Routines
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610B: Print a tile
Used by the routines at 6000 and 6992. Copies a tile of the skool into the back buffer at 7F01, superimposes character sprite tiles as appropriate, and then copies the resultant tile to the screen.
Input
D Skool y-coordinate (152-172)
E Skool x-coordinate (0-95)
610B LD A,E A=skool x-coordinate (0-95)
610C AND \$60 Now H is 0x80 if 0<=E<32; 0x88 if 32<=E<64; or 0x90 if 64<=E<96
610E RRCA
610F INC A
6110 RRCA
6111 LD H,A
6112 LD A,(DE) A=skool UDG reference for the character square at (E,D)
6113 PUSH DE
6114 LD L,A HL=base address of the UDG bytes for the character square at (E,D)
6115 LD BC,\$0800 B=8, C=0
6118 LD DE,\$7F01 Copy the 8 bytes of the skool UDG into the back buffer at 7F01
611B LD A,(HL)
611C LD (DE),A
611D INC H
611E INC E
611F DJNZ \$611B
6121 NOP
6122 POP DE Restore the skool coordinates to DE
6123 LD B,\$15 There are 21 characters to consider
6125 LD H,\$98 0x98=little boy no. 1
Now we enter a loop that checks which part (if any) of each character's sprite appears at the skool coordinates in DE.
6127 LD L,\$62 Byte 0x62 of the character's buffer holds his x-coordinate
6129 LD A,E A=skool x-coordinate (0-95)
612A SUB (HL) Subtract the character's x-coordinate
612B DEC HL Byte 0x61 of the character's buffer holds his y-coordinate
612C JR C,\$615E Jump if no part of the character's sprite appears at this x-coordinate
612E CP \$03
6130 JR NC,\$615E
6132 ADD A,A Now C is 0, 4 or 8, corresponding to the column (left, middle or right) of the character's sprite that aligns with x-coordinate E
6134 LD C,A
6135 LD A,D A=skool y-coordinate (152-172)
6136 SUB (HL) Subtract the character's y-coordinate
6137 JR C,\$615E Jump if no part of the character's sprite appears at this y-coordinate
6139 CP \$04
613B JR NC,\$615E
613D ADD A,C A=0x00-0x0B, corresponding to the sprite tile that appears at (E,D)
613E EXX
6142 EXX
6143 LD L,\$60 Pick up the character's animatory state in A
6145 LD A,(HL)
6146 EXX
6147 LD L,A L'=character's animatory state
6148 LD A,(HL) A=sprite tile UDG reference
6149 AND A Is it the blank UDG?
614A JR Z,\$615D Jump if so
614C CALL \$6D0A Get the base address of the sprite tile UDG bytes in HL'
The following inner loop superimposes the sprite tile onto the tile built up so far in the back buffer at 7F01 (which was originally populated with the relevant skool UDG).
614F LD B,\$08 There are 8 bytes in the UDG
6151 LD DE,\$7F01
6154 LD A,(DE) A=back buffer UDG byte
6155 OR (HL) OR in the sprite tile UDG byte
6156 INC H
6157 AND (HL) AND in the sprite tile UDG mask
6158 INC H
6159 LD (DE),A
615A INC E
615B DJNZ \$6154 Jump back until the UDG is done
615D EXX
615E INC H Next character
615F DJNZ \$6127 Jump back to consider the next character
The display tile (consisting of the skool UDG with all relevant sprite tiles superimposed) has been built. Figure out where in the display file it should be printed.
6161 LD HL,\$7F00 7F00 holds the leftmost column of the skool on-screen (0-64)
6164 LD A,D A=skool y-coordinate (152-172)
6165 AND \$07 Set C to the LSB of the display file address corresponding to the screen coordinates (0,D-152)
6167 RRCA
6168 RRCA
6169 RRCA
616A LD C,A
616B LD A,E A=screen x-coordinate (0-31)
616C SUB (HL)
616D ADD A,C Set L to the LSB of the display file address corresponding to the skool coordinates (E,D)
616E LD L,A
616F LD A,D A=screen y-coordinate (0-20)
6170 SUB \$98
6172 AND \$18 Set H to the MSB of the display file address corresponding to the skool coordinates (E,D)