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Used by the routines at 25026, 25080 and 25248. Copies a tile of the play area into the back buffer at 57712, superimposes character sprite tiles as appropriate, and then copies the resultant tile to the screen. Also sets the corresponding attribute byte.
The play area graphic data is laid out across pages 128 to 181:
For the tile at skool coordinates (X,Y) (0<=X<=191, 0<=Y<=20), the column pointer Q (0<=Q<=143) is collected from byte X of page 181. Then Q and Y together determine the location of the attribute byte, and where to look up the base address of the tile graphic data.
The BRIGHT/PAPER bits for row Y are arranged in a set of 144 nibbles in bytes 180-251 of page Y+160, from bits 7-4 of byte 180 (nibble 0) to bits 3-0 of byte 251 (nibble 143). Q is the index of the nibble that corresponds to the tile at (X,Y). In other words, the BRIGHT/PAPER bits for the skool location (X,Y) can be found in byte 180+INT(Q/2) of page Y+160: bits 4-7 if Q is even, bits 0-3 if Q is odd.
By default, the INK is 0 (black). However, if the BRIGHT/PAPER bits are all 0, this indicates a tile with non-black INK, of which there are normally four: the three shields in MR WACKER's study (which are red, blue, and red), and the top of the entrance to the girls' skool (which is blue). (However, when the left study door is open - which never happens in an unhacked game - there is a fifth tile with non-black INK: the top of the left study doorway, which is magenta.) When a tile has non-black INK, the BRIGHT/PAPER bits are taken from byte 180+INT(Q/2) of page Y+161 (i.e. the attribute byte corresponding to the skool location (X,Y+1)); then the INK is 2 (red) if Q is even, 1 (blue) if 4|Q-1, or 3 (magenta) if 4|Q-3.
So much for the attributes. The LSB of the base address of the tile graphic data is found in byte Q of page Y+160. Information on which MSB (128, 136, 144 or 152) applies is arranged in a set of 144 bit-pairs in bytes 144-179 of page Y+160. Bit-pair 0 is bits 7 and 3 of byte 144; bit-pair 1 is bits 6 and 2; bit-pair 2 is bits 5 and 1; bit-pair 3 is bits 4 and 0; bit-pair 4 is bits 7 and 3 of byte 145; and so on up to bit-pair 143, which is bits 4 and 0 of byte 179. Q is the index of the bit-pair that corresponds to the tile at (X,Y). In other words, the tile MSB indicator for the skool location (X,Y) can be found in byte 144+INT(Q/4) of page Y+160: bits 7 and 3 if 4|Q, 6 and 2 if 4|Q-1, 5 and 1 if 4|Q-2, 4 and 0 if 4|Q-3. The bit-pair forms a 2-digit binary number from 0 to 3; if we call this P, then the MSB is 128+8P.
To summarise, then, the attributes and graphic data for the tile at skool location (X,Y) can be found thus, where Q=PEEK(46336+X):
|24685||LD A,H||Compute the appropriate attribute file address in BC and the appropriate display file address in DE|
|24707||EX DE,HL||Place the display file address on the stack and get (row,col) back in HL|
|24709||LD A,(32767)||A=leftmost column of the play area on screen (0-160)|
|24712||ADD A,L||L=X: column of the play area corresponding to the character square under consideration (0-191)|
|24714||PUSH HL||Push the skool coordinates (X,Y) onto the stack|
|24715||LD A,H||A=Y (0-20)|
|24718||LD L,(HL)||L=Q (0<=Q<=143)|
|24722||LD E,L||E=Q (0<=Q<=143)|
|24724||RRA||Is Q odd?|
|24725||JR C,24754||Jump if so|
Q (the contents of (46336+X)) is even, which means the PAPER colour and BRIGHT status can be found in bits 4-7 of byte 180+Q/2 in page Y+160. At this point A=Q/2.
|24729||LD L,A||Point HL at the attribute byte|
|24730||LD A,(HL)||Pick up the attribute byte in A|
|24731||AND 240||The PAPER colour and BRIGHT status are in bits 4-7|
|24733||RR A||Shift them into bits 3-6|
|24735||JR NZ,24774||Jump if this character square has INK 0|
We are dealing with one of the few character squares with non-black INK. In this case the PAPER colour and BRIGHT status can be found in bits 4-7 (since Q is even) of byte 180+Q/2 in page Y+161.
|24737||INC H||Point HL at the attribute byte in page Y+161|
|24738||LD A,(HL)||Pick up the attribute byte in A|
|24739||AND 240||The PAPER colour and BRIGHT status are in bits 4-7|
|24741||RRA||Shift them into bits 3-6|
|24742||LD L,A||L holds the PAPER colour and BRIGHT status|
|24743||LD A,E||A=Q (0<=Q<=143)|
|24744||AND 3||A=0, 1, 2 or 3|
|24746||JR NZ,24750||Jump if the INK is blue, red, or magenta|
|24748||ADD A,2||A=2: INK 2 (red)|
|24750||OR L||OR on the PAPER colour and BRIGHT status|
|24752||JR 24774||Jump forward to transfer the attribute byte in A onto the screen|
Q (the contents of (46336+X)) is odd, which means the PAPER colour and BRIGHT status can be found in bits 0-3 of byte 180+(Q-1)/2 in page Y+160. At this point A=(Q-1)/2.
