Routines
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876B: Scroll Line
Used by the routine at PrintWriteACharacterToTheScreen.
Scrolls the text window up by one line.
ScrollLine 876B PUSH AF Save AF, BC, HL and DE, and point HL at screen line 4020.
876C PUSH BC
876D PUSH HL
876E PUSH DE
Set up the initial values.
876F LD HL,$4020
8772 LD DE,$4000 Point DE at the display file.
8775 LD A,$11 Load 11 for the scroll.
8777 LD B,$00 Start the pass count at zero.
ScrollLine_Loop 8779 PUSH HL Save the screen pointers and count 08 pixel rows.
877A PUSH DE
877B LD C,$08
ScrollLine_Line 877D PUSH HL Save the row addresses and the pixel count, and set the byte count to 20.
877E PUSH DE
877F PUSH BC
8780 LD C,$20
8782 LDIR Copy the row up one.
8784 POP BC Increment the MSB of HL and DE by one to point to the next line down.
8785 POP DE
8786 POP HL
8787 INC H
8788 INC D
8789 DEC C Drop one pixel row.
878A JR NZ,ScrollLine_Line Jump back to ScrollLine_Line unless the whole line is moved (i.e. C is now zero).
878C POP DE Restore the saved screen pointers from the stack and set BC to 20, one screen row.
878D POP HL
878E LD C,$20
8790 ADD HL,BC Step one screen row, 20 bytes.
8791 EX DE,HL Swap the two screen pointers.
8792 ADD HL,BC Step the other pointer one screen row, 20 bytes.
8793 EX DE,HL Swap the two screen pointers.
8794 PUSH AF Save AF and use D as the next byte.
8795 LD A,D
8796 AND %00000111 Keep the selected bits.
8798 JR Z,ScrollLine_SkipDE Skip to ScrollLine_SkipDE if zero.
Handle changing the target between each screen buffer area.
879A LD A,D Take the high byte of DE.
879B ADD A,$07 Add 07 onto it.
879D LD D,A Store the row address past the screen-third gap.
ScrollLine_SkipDE 879E LD A,H Take the high byte of the screen address.
879F AND %00000111 Keep the selected bits.
87A1 JR Z,ScrollLine_SkipHL Skip to ScrollLine_SkipHL if zero.
Handle changing the source between each screen buffer area.
87A3 LD A,H Take the high byte of the screen address.
87A4 ADD A,$07 Add 07 onto it.
87A6 LD H,A Store the other row address past the screen-third gap.
ScrollLine_SkipHL 87A7 POP AF Restore the line counter and drop one line.
87A8 DEC A
87A9 JR NZ,ScrollLine_Loop Loop back to ScrollLine_Loop until the line counter is zero.
Handle the attributes.
87AB LD HL,$5820 Move the attributes up one character block.
87AE LD DE,$5800
87B1 LD BC,$0220
87B4 LDIR
This creates the indent.
87B6 LD B,$2A Set a counter for $2A spaces.
87B8 LD HL,$5020 Set the target screen location to $5020.
87BB LD C,$01 One column of the scrolled line.
87BD LD A,$20 Use the character ' '.
ScrollLine_Spacing 87BF CALL Print_ProportionalCharacter Call Print_ProportionalCharacter to draw this character with the game's font.
87C2 DJNZ ScrollLine_Spacing Decrease counter by one and loop back to ScrollLine_Spacing until counter is zero.
Housekeeping and return.
87C4 POP DE Restore DE, HL, BC and AF from the stack.
87C5 POP HL
87C6 POP BC
87C7 POP AF
87C8 RET Return.
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