|
Routines |
| Prev: 38356 | Up: Map | Next: 38486 |
|
We write zeroes to the display file in order to blank out a region, for example the interior of a window. The top-left and bottom-right coordinates of the area to be blanked by this routine are stored at 23493 - 23496).
Used by the routine at 36987.
|
||||
| 38417 | LD BC,(23493) | Load B with y-coordinate of top edge and C with x-coordinate of left edge | ||
| 38421 | LD HL,(36488) | Load HL with current virtual text cursor location... | ||
| 38424 | PUSH HL | ...and store on the stack | ||
| 38425 | CALL 36551 | Set virtual cursor location (bitmap area) | ||
| 38428 | LD A,(23495) | Load A with x-coordinate of right-edge | ||
| 38431 | SUB C | Subtract x-coordinate of left-edge... | ||
| 38432 | INC A | ...and add one to obtain width | ||
| 38433 | LD D,A | Store width in D | ||
| 38434 | LD A,(23496) | Load A with y-coordinate of bottom | ||
| 38437 | SUB B | Subtract y-coordinate of top-edge... | ||
| 38438 | INC A | ...and add one to obtain height | ||
| 38439 | LD E,A | Store height in E | ||
| 38440 | PUSH DE | Copy width and height... | ||
| 38441 | POP BC | ...into BC | ||
| 38442 | PUSH HL | Store (initial) HL on stack | ||
| 38443 | LD A,B | Copy width into A | ||
| 38444 | EX AF,AF' | Exchange AF register (preserve width) | ||
| 38445 | PUSH BC | Store width and height on stack | ||
| 38446 | PUSH HL | Store (current) HL on stack | ||
| 38447 | XOR A | Set A to zero | ||
| 38448 | LD B,8 | Set B to 8 | ||
| 38450 | LD (HL),A | Write zero to current display file address | ||
| 38451 | INC H | Move down one pixel row (by adding 256 bytes to current display file address) | ||
| 38452 | DJNZ 38450 | Repeat for other 7 pixel rows | ||
| 38454 | POP HL | Recover previous (current) value of HL (start of previous character block)... | ||
| 38455 | INC HL | ...and increase to obtain next block | ||
| 38456 | POP BC | Restore width and height... | ||
| 38457 | DJNZ 38445 | ...reduce width by one and loop back to clear next block (until remaining width = 0) | ||
| 38459 | POP HL | Recover (initial) value of HL | ||
| 38460 | DEC C | Reduce remaining height by one... | ||
| 38461 | JR Z,38481 | ...and if it is now zero, then exit loop... | ||
| 38463 | LD A,32 | ...else... | ||
| 38465 | ADD A,L | ...add 32 (screen width) to current display file pointer to move down to next character row... | ||
| 38466 | LD L,A | ... | ||
|
Here we check whether moving down a character row is valid, i.e. that we're not moving down one row from the bottom row of one of the three thirds of the bitmap area, which would actually wrap back up to the second pixel row of the top character row of the current third.
|
||||
| 38467 | JP NC,38474 | If moving down a character row is valid then skip ahead to 38474 | ||
| 38470 | LD A,8 | We need to move down from the bottom character row of one third of the screen to the top character row of the third below so... | ||
| 38472 | ADD A,H | ...increase HL accordingly... | ||
| 38473 | LD H,A | ... | ||
| 38474 | EX AF,AF' | Swap AF register | ||
| 38475 | LD B,A | Recover width (stored earlier, at instr. 38444) into B | ||
| 38476 | EX AF,AF' | Preserve width again | ||
| 38477 | PUSH HL | Store current display file address on stack... | ||
| 38478 | JP 38445 | ...and loop back to 38445 to write another row of zeroes | ||
| 38481 | POP HL | Restore virtual bitmap cursor location... | ||
| 38482 | LD (36488),HL | ...and store | ||
| 38485 | RET | Return | ||
| Prev: 38356 | Up: Map | Next: 38486 |