|24756||LD L,A||Point HL at the attribute byte|
|24757||LD A,(HL)||Pick up the attribute byte in A|
|24758||AND 15||The PAPER colour and BRIGHT status are in bits 0-3|
|24760||JR NZ,24771||Jump if this character square has INK 0|
We are dealing with one of the few character squares with non-black INK. In this case the PAPER colour and BRIGHT status can be found in bits 0-3 (since Q is odd) of byte 180+(Q-1)/2 in page Y+161.
|24762||INC H||Point HL at the attribute byte in page Y+161|
|24763||LD A,(HL)||Pick up the attribute byte in A|
|24764||AND 15||The PAPER colour and BRIGHT status are in bits 0-3|
|24766||ADD A,A||Shift the PAPER and BRIGHT bits from bits 0-3 to bits 3-6|
|24769||JR 24742||Jump back to fill in the INK bits|
|24771||ADD A,A||Shift the PAPER and BRIGHT bits from bits 0-3 to bits 3-6, and set the INK to 0|
|24774||NOP||Do nothing (during the startup sequence), or poke the attribute byte onto the screen (this instruction is set to LD (BC),A before the game starts; see 21408)|
The appropriate attribute byte has been poked onto the screen. Now to find the appropriate tile graphic data.
|24775||LD L,E||L=Q (0<=Q<=143), H=Y+160|
|24776||LD E,(HL)||E=LSB of the base address of the tile graphic data|
|24779||SRL L||Shift A right (Q AND 3) times|
|24790||LD C,A||C=10001000, 01000100, 00100010 or 00010001|
|24791||LD A,L||Point HL at the byte containing the MSB indicator bit-pair: H=Y+160, 144<=L=144+INT(Q/4)<=179|
|24795||LD A,C||A=10001000, 01000100, 00100010 or 00010001|
|24796||AND (HL)||Keep only the bits of the bit-pair|
The base page of the tile graphic data depends on the bits set in A.
|24799||CP 16||Are any of bits 4-7 in A set?|
|24801||JR C,24805||Jump if not|
|24805||AND 15||Are any of bits 0-3 in A set?|
|24807||JR Z,24811||Jump if not|
Now D=128, 136, 144 or 152, and DE points at the first byte of the graphic data for the tile at skool coordinates (X,Y).
|24811||LD HL,57712||We're going to copy the tile into the 8-byte back buffer at 57712|
|24816||XOR A||Use a blank graphic byte (during startup), or collect the tile graphic byte (this instruction is set to LD A,(DE) before the game starts; see 21408)|
|24817||INC D||Point DE at the next graphic byte|
|24818||LD (HL),A||Copy the graphic byte into the back buffer|
|24819||INC L||Move to the next slot in the back buffer|
|24820||DJNZ 24816||Jump back until the entire tile has been copied|
The appropriate play area tile has been copied into the back buffer at 57712. Now it's time to superimpose game character sprite tiles (if any).
|24822||LD H,183||183=character number of little girl no. 1|
|24824||POP DE||D=Y (0-20), E=X (0-191)|
|24825||LD A,(32767)||A=leftmost column of the play area on screen (0-160)|
|24828||CP 120||This x-coordinate is roughly halfway between the tree and the gate|
|24830||JR C,24843||Jump if columns 144 onwards (girls' skool + half the girls' playground) are off-screen|
|24832||LD B,7||7 little girls|
|24834||CALL 24576||Superimpose graphic bytes for the little girls if necessary|
|24837||LD H,198||198=character number of little boy no. 9|
|24839||LD B,17||2 little boys, 11 main characters, plus the objects using character buffers 211-214|
|24843||CP 80||This is the x-coordinate of the assembly hall stage (or thereabouts)|
|24845||JR C,24858||Jump if columns 104 onwards (everything to the right of the tree, roughly) are off-screen|
|24847||LD B,3||3 little girls (183-185)|
|24849||CALL 24576||Superimpose graphic bytes for these little girls if necessary|
|24852||LD H,193||193=character number of little boy no. 4|
|24854||LD B,22||7 little boys, 11 main characters, plus the objects using character buffers 211-214|
|24858||LD H,190||190=character number of little boy no. 1|
|24860||LD B,25||All 10 little boys, 11 main characters, plus the objects using character buffers 211-214|
|24862||CALL 24576||Superimpose graphic bytes for these characters if necessary|
|24865||LD HL,57712||The 8-byte back buffer at 57712 now contains the tile to be copied to the screen|
|24868||POP DE||Retrieve the appropriate display file address in DE|
This entry point is used by the routine at 26941.
|24869||LD B,8||There are 8 bytes per character square|
|24871||LD A,(HL)||Transfer the graphic bytes to the screen|
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