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 | 0E44 | PUSH BC | The line number is saved for the duration of the subroutine. | |||||
0E45 | CALL CL_ADDR | The starting address for the line is formed in HL. | ||||||
0E48 | LD C,$08 | 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 | 0E4A | PUSH BC | Save the line number and the pixel line counter. | |||||
0E4B | PUSH HL | Save the address. | ||||||
0E4C | LD A,B | Save the line number in A. | ||||||
CL_LINE_2 | 0E4D | AND $07 | 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, for a 'third'.) | |||||
0E4F | RRCA | |||||||
0E50 | RRCA | |||||||
0E51 | RRCA | |||||||
0E52 | LD C,A | |||||||
0E53 | LD A,B | Fetch the line number. | ||||||
0E54 | LD B,$00 | Make the BC register pair hold one less than the number of characters. | ||||||
0E56 | DEC C | |||||||
0E57 | LD D,H | Make DE point to the first character. | ||||||
0E58 | LD E,L | |||||||
0E59 | LD (HL),$00 | Clear the pixel-byte of the first character. | ||||||
0E5B | INC DE | Make DE point to the second character and then clear the pixel-bytes of all the other characters. | ||||||
0E5C | LDIR | |||||||
0E5E | LD DE,$0701 | For each 'third' of the display HL has to be increased by +0701. | ||||||
0E61 | ADD HL,DE | |||||||
0E62 | DEC A | Now decrease the line number. | ||||||
0E63 | AND $F8 | Discard any extra lines and pass the 'third' count to B. | ||||||
0E65 | LD B,A | |||||||
0E66 | 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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0E68 | POP HL | Update the address for each pixel line. | ||||||
0E69 | INC H | |||||||
0E6A | POP BC | Fetch the counters. | ||||||
0E6B | DEC C | Decrease the pixel line counter and jump back unless finished. | ||||||
0E6C | JR NZ,CL_LINE_1 | |||||||
0E6E | CALL CL_ATTR | The address of the first attribute byte and the number of bytes are found. | ||||||
0E71 | LD H,D | HL will point to the first attribute byte and DE the second. | ||||||
0E72 | LD L,E | |||||||
0E73 | INC DE | |||||||
0E74 | LD A,($5C8D) | Fetch the value in ATTR-P. | ||||||
0E77 | BIT 0,(IY+$02) | Jump forward if handling the main part of the screen (bit 0 of TV-FLAG reset). | ||||||
0E7B | JR Z,CL_LINE_3 | |||||||
0E7D | LD A,($5C48) | Otherwise use BORDCR instead. | ||||||
CL_LINE_3 | 0E80 | LD (HL),A | Set the attribute byte. | |||||
0E81 | DEC BC | One byte has been done. | ||||||
0E82 | LDIR | Now copy the value to all the attribute bytes. | ||||||
0E84 | POP BC | Restore the line number. | ||||||
0E85 | LD C,$21 | Set the column number to the lefthand column and return. | ||||||
0E87 | RET |
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