Spectrum ROM | Routines |
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The routine at CL_SC_ALL continues here.
This subroutine will clear the bottom B lines of the display.
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CL_LINE | 03652 | PUSH BC | The line number is saved for the duration of the subroutine. | ||||
03653 | CALL CL_ADDR | The starting address for the line is formed in HL. | |||||
03656 | LD C,8 | Again there are eight pixel lines to be considered. | |||||
Now enter a loop to clear all the pixel lines.
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CL_LINE_1 | 03658 | PUSH BC | Save the line number and the pixel line counter. | ||||
03659 | PUSH HL | Save the address. | |||||
03660 | LD A,B | Save the line number in A. | |||||
CL_LINE_2 | 03661 | AND 7 | Find how many characters are involved in 'B mod 8' lines. Pass the result to the C register. (C will hold +00 i.e. 256 dec. for a 'third'.) | ||||
03663 | RRCA | ||||||
03664 | RRCA | ||||||
03665 | RRCA | ||||||
03666 | LD C,A | ||||||
03667 | LD A,B | Fetch the line number. | |||||
03668 | LD B,0 | Make the BC register pair hold one less than the number of characters. | |||||
03670 | DEC C | ||||||
03671 | LD D,H | Make DE point to the first character. | |||||
03672 | LD E,L | ||||||
03673 | LD (HL),0 | Clear the pixel-byte of the first character. | |||||
03675 | INC DE | Make DE point to the second character and then clear the pixel-bytes of all the other characters. | |||||
03676 | LDIR | ||||||
03678 | LD DE,1793 | For each 'third' of the display HL has to be increased by +0701. | |||||
03681 | ADD HL,DE | ||||||
03682 | DEC A | Now decrease the line number. | |||||
03683 | AND 248 | Discard any extra lines and pass the 'third' count to B. | |||||
03685 | LD B,A | ||||||
03686 | JR NZ,CL_LINE_2 | Jump back if there are still 'thirds' to be dealt with. | |||||
Now find if the loop has been used eight times.
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03688 | POP HL | Update the address for each pixel line. | |||||
03689 | INC H | ||||||
03690 | POP BC | Fetch the counters. | |||||
03691 | DEC C | Decrease the pixel line counter and jump back unless finished. | |||||
03692 | JR NZ,CL_LINE_1 | ||||||
03694 | CALL CL_ATTR | The address of the first attribute byte and the number of bytes are found. | |||||
03697 | LD H,D | HL will point to the first attribute byte and DE the second. | |||||
03698 | LD L,E | ||||||
03699 | INC DE | ||||||
03700 | LD A,(23693) | Fetch the value in ATTR-P. | |||||
03703 | BIT 0,(IY+2) | Jump forward if handling the main part of the screen. | |||||
03707 | JR Z,CL_LINE_3 | ||||||
03709 | LD A,(23624) | Otherwise use BORDCR instead. | |||||
CL_LINE_3 | 03712 | LD (HL),A | Set the attribute byte. | ||||
03713 | DEC BC | One byte has been done. | |||||
03714 | LDIR | Now copy the value to all the attribute bytes. | |||||
03716 | POP BC | Restore the line number. | |||||
03717 | LD C,33 | Set the column number to the lefthand column and return. | |||||
03719 | RET |
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