Routines
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6A8C: Save the area of the screen that will be overwritten by a message box
Used by the routines at 7414, 74C8 and F4CC. Returns with the carry flag set if the message box would be off-screen. Otherwise copies the area of the screen that will be overwritten by the message box into the buffer at E328, and returns with the attribute file address for the top-left corner of the message box in DE.
Input
DE Coordinates of the point above the character's head
6A8C LD A,($7FFF) B=leftmost column of the play area on screen
6A8F LD B,A
6A90 LD A,E A=x-coordinate of the character's head
6A91 AND $F8 A=x-coordinate of the left edge of the message box
6A93 SUB B Return with the carry flag set if the message box would be off-screen to the right
6A94 CP $20
6A96 CCF
6A97 RET C
6A98 LD E,A E=0, 8, 16 or 24: screen x-coordinate of the left edge of the message box
6A99 LD A,D A=y-coordinate of the point above the character's head
6A9A SUB $03 A=y-coordinate of the top line of the message box
6A9C RET C Return with the carry flag set if this would be off the top of the screen
The message box is going to be printed. Prepare for that by saving the area of the screen that will be overwritten.
6A9D LD D,$00 DE=0, 8, 16 or 24
6A9F PUSH HL Save HL
6AA0 ADD A,A Set HL the to the attribute file address for the top left corner of the message box
6AA1 ADD A,A
6AA2 ADD A,A
6AA3 LD L,A
6AA4 LD H,$16
6AA6 ADD HL,HL
6AA7 ADD HL,HL
6AA8 ADD HL,DE
6AA9 PUSH HL Save this base attribute file address
6AAA LD DE,$E328 Point DE at the buffer used to save the screen area overwritten by the message box (at E328)
6AAD LD A,$03 Copy three rows of eight attribute bytes each from the screen into the first 24 bytes of the buffer
6AAF LD BC,$0008
6AB2 LDIR
6AB4 LD C,$18
6AB6 ADD HL,BC
6AB7 DEC A
6AB8 JR NZ,$6AAF
6ABA POP HL Restore the base attribute file address to HL
6ABB PUSH HL And save it again
6ABC LD A,H Set HL to the display file address for the top left corner of the message box
6ABD SUB $50
6ABF ADD A,A
6AC0 ADD A,A
6AC1 ADD A,A
6AC2 LD H,A
6AC3 LD C,$03 There are 3 rows of UDGs to copy
6AC5 LD A,$08 8 pixel rows per UDG
6AC7 PUSH BC
6AC8 PUSH HL
6AC9 LD C,$08 8 UDGs per row
6ACB LDIR Copy one row of pixels into the buffer
6ACD POP HL
6ACE INC H Move HL to the next row of pixels on screen
6ACF DEC A
6AD0 JR NZ,$6AC8 Jump back until all 8 pixel rows for this row of UDGs have been copied into the buffer
6AD2 LD A,L Move HL to the display file address for the start of the top row of pixels for the next row of UDGs
6AD3 ADD A,$20
6AD5 LD L,A
6AD6 JR C,$6ADC
6AD8 LD A,H
6AD9 SUB $08
6ADB LD H,A
6ADC POP BC
6ADD DEC C One more row of UDGs copied
6ADE JR NZ,$6AC5 Jump back until all 3 rows have been done
6AE0 POP DE Drop the base attribute file address into DE
6AE1 POP HL Restore the original contents of HL
6AE2 RET Return with the carry flag reset
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