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Routines |
Prev: 607B | Up: Map |
Used by the routine at 64DC.
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609B | EXX | |||||||
609C | LD HL,$5801 | Point HL' and DE' at the leftmost end of the top row of attribute bytes on the screen | ||||||
609F | LD DE,$5800 | |||||||
60A2 | LD BC,$001F | We need to scroll 31 attribute bytes at a time | ||||||
60A5 | EXX | |||||||
60A6 | LD HL,$4001 | Point HL and DE at the leftmost end of the top row of pixels on the screen | ||||||
60A9 | LD DE,$4000 | |||||||
60AC | LD BC,$001F | We need to scroll 31 display file bytes at a time | ||||||
60AF | LD A,$15 | There are 21 rows of character squares to scroll | ||||||
60B1 | EX AF,AF' | |||||||
60B2 | LD A,$08 | Each row of character squares contains 8 rows of pixels | ||||||
60B4 | PUSH HL | |||||||
60B5 | PUSH DE | |||||||
60B6 | LDIR | Scroll a row of pixels left one column | ||||||
60B8 | POP DE | |||||||
60B9 | POP HL | |||||||
60BA | INC D | Point HL and DE at the next row of pixels | ||||||
60BB | INC H | |||||||
60BC | LD C,$1F | BC=001F | ||||||
60BE | DEC A | Next row in this block of 8x32 | ||||||
60BF | JR NZ,$60B4 | Jump back until this 8x32 pixel block has been scrolled | ||||||
60C1 | EXX | |||||||
60C2 | LDIR | Scroll a row of attribute bytes left one column | ||||||
60C4 | INC HL | Point HL' and DE' at the leftmost end of the next row of attribute bytes on the screen | ||||||
60C5 | INC DE | |||||||
60C6 | LD C,$1F | BC'=001F | ||||||
60C8 | EXX | |||||||
60C9 | LD A,$20 | Point HL and DE at the leftmost end of the top row of the next 8x32 block of pixels | ||||||
60CB | ADD A,E | |||||||
60CC | LD E,A | |||||||
60CD | LD A,$20 | |||||||
60CF | ADD A,L | |||||||
60D0 | LD L,A | |||||||
60D1 | EX AF,AF' | |||||||
60D2 | DEC A | Next row of character squares | ||||||
60D3 | JR Z,$60E5 | Exit this loop if all 21 rows have been scrolled left | ||||||
60D5 | CP $05 | Have we reached the bottom third of the screen? | ||||||
60D7 | JR Z,$60B1 | Jump if so | ||||||
60D9 | CP $0D | Have we reached the middle third of the screen? | ||||||
60DB | JR Z,$60B1 | Jump if so | ||||||
60DD | EX AF,AF' | |||||||
60DE | LD A,H | Point HL and DE at the leftmost end of the top row of the next 8x32 block of pixels | ||||||
60DF | SUB $08 | |||||||
60E1 | LD H,A | |||||||
60E2 | LD D,A | |||||||
60E3 | JR $60B2 | Jump back to deal with the next row of character squares | ||||||
The 21 rows of character squares have been scrolled one place to the left. Now the rightmost column needs to be filled.
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60E5 | EXX | |||||||
60E6 | LD HL,$581F | Point HL' and DE' at the attribute file and display file addresses for the top right of the screen, and set BC'=0020; however, these values are not used | ||||||
60E9 | LD DE,$401F | |||||||
60EC | INC C | |||||||
60ED | EXX | |||||||
60EE | LD A,($7F00) | A=leftmost column of the skool on-screen (0-64) | ||||||
60F1 | INC A | We are scrolling one column left | ||||||
60F2 | LD ($7F00),A | |||||||
60F5 | ADD A,C | A=rightmost column of the skool on-screen (31-95) | ||||||
60F6 | JP $605E | Fill in the rightmost column |
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