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Routines |
| Prev: ABA0 | Up: Map | Next: AC6F |
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This draws the zoombox contents.
Used by the routine at zoombox.
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| zoombox_fill | ABF9 | LD A,($AB68) | Fetch zoombox_y | |
| ABFC | LD H,A | Multiply it by 128 and store it in DE | ||
| ABFD | XOR A | |||
| ABFE | SRL H | |||
| AC00 | RRA | |||
| AC01 | LD E,A | |||
| AC02 | LD D,H | |||
| AC03 | SRL H | Multiply it by 64 and store it in HL | ||
| AC05 | RRA | |||
| AC06 | LD L,A | |||
| AC07 | ADD HL,DE | Sum the two, producing zoombox_y * 192 | ||
| AC08 | LD A,($AB66) | Fetch zoombox_x | ||
| AC0B | ADD A,L | Add zoombox_x to zoombox_y * 192 producing the window buffer source offset | ||
| AC0C | LD L,A | |||
| AC0D | JR NC,zoombox_fill_0 | |||
| AC0F | INC H | |||
| zoombox_fill_0 | AC10 | LD DE,$F291 | Point DE at window buffer + 1 (TODO: Explain the +1) | |
| AC13 | ADD HL,DE | Form the final source pointer | ||
| AC14 | EX DE,HL | Free up HL for the next chunk | ||
| AC15 | LD A,($AB68) | Fetch zoombox_y again | ||
| AC18 | ADD A,A | Multiply it by 16 producing an offset into the game_window_start_addresses array | ||
| AC19 | ADD A,A | |||
| AC1A | ADD A,A | |||
| AC1B | ADD A,A | |||
| AC1C | LD HL,$EDD3 | Point HL at the game_window_start_addresses array | ||
| AC1F | ADD A,L | Combine it with the offset | ||
| AC20 | LD L,A | |||
| AC21 | JR NC,zoombox_fill_1 | |||
| AC23 | INC H | |||
| zoombox_fill_1 | AC24 | LD A,(HL) | Fetch the game window start address | |
| AC25 | INC HL | |||
| AC26 | LD H,(HL) | |||
| AC27 | LD L,A | |||
| AC28 | LD A,($AB66) | Fetch zoombox_x again | ||
| AC2B | ADD A,L | Combine with game window start address | ||
| AC2C | LD L,A | |||
| AC2D | EX DE,HL | Get the source pointer back in HL | ||
| AC2E | LD A,($AB67) | Fetch zoombox_width | ||
| AC31 | LD ($AC55),A | Self modify the 'SUB $00' at AC54 | ||
| AC34 | NEG | Compute source skip (24 - width) | ||
| AC36 | ADD A,$18 | |||
| AC38 | LD ($AC4D),A | Self modify the 'LD A,$00' at AC4C | ||
| AC3B | LD A,($AB69) | Fetch zoombox_height (number of rows to copy) | ||
| AC3E | LD B,A | Set outer iterations | ||
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Start loop (outer) -- once for every row
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| zoombox_fill_2 | AC3F | PUSH BC | Preserve iteration counter | |
| AC40 | PUSH DE | Preserve destination pointer | ||
| AC41 | LD A,$08 | 8 iterations / 1 row | ||
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Start loop (inner) -- once for every line
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| zoombox_fill_3 | AC43 | EX AF,AF' | Bank the counter | |
| AC44 | LD A,($AB67) | Fetch zoombox_width into BC | ||
| AC47 | LD C,A | |||
| AC48 | LD B,$00 | |||
| AC4A | LDIR | Copy zoombox_width bytes from HL to DE, then advance those pointers | ||
| AC4C | LD A,$00 | Load the (self modified) source skip | ||
| AC4E | ADD A,L | Advance the source pointer by source skip bytes | ||
| AC4F | LD L,A | |||
| AC50 | JR NC,zoombox_fill_4 | |||
| AC52 | INC H | |||
| zoombox_fill_4 | AC53 | LD A,E | Subtract the (self modified) zoombox_width from the destination pointer to undo LDIR's post-increment | |
| AC54 | SUB $00 | |||
| AC56 | LD E,A | |||
| AC57 | INC D | Move to the next scanline by incrementing high byte | ||
| AC58 | EX AF,AF' | Unbank the inner loop counter | ||
| AC59 | DEC A | ...loop (inner) | ||
| AC5A | JP NZ,zoombox_fill_3 | |||
| AC5D | POP DE | Restore the destination pointer | ||
| AC5E | EX DE,HL | Exchange | ||
| AC5F | LD BC,$0020 | Set the row-to-row delta to 32 | ||
| AC62 | LD A,L | Isolate | ||
| AC63 | CP $E0 | Is A < 224? This sets the C flag if we're NOT at the end of the current third of the screen | ||
| AC65 | JR C,zoombox_fill_5 | Jump over if so | ||
| AC67 | LD B,$07 | Otherwise set the stride to $0720 so we will step forward to the next third of the screen. In this case HL is of the binary form 010ttyyy111xxxxx. Adding $0720 - binary 0000011100100000 - will result in 010TTyyy000xxxxx where the 't' bits are incremented, moving the pointer forward to the next screen third | ||
| zoombox_fill_5 | AC69 | ADD HL,BC | Step | |
| AC6A | EX DE,HL | Exchange back | ||
| AC6B | POP BC | Pop the outer loop counter | ||
| AC6C | DJNZ zoombox_fill_2 | ...loop (outer) | ||
| AC6E | RET | Return | ||
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