Routines
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817C: Launch the player's weapon
Used by the routines at KNIGHT_FIRE, WIZARD_FIRE and SERF_FIRE.
Sets the weapon's velocity from the direction the player is facing: +4, -4 or nothing on each axis, taken from the sign of +$06 and +$07. A shot therefore travels four pixels a frame along whichever axes the player was moving on, and a diagonal shot moves on both.
Confirmed live -- firing and reading the weapon's velocity field back gives exactly $04 and $FC, and the shot's x walks across the room four pixels at a time.
Input
IX The player
FIRE_WEAPON 817C LD HL,$EA9E The weapon's velocity field, +$0E of the player's record.
817F LD A,$30 A lifetime of 48 frames, in +$0F of the weapon's record (WEAPON_LIFE, which lies in the sound slot's unused tail).
8181 LD ($EAA7),A
8184 LD A,$00 And clear its contact flag.
8186 LD ($EA9A),A
8189 LD A,(IX+$06) Standing still: nothing to fire along.
818C OR (IX+$07)
818F JR Z,FIRE_WEAPON_4
8191 LD A,(IX+$06) Which way on this axis?
8194 AND A
8195 JR Z,FIRE_WEAPON_1
8197 JP M,FIRE_WEAPON_0
819A LD A,$04 Right or down.
819C JR FIRE_WEAPON_1
FIRE_WEAPON_0 819E LD A,$FC Left or up: $FC is minus four.
FIRE_WEAPON_1 81A0 LD (HL),A Store it and do the other axis the same way.
81A1 INC HL
81A2 LD A,(IX+$07)
81A5 AND A
81A6 JR Z,FIRE_WEAPON_3
81A8 JP M,FIRE_WEAPON_2
81AB LD A,$04
81AD JR FIRE_WEAPON_3
FIRE_WEAPON_2 81AF LD A,$FC
FIRE_WEAPON_3 81B1 LD (HL),A
81B2 RET
FIRE_WEAPON_4 81B3 LD A,(IX+$00)
81B6 DEC A
81B7 AND $0C
81B9 JR Z,FIRE_WEAPON_5
81BB CP $04
81BD JR Z,FIRE_WEAPON_6
81BF CP $08
81C1 JR NZ,FIRE_WEAPON_7
81C3 LD (HL),$00
81C5 INC HL
81C6 LD (HL),$FC
81C8 RET
FIRE_WEAPON_5 81C9 LD (HL),$FC
81CB INC HL
81CC LD (HL),$00
81CE RET
FIRE_WEAPON_6 81CF LD (HL),$04
81D1 INC HL
81D2 LD (HL),$00
81D4 RET
FIRE_WEAPON_7 81D5 LD (HL),$00
81D7 INC HL
81D8 LD (HL),$04
81DA RET
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