for all blackboard tile UDGs is 0x80
686A LD D,H
686B LD B,\$08 8 lines per character square
686D LD A,\$FF 0xFF=blank blackboard byte
686F LD (HL),A Clear a pixel line in the skool UDG for the lower blackboard tile
6870 LD (DE),A Clear a pixel line in the skool UDG for the upper blackboard tile
6871 INC H Next pixel row of the lower blackboard tile UDG
6872 INC D Next pixel row of the upper blackboard tile UDG
6873 DJNZ \$686F Jump back until the two blackboard tile UDGs have been cleared
6875 RET
The teacher is either wiping or midstride while on his way to the next spot along the board. At this point, A holds the teacher's base animatory state.
6876 BIT 1,(HL) Is the teacher wiping?
6878 JR Z,\$687B Jump if so
687A DEC E The teacher is midstride, so now move him one space to the left
687B JP \$6130 Update the teacher's animatory state and location and update the SRB
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