| Chase H.Q. | Routines |
| Prev: 860F | Up: Map | Next: 86F6 |
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The pre-game screen ("CHASE H.Q. MONITORING SYSTEM") is drawn by following the commands stored at pregame_data. Possible commands are "plot tile" (single or up to 30 repetitions), "set background colour", "set address", "draw horizontally", "draw vertically" or "stop". The tiles are the 45 8x8 pixel tiles that live at pregame_tiles.
Once the tiles are drawn the strings at pre_game_messages are drawn by calling print_message.
Used by the routine at run_pregame_screen.
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| draw_pregame | 865A | LD HL,$77D8 | Address of pre-game screen data | |
| dp_get_command | 865D | LD A,(HL) | Read a command byte | |
| 865E | AND A | Set flags | ||
| 865F | JP Z,dp_print_strings | Zero is the "Stop" command so jump to rendering messages | ||
| 8662 | INC HL | Advance to next command byte | ||
| 8663 | CP $D0 | Jump if < $D0 (either "Repeat" or "Plot tile") | ||
| 8665 | JR C,dp_repeat_or_plot_tile | |||
| 8667 | CP $E0 | Jump if < $D0..$DF ("Set background colour") | ||
| 8669 | JR C,dp_set_bg_colour | |||
| 866B | CP $F0 | Jump if >= $F0 ("Set address") | ||
| 866D | JR NC,dp_set_address | |||
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$E1 is "Draw horizontally". $E2 is "Draw vertically".
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| dp_set_direction | 866F | SUB $E0 | Self modify dp_direction with (A - $E0) | |
| 8671 | LD ($86C9),A | |||
| 8674 | JR dp_get_command | Loop dp_get_command | ||
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$D0..$DF is "Set background colour".
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| dp_set_bg_colour | 8676 | SUB $D0 | Self modify 86C2 with (A - $D0) << 3 | |
| 8678 | RLCA | |||
| 8679 | RLCA | |||
| 867A | RLCA | |||
| 867B | LD ($86C3),A | |||
| 867E | JR dp_get_command | Loop dp_get_command | ||
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$F0..$FF is "Set address".
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| dp_set_address | 8680 | LD D,A | Load new backbuffer address into DE | |
| 8681 | LD E,(HL) | |||
| 8682 | INC HL | |||
| 8683 | JR dp_get_command | Loop dp_get_command | ||
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$00..$CF could be either "Repeat" or "Plot tile".
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| dp_repeat_or_plot_tile | 8685 | CP $1F | Command bytes >= $1F emit single tiles | |
| 8687 | LD B,$01 | Set count to 1 irrespective | ||
| 8689 | JR NC,dp_do_plot | Jump if so | ||
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Plot multiple tiles.
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| 868B | LD B,A | Command bytes <= $1E emit repeated tiles | ||
| 868C | LD A,(HL) | Read tile index | ||
| 868D | INC HL | Advance to next command | ||
| dp_do_plot | 868E | PUSH HL | Preserve command pointer | |
| 868F | SUB $1F | Turn command byte into a tile index | ||
| 8691 | RLCA | Multiply by 8 - size of tile. Carry flag is updated | ||
| 8692 | RLCA | |||
| 8693 | RLA | |||
| 8694 | PUSH BC | Preserve count | ||
| 8695 | LD C,A | Expand tile offset into BC | ||
| 8696 | LD B,$00 | |||
| 8698 | RL B | Add in carry flag | ||
| 869A | LD HL,$78A7 | HL = &pregame_tiles[A] | ||
| 869D | ADD HL,BC | |||
| 869E | POP BC | Restore count | ||
| 869F | EX DE,HL | DE becomes tile ptr, HL becomes backbuffer ptr | ||
| dp_line_loop | 86A0 | PUSH BC | Count | |
| 86A1 | PUSH DE | Tile ptr | ||
| 86A2 | PUSH HL | Backbuffer ptr | ||
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Plot a tile.
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| 86A3 | LD B,$08 | 8 rows per tile | ||
| dp_tile_loop | 86A5 | LD A,(DE) | Copy a row of tile pixels to the screen | |
| 86A6 | LD (HL),A | |||
| 86A7 | INC DE | Move to next tile row | ||
| 86A8 | INC H | Move to next scanline | ||
| 86A9 | DJNZ dp_tile_loop | Loop dp_tile_loop | ||
| 86AB | POP BC | Pop backbuffer ptr | ||
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Seems to be building an attribute address?
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| 86AC | LD A,B | Get bit 3 of Y | ||
| 86AD | RLA | |||
| 86AE | RLA | |||
| 86AF | AND $20 | |||
| 86B1 | LD D,A | |||
| 86B2 | LD A,C | Add to X bits | ||
| 86B3 | AND $1F | |||
| 86B5 | ADD A,D | |||
| 86B6 | LD E,A | |||
| 86B7 | LD A,C | Get bits 4..6 of Y. Bit 6 goes to carry | ||
| 86B8 | AND $E0 | |||
| 86BA | ADD A,A | |||
| 86BB | LD D,$59 | Select second third of attributes | ||
| 86BD | JR NC,dp_attrs | |||
| 86BF | INC D | Otherwise select final third of attributes | ||
| dp_attrs | 86C0 | ADD A,E | DE is now the attribute byte address | |
| 86C1 | LD E,A | |||
| 86C2 | LD A,$00 | Load <self modified> attribute value, as set by the $Dx commands | ||
| 86C4 | AND A | Set attribute byte if non-zero | ||
| 86C5 | JR Z,dp_direction | |||
| 86C7 | LD (DE),A | |||
| dp_direction | 86C8 | LD A,$00 | Load <self modified - 1> direction value, as set by the $Ex commands | |
| 86CA | DEC A | |||
| 86CB | JR Z,dp_direction_horizontal | Jump if horizontal | ||
| dp_direction_vertical | 86CD | LD A,H | Get bits 0..3 of Y | |
| 86CE | AND $0F | |||
| 86D0 | JR NZ,dp_nextone | Jump to dp_nextone if non-zero | ||
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H is incremented by 1 earlier. So if it was 0b11111111 it's now 0b00000000 so make it 0b11110000. Does this happen in any other case?
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| 86D2 | LD A,H | H -= 16 | ||
| 86D3 | SUB $10 | |||
| 86D5 | LD H,A | |||
| 86D6 | LD A,L | Increment bits 4..6 of Y (visually downwards by 16?) | ||
| 86D7 | ADD A,$20 | |||
| 86D9 | LD L,A | |||
| 86DA | JR dp_nextone | Jump to dp_nextone | ||
| dp_direction_horizontal | 86DC | LD H,B | Move one character to the right | |
| 86DD | LD L,C | |||
| 86DE | INC L | |||
| dp_nextone | 86DF | POP DE | Restore tile ptr | |
| 86E0 | POP BC | Restore count | ||
| 86E1 | DJNZ dp_line_loop | Loop dp_line_loop while B > 0 | ||
| 86E3 | EX DE,HL | DE = Back buffer ptr | ||
| 86E4 | POP HL | Restore command pointer | ||
| 86E5 | JP dp_get_command | Loop dp_get_command | ||
| dp_print_strings | 86E8 | LD B,$04 | Four messages to print | |
| 86EA | LD HL,$7798 | Address of pre_game_messages | ||
| dp_print_string | 86ED | LD A,$04 | Flags TBD | |
| 86EF | DEC HL | HL-- | ||
| 86F0 | CALL print_message | Call print_message | ||
| 86F3 | DJNZ dp_print_string | Loop to dp_print_string while B > 0 | ||
| 86F5 | RET | Return | ||
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