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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 | 36224 | LD B,69 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 36226 | LD C,H | |||||||||
| 36227 | SLA C | |||||||||
| 36229 | RL B | |||||||||
| 36231 | LD H,70 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 36233 | SLA L | |||||||||
| 36235 | RL H | |||||||||
| 36237 | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 36238 | INC HL | |||||||||
| 36239 | LD H,(HL) | |||||||||
| 36240 | LD L,A | |||||||||
| 36241 | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 36242 | ADD A,L | |||||||||
| 36243 | LD L,A | |||||||||
| 36244 | INC BC | |||||||||
| 36245 | LD A,(BC) | Set the pixel on screen by ORing the column mask. | ||||||||
| 36246 | OR (HL) | |||||||||
| 36247 | LD (HL),A | |||||||||
| 36248 | 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 | 36249 | LD B,69 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 36251 | LD C,H | |||||||||
| 36252 | SLA C | |||||||||
| 36254 | RL B | |||||||||
| 36256 | LD H,70 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 36258 | SLA L | |||||||||
| 36260 | RL H | |||||||||
| 36262 | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 36263 | INC HL | |||||||||
| 36264 | LD H,(HL) | |||||||||
| 36265 | LD L,A | |||||||||
| 36266 | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 36267 | ADD A,L | |||||||||
| 36268 | LD L,A | |||||||||
| 36269 | INC BC | |||||||||
| 36270 | LD A,(BC) | Invert the column mask and clear the matching bits on screen. | ||||||||
| 36271 | CPL | |||||||||
| 36272 | AND (HL) | |||||||||
| 36273 | LD (HL),A | |||||||||
| 36274 | 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 | 36275 | LD B,69 | Compute the column entry address in Column_Pixel_Lookup_Table. | |||||||
| 36277 | LD C,H | |||||||||
| 36278 | SLA C | |||||||||
| 36280 | RL B | |||||||||
| 36282 | LD H,70 | Compute the row entry address in Screen_Row_Address_Lookup_Table. | ||||||||
| 36284 | SLA L | |||||||||
| 36286 | RL H | |||||||||
| 36288 | LD A,(HL) | Read the scan-line base address from Screen_Row_Address_Lookup_Table. | ||||||||
| 36289 | INC HL | |||||||||
| 36290 | LD H,(HL) | |||||||||
| 36291 | LD L,A | |||||||||
| 36292 | LD A,(BC) | Apply the column byte offset from Column_Pixel_Lookup_Table to the scan-line address and advance to the pixel mask. | ||||||||
| 36293 | ADD A,L | |||||||||
| 36294 | LD L,A | |||||||||
| 36295 | INC BC | |||||||||
| 36296 | LD A,(BC) | Toggle the matching bits on screen using the column mask. | ||||||||
| 36297 | XOR (HL) | |||||||||
| 36298 | LD (HL),A | |||||||||
| 36299 | RET | Return. | ||||||||
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