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Routines |
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Renders one pixel at screen position (H = pixel column, L = pixel row). Derives the column byte offset and pixel mask by indexing into Column_Pixel_Lookup_Table, and reads the screen scan-line address from Screen_Row_Address_Lookup_Table. Three entry points: PlotCharacterCell_Draw draws by setting bits, PlotCharacterCell_Erase erases by clearing bits, PlotCharacterCell_Toggle toggles bits.
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| PlotCharacterCell_Draw | 8D80 | LD B,$45 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 8D82 | LD C,H | |||||||||
| 8D83 | SLA C | |||||||||
| 8D85 | RL B | |||||||||
| 8D87 | LD H,$46 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 8D89 | SLA L | |||||||||
| 8D8B | RL H | |||||||||
| 8D8D | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 8D8E | INC HL | |||||||||
| 8D8F | LD H,(HL) | |||||||||
| 8D90 | LD L,A | |||||||||
| 8D91 | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 8D92 | ADD A,L | |||||||||
| 8D93 | LD L,A | |||||||||
| 8D94 | INC BC | |||||||||
| 8D95 | LD A,(BC) | Set the pixel on screen by ORing the column mask. | ||||||||
| 8D96 | OR (HL) | |||||||||
| 8D97 | LD (HL),A | |||||||||
| 8D98 | RET | Return. | ||||||||
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Alternate entry: erase the pixel by inverting the column mask and clearing the matching bits on screen.
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| PlotCharacterCell_Erase | 8D99 | LD B,$45 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 8D9B | LD C,H | |||||||||
| 8D9C | SLA C | |||||||||
| 8D9E | RL B | |||||||||
| 8DA0 | LD H,$46 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 8DA2 | SLA L | |||||||||
| 8DA4 | RL H | |||||||||
| 8DA6 | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 8DA7 | INC HL | |||||||||
| 8DA8 | LD H,(HL) | |||||||||
| 8DA9 | LD L,A | |||||||||
| 8DAA | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 8DAB | ADD A,L | |||||||||
| 8DAC | LD L,A | |||||||||
| 8DAD | INC BC | |||||||||
| 8DAE | LD A,(BC) | Invert the column mask and clear the matching bits on screen. | ||||||||
| 8DAF | CPL | |||||||||
| 8DB0 | AND (HL) | |||||||||
| 8DB1 | LD (HL),A | |||||||||
| 8DB2 | RET | Return. | ||||||||
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Alternate entry: toggle the pixel by flipping the matching bits using the column mask.
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| PlotCharacterCell_Toggle | 8DB3 | LD B,$45 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 8DB5 | LD C,H | |||||||||
| 8DB6 | SLA C | |||||||||
| 8DB8 | RL B | |||||||||
| 8DBA | LD H,$46 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 8DBC | SLA L | |||||||||
| 8DBE | RL H | |||||||||
| 8DC0 | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 8DC1 | INC HL | |||||||||
| 8DC2 | LD H,(HL) | |||||||||
| 8DC3 | LD L,A | |||||||||
| 8DC4 | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 8DC5 | ADD A,L | |||||||||
| 8DC6 | LD L,A | |||||||||
| 8DC7 | INC BC | |||||||||
| 8DC8 | LD A,(BC) | Toggle the matching bits on screen using the column mask. | ||||||||
| 8DC9 | XOR (HL) | |||||||||
| 8DCA | LD (HL),A | |||||||||
| 8DCB | RET | Return. | ||||||||
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