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Routines |
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The first check is to see if the item's horizontal/vertical pixel co-ordinates fall within the bounds of the visible playing area.
The checks also test for cases where part of the graphic is on-screen - it's only considered 'off-screen' if none of it needs drawing.
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| DB98 | LD E,(HL) | Transfer the address (from 97C3) pointed to by HL into the DE register | ||||||
| DB99 | INC HL | |||||||
| DB9A | LD D,(HL) | |||||||
| DB9B | EX DE,HL | Swap back so HL now points to the graphics data, e.g. 9950 | ||||||
| DB9C | LD A,$00 | This is the flag indicating whether (any part of) the graphic is visible on-screen (0 = not visible, 1 = visible) It'll be updated shortly once we've worked out if any part of the graphic is on-screen or not |
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| DB9E | LD ($EAD5),A | This is set to not visible (0) for off-screen objects, but if any part of the item needs drawing, it's set to 1 (visible) at DBF5 | ||||||
| DBA1 | LD A,(HL) | Number of tiles that make up the graphic/sprite | ||||||
| DBA2 | LD ($EAD0),A | ...and store... | ||||||
| DBA5 | INC HL | HL now at the start of the graphic's first tile data | ||||||
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This entry point is used by the routine at DC77.
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| DBA6 | LD C,(HL) | Store this byte in C register. This is the horizontal pixel offset for drawing this tile from its screen X/Y co-ordinates | ||||||
| DBA7 | INC HL | Move onto the next byte | ||||||
| DBA8 | EX DE,HL | Swap the graphics address pointer back into the DE register | ||||||
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Calculate horizontal (X) pixel offset, check for out of bounds
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| DBA9 | LD HL,($EAD3) | Get graphic X horizontal (pixel) position from character column 1 on the left of the screen (the right hand part of the left border). | ||||||
| DBAC | LD A,C | Graphic's horizontal offset in pixels | ||||||
| DBAD | RLA | If the offset is >=128, it's a negative pixel offset (e.g. 253 = -3 pixels) | ||||||
| DBAE | SBC A,A | ...So if the RLA creates a carry, the SBC will set A to to 255 (-1). Otherwise 0. | ||||||
| DBAF | LD B,A | Transfer to B. This determines if the offset value gets subtracted or added in the next instruction. | ||||||
| DBB0 | ADD HL,BC | Add horizontal pixel offset to horizontal pixel drawing position | ||||||
| DBB1 | EX DE,HL | Swap into DE | ||||||
| DBB2 | LD A,D | Get the high byte of the horizontal position as just calculated | ||||||
| DBB3 | CP $00 | If high byte changed from 0 to 1, the graphic has gone out of the screen's viewport boundary | ||||||
| DBB5 | JP NZ,$DC77 | ...if so, skip out of this routine to deal with the out-of-bounds graphic | ||||||
| DBB8 | LD A,E | Get horizontal pixel position | ||||||
| DBB9 | CP $E8 | Check to see if this is in the right hand border screen position - 232/8 = 29, but as the counting starts at pixel column 8 (character column 1, rather than 0), this will be character column 30 in the right hand border | ||||||
| DBBB | JP NC,$DC77 | If there's no carry, jump out of this routine to deal with the out-of-bounds graphic | ||||||
| DBBE | ADD A,$08 | The screen display address is calculated (in the next routine) from the leftmost screen column - i.e. column 0, not column 1 as currently stored - so add 8 so that this is taken into account. | ||||||
| DBC0 | LD E,A | Store in E register ready for the next part of the routine at DBF5 | ||||||
| DBC1 | AND $07 | Take lowest 3 bits (values 0-7) | ||||||
| DBC3 | LD B,A | Store in B register - this is the number of pixel shifts needed for the graphic at DC32 | ||||||
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Calculate vertical (Y) pixel offset, check for out of bounds
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| DBC4 | LD A,($EAD2) | Get graphic Y vertical (pixel) position, e.g. 80 for character row 10 (80/8) | ||||||
| DBC7 | ADD A,(HL) | Add to the graphic vertical offset, e.g. an offset of 231 means a -25 vertical pixel offset to the existing co-ordinate | ||||||
| DBC8 | INC HL | Move HL to the next pre-graphics byte address | ||||||
| DBC9 | LD C,(HL) | Number of bytes in the next tile graphic | ||||||
| DBCA | INC HL | HL now points to first graphic tile byte ready for drawing | ||||||
| DBCB | CP $70 | Vertical pixel position 112 is the top of the scroll, which is out of bounds for drawing graphics | ||||||
| DBCD | JP C,$DBEA | Carry means (at least the top part of) the item is within the screen viewport and drawable, so jump to DBEA | ||||||
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A no-carry indicates the vertical-pixel-position-plus-offset is either above the top of the playing area or into the scroll area.
However, some part of the item may be visible; for example the vertical corner floor/wall connectors can be partly off the top of the playing area, but the rest of them may be visible on-screen.
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| DBD0 | LD D,A | Store vertical (Y) pixel position | ||||||
| DBD1 | ADD A,C | Add the number of bytes in the tile to the current out-of-bounds position | ||||||
| DBD2 | JP NC,$DC7A | |||||||
| DBD5 | JP Z,$DC7A | A no-carry or zero result means that no part of the object is visible, so this graphic doesn't need drawing. Jump out here to re-calculate address pointers etc. | ||||||
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Some (lower) part of this graphic is visible at the top of the screen. Put the graphics address pointer to the right part of the graphics and set the pixel row number in the line.
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| DBD8 | LD ($EAD1),A | Store the number of bytes in the visible part of the graphic tile left to draw | ||||||
| DBDB | LD A,D | Retrieve the vertical pixel co-ordinate (plus offset). This will be a negative number... | ||||||
| DBDC | NEG | So make it positive... | ||||||
| DBDE | ADD A,L | HL is pointing at the address for the tile graphics... | ||||||
| DBDF | LD L,A | |||||||
| DBE0 | LD A,$00 | |||||||
| DBE2 | ADC A,H | ...So increase it by the number of bytes that don't need to be drawn as they're in/above the decorative border | ||||||
| DBE3 | LD H,A | |||||||
| DBE4 | LD D,$10 | Set the vertical screen position to 16 pixel rows down - the first byte of character row 2, just below the top border | ||||||
| DBE6 | LD C,$00 | C register contains the pixel row; as we're starting from character row 2 means that the first byte will be pixel row 0 | ||||||
| DBE8 | JR $DBF5 | Ready to calculate screen address low/high bytes | ||||||
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Jump from DBCD. The graphic (or at least the top of it) are drawable. Set the screen vertical pixel position and pixel row in the line (0-7).
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| DBEA | ADD A,$10 | Vertical pixel position starts from 0 (16 pixels from top of screen... | ||||||
| DBEC | LD D,A | D contains vertical Y screen pixel position | ||||||
| DBED | LD A,C | Get number of tile bytes to draw | ||||||
| DBEE | LD ($EAD1),A | ...and store... | ||||||
| DBF1 | LD A,D | Get A back from D (mystery value - now 71) | ||||||
| DBF2 | AND $07 | Take lowest 3 bits (0,1,2) - pixel row number in line | ||||||
| DBF4 | LD C,A | Store in C register for the next part of the routine | ||||||
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