Routines
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7FA7: Interpret a picture stream
Used by the routine at DRAW_LOCATION_PICTURE.
IY walks the stream; D and E are the current point, in a 256x128 space with y measured upwards from scanline 127. The opcodes are: $00 end, $08 move, $2x paint attributes, $4x flood fill, and anything with bit 7 set a line. Bits 0-2 of a fill or paint opcode are its colour.
A line is two bytes, and both are split: the opcode byte carries the direction in bits 0-2 and part of the minor-axis step in bits 2-5, while the second byte carries the length in bits 0-5 and the last two step bits in bits 6-7. That is a Bresenham line of up to 64 pixels in 16 bits.
Input
HL The start of the stream
RUN_PICTURE 7FA7 PUSH IY
7FA9 PUSH HL
7FAA PUSH HL
7FAB POP IY
7FAD PUSH DE
7FAE PUSH BC
7FAF CALL CLEAR_CANVAS Clear the canvas and take the border and attribute bytes off the front
7FB2 LD D,$7F The point a stream starts from if it draws before it moves: (127, 63)
7FB4 LD E,$3F
7FB6 LD B,$01
7FB8 LD C,$01
7FBA LD L,$01
RUN_PICTURE_0 7FBC LD A,(IY+$00)
7FBF AND A
7FC0 JP Z,PICTURE_DONE $00: end of the picture
7FC3 INC IY
7FC5 CP $08 $08: move...
7FC7 JR NZ,RUN_PICTURE_1
7FC9 LD D,(IY+$00) ...to the point in the next two bytes
7FCC INC IY
7FCE LD E,(IY+$00)
7FD1 INC IY
7FD3 JR RUN_PICTURE_0
RUN_PICTURE_1 7FD5 BIT 7,A
7FD7 JR Z,RUN_PICTURE_2
7FD9 LD B,A
7FDA AND $07 A line: bits 0-2 of the opcode are the direction
7FDC LD C,A
7FDD LD A,B Bits 2-5 of it are part of the minor-axis step
7FDE RRCA
7FDF AND $3C
7FE1 LD B,A
7FE2 LD A,(IY+$00) The second byte's bits 0-5 are the length, plus one
7FE5 AND $3F
7FE7 LD L,A
7FE8 INC L
7FE9 LD A,(IY+$00) ...and its bits 6-7 are the rest of the step
7FEC INC IY
7FEE RLCA
7FEF RLCA
7FF0 AND $03
7FF2 OR B
7FF3 LD B,A
7FF4 INC B
7FF5 CALL DRAW_LINE Draw it
7FF8 JR RUN_PICTURE_0
RUN_PICTURE_2 7FFA BIT 6,A
7FFC JR Z,RUN_PICTURE_3
7FFE AND $07 Bit 6 set: a flood fill; bits 0-2 are its colour
8000 PUSH DE
8001 LD D,(IY+$00) The fill's seed point, from the next two bytes
8004 INC IY
8006 LD E,(IY+$00)
8009 INC IY
800B CALL FLOOD_FILL Fill from there
800E POP DE The fill does not move the current point
800F JP RUN_PICTURE_0
RUN_PICTURE_3 8012 BIT 5,A
8014 JP Z,RUN_PICTURE_0
8017 AND $07 Bit 5 set: paint attribute cells; bits 0-2 are the colour
8019 RLCA
801A RLCA
801B RLCA
801C PUSH HL
801D PUSH DE
801E PUSH BC
801F LD C,A
8020 LD H,(IY+$00) HL = the first attribute address, stored high byte first
8023 INC IY
8025 LD L,(IY+$00)
8028 INC IY
RUN_PICTURE_4 802A LD A,(IY+$00)
802D INC IY
802F CP $FF $FF ends the path
8031 JR Z,RUN_PICTURE_7
8033 LD B,A
8034 AND $03
8036 LD E,A
8037 LD A,B Each path item: bits 0-1 the direction, bits 2-7 the count
8038 RRCA
8039 RRCA
803A AND $3F
803C INC A
803D LD B,A
RUN_PICTURE_5 803E LD A,(HL) Keep the cell's ink out of its paper, so a colour can never hide the lines
803F AND $07
8041 RLCA
8042 RLCA
8043 RLCA
8044 CP C
8045 JR NZ,RUN_PICTURE_6
8047 XOR $38
RUN_PICTURE_6 8049 RRCA
804A RRCA
804B RRCA
804C OR C
804D LD (HL),A
804E LD A,E
804F AND A
8050 CALL Z,ATTR_UP
8053 DEC A
8054 CALL Z,ATTR_RIGHT
8057 DEC A
8058 CALL Z,ATTR_DOWN
805B DEC A
805C CALL Z,ATTR_LEFT
805F DJNZ RUN_PICTURE_5
8061 JR RUN_PICTURE_4
RUN_PICTURE_7 8063 POP BC
8064 POP DE
8065 POP HL
8066 JP RUN_PICTURE_0
This entry point is used by the routine at CLEAR_CANVAS.
PICTURE_DONE 8069 POP BC
806A POP DE
806B POP HL
806C POP IY
806E RET
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