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6390: Guide a character down a staircase
The address of this interruptible subcommand routine is placed into bytes 0x69 and 0x6A of a character's buffer by the routine at 63BE when the character has reached the top of a staircase he needs to descend to reach his destination.
H Character number (0x98-0xA9)
6390 LD B,$01 Signal that the character is going down a staircase
This entry point is used by the routine at 638C with B=0xFF.
6392 LD L,$6B Byte 0x6B of the character's buffer holds the number of movements remaining
6394 DEC (HL) Has the character finished going up or down the stairs?
6395 JP Z,$62A4 Terminate this interruptible subcommand if so
6398 PUSH BC Save the direction indicator (in B) briefly
6399 CALL $6214 Update the SRB for the character's current location
639C POP BC Restore the direction indicator to B
639D LD C,A C=character's current animatory state
639E INC A A=character's next animatory state
639F BIT 0,A Is the character midstride right now?
63A1 JR Z,$63AB Jump if so
The following four instructions calculate the character's next y-coordinate. The y-coordinate will be decremented if the character is going up the stairs and he's not midstride at the moment, incremented if he's going down the stairs and he is midstride at the moment, and otherwise unchanged.
63A3 DEC D
63A4 INC B Is the character going up the stairs?
63A5 JR Z,$63A8 Jump if so
63A7 INC D
At this point A holds the character's new animatory state, and DE his new location.
63A8 JP $61B0 Update the character's animatory state and location and update the SRB
The character is midstride. Now we calculate his next x-coordinate.
63AB RLCA Set the carry flag if the character is facing left
63AD SBC A,A A=0 (facing right) or -2 (facing left)
63AF ADD A,E Set E equal to the character's new x-coordinate
63B0 LD E,A
63B1 INC E
63B2 LD A,C A=character's current animatory state
63B3 INC A A=character's next animatory state
63B4 BIT 1,A Will the character be standing (phase 3)?
63B6 JR NZ,$63A4 Jump if so
63B8 SUB $04 Adjust the animatory state to standing (phase 1)
63BA JR $63A4
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