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Routines |
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Used by the routine at 64D2.
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6000 | EXX | |||||||
6001 | LD HL,$581E | Point HL' and DE' at the rightmost end of the top row of attribute bytes on the screen | ||||||
6004 | LD DE,$581F | |||||||
6007 | LD BC,$001F | We need to scroll 31 attribute bytes at a time | ||||||
600A | EXX | |||||||
600B | LD HL,$401E | Point HL and DE at the rightmost end of the top row of pixels on the screen | ||||||
600E | LD DE,$401F | |||||||
6011 | LD BC,$001F | We need to scroll 31 display file bytes at a time | ||||||
6014 | LD A,$15 | There are 21 rows of character squares to scroll | ||||||
6016 | EX AF,AF' | |||||||
6017 | LD A,$08 | Each row of character squares contains 8 rows of pixels | ||||||
6019 | PUSH HL | |||||||
601A | PUSH DE | |||||||
601B | LDDR | Scroll a row of pixels right one column | ||||||
601D | POP DE | |||||||
601E | POP HL | |||||||
601F | LD C,$1F | BC=001F | ||||||
6021 | INC H | Point HL and DE at the next row of pixels | ||||||
6022 | INC D | |||||||
6023 | DEC A | Next row in this block of 8x32 | ||||||
6024 | JR NZ,$6019 | Jump back until this 8x32 pixel block has been scrolled | ||||||
6026 | EXX | |||||||
6027 | LDDR | Scroll a row of attribute bytes right one column | ||||||
6029 | LD C,$3F | Point HL' and DE' at the rightmost end of the next row of attribute bytes on the screen | ||||||
602B | ADD HL,BC | |||||||
602C | EX DE,HL | |||||||
602D | ADD HL,BC | |||||||
602E | EX DE,HL | |||||||
602F | LD C,$1F | BC'=001F | ||||||
6031 | EXX | |||||||
6032 | LD A,$20 | Point HL and DE at the rightmost end of the top row of the next 8x32 block of pixels | ||||||
6034 | ADD A,E | |||||||
6035 | LD E,A | |||||||
6036 | LD A,$20 | |||||||
6038 | ADD A,L | |||||||
6039 | LD L,A | |||||||
603A | EX AF,AF' | |||||||
603B | DEC A | Next row of character squares | ||||||
603C | JR Z,$604E | Exit this loop if all 21 rows have been scrolled right | ||||||
603E | CP $0D | Have we reached the middle third of the screen? | ||||||
6040 | JR Z,$6016 | Jump if so | ||||||
6042 | CP $05 | Have we reached the bottom third of the screen? | ||||||
6044 | JR Z,$6016 | Jump if so | ||||||
6046 | EX AF,AF' | |||||||
6047 | LD A,H | Point HL and DE at the rightmost end of the top row of the next 8x32 block of pixels | ||||||
6048 | SUB $08 | |||||||
604A | LD H,A | |||||||
604B | LD D,A | |||||||
604C | JR $6017 | Jump back to deal with the next row of character squares | ||||||
The 21 rows of character squares have been scrolled one place to the right. Now the leftmost column needs to be filled.
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604E | EXX | |||||||
604F | LD HL,$5800 | Point HL' and DE' at the attribute file and display file addresses for the top left of the screen, and set BC'=0020; however, these values are not used | ||||||
6052 | LD DE,$4000 | |||||||
6055 | INC C | |||||||
6056 | EXX | |||||||
6057 | LD A,($7F00) | A=leftmost column of the skool on-screen (0-64) | ||||||
605A | DEC A | We are scrolling one column right | ||||||
605B | LD ($7F00),A | |||||||
This entry point is used by the routine at 609B with A holding the x-coordinate of the rightmost column of the skool on-screen (31-95).
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605E | LD E,A | E=X (x-coordinate of the far left or far right column of the skool currently on-screen) | ||||||
605F | LD D,$98 | D=Y (this is the y-coordinate of the top row of the screen) | ||||||
6061 | LD B,$15 | There are 21 rows on the screen | ||||||
6063 | PUSH DE | |||||||
6064 | PUSH BC | |||||||
6065 | CALL $610B | Print the character square at (X,Y) | ||||||
6068 | POP BC | |||||||
6069 | POP DE | |||||||
606A | LD A,H | Point HL at the attribute file address corresponding to the skool coordinates (X,Y) | ||||||
606B | RRCA | |||||||
606C | RRCA | |||||||
606D | RRCA | |||||||
606E | ADD A,$4F | |||||||
6070 | LD H,A | |||||||
6071 | SET 7,E | Point DE at the attribute byte for the skool coordinates (X,Y) | ||||||
6073 | LD A,(DE) | Pick the attribute byte up in A | ||||||
6074 | RES 7,E | Restore the skool coordinates (X,Y) to DE | ||||||
6076 | LD (HL),A | Poke the attribute byte onto the screen | ||||||
6077 | INC D | Next row (Y) of the screen | ||||||
6078 | DJNZ $6063 | Jump back until all 21 rows are done | ||||||
607A | RET |
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