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A C64 Game - Step 47

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Endurion

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Poor Sam got nerfed. Now his force range starts out really low (about 5 characters), but can be improved by picking the proper extra. And finally cheat keys appear. Press 1 to advance to the next stage.

step47.png

We modify the existing SamUseForce routine to check for a max range (stored in PLAYER_FORCE_RANGE):


;------------------------------------------------------------
;sam uses power
;returns 1 when holding an enemy
;------------------------------------------------------------
!zone SamUseForce
SamUseForce
lda SPRITE_HELD
beq .NoSpriteHeldNow

lda #1
rts

.NoSpriteHeldNow
stx PARAM6

ldy SPRITE_CHAR_POS_Y,x
dey
lda SCREEN_LINE_OFFSET_TABLE_LO,y
sta ZEROPAGE_POINTER_1
lda SCREEN_BACK_LINE_OFFSET_TABLE_HI,y
sta ZEROPAGE_POINTER_1 + 1

ldy SPRITE_CHAR_POS_X,x
lda #0
sta PARAM7

.ShotContinue
lda PARAM7
cmp PLAYER_FORCE_RANGE
beq .OutOfRange
inc PARAM7
;y contains shot X pos
;PARAM6 contains x sprite index of player
ldx PARAM6
lda SPRITE_DIRECTION,x
beq .ShootRight
;shooting left
dey

lda (ZEROPAGE_POINTER_1),y
jsr IsCharBlocking
beq .CheckHitEnemy
ldx PARAM6
.ShotDoneMiss
.OutOfRange
lda #0
.ShotDoneHit
rts

.ShootRight
iny

lda (ZEROPAGE_POINTER_1),y
jsr IsCharBlocking
bne .ShotDoneMiss

.CheckHitEnemy
;hit an enemy?
ldx #0

.CheckEnemy
stx PARAM2
sty PARAM1
lda SPRITE_ACTIVE,x
beq .CheckNextEnemy
tax
lda IS_TYPE_ENEMY,x
beq .CheckNextEnemy
ldx PARAM2
;is vulnerable?
lda SPRITE_STATE,x
cmp #128
bpl .CheckNextEnemy
;sprite pos matches on x?
lda SPRITE_CHAR_POS_X,x
cmp PARAM1
bne .CheckNextEnemy

;sprite pos matches on y?
ldy PARAM6
lda SPRITE_CHAR_POS_Y,x
cmp SPRITE_CHAR_POS_Y,y
beq .EnemyHit
;sprite pos matches on y + 1?
clc
adc #1
cmp SPRITE_CHAR_POS_Y,y
beq .EnemyHit
;sprite pos matches on y - 1?
sec
sbc #2
cmp SPRITE_CHAR_POS_Y,y
bne .CheckNextEnemy

.EnemyHit
;enemy hit!
stx SPRITE_HELD
inc SPRITE_HELD

;call enemy hit behaviour
ldy SPRITE_ACTIVE,x
;enemy is active
dey
dey
lda ENEMY_HIT_BEHAVIOUR_TABLE_LO,y
sta ZEROPAGE_POINTER_1
lda ENEMY_HIT_BEHAVIOUR_TABLE_HI,y
sta ZEROPAGE_POINTER_1 + 1

;set up return address for rts
lda #>( .ShotDoneHit - 1 )
pha
lda #<( .ShotDoneHit - 1 )
pha

;1 as return value
lda #1
jmp (ZEROPAGE_POINTER_1)

.CheckNextEnemy
ldx PARAM2
ldy PARAM1
inx
cpx #8
beq .NoEnemyHit

jmp .CheckEnemy
.NoEnemyHit
jmp .ShotContinue



If the extra is picked up the range is simply increased to a max of 38 (Remember, screen width is 40 characters):


.EffectIncForceRange
cpx #0
beq .DeanDoesNotUseForce

lda PLAYER_FORCE_RANGE
clc
adc #2
sta PLAYER_FORCE_RANGE

cmp #38
bcs .NotTooLong

lda #38
sta PLAYER_FORCE_RANGE

.NotTooLong
jmp .RemoveItem



Adding cheat keys is way easier. Since the Kernal (yes, with an 'a') comes with a keyboard check routine we just call that. Notice that for this to work you must not have the Kernal disabled (remember the memory layout in the beginning):


JSR $FFE4 ;GETIN
BEQ .NOCHEAT
CMP #49
bne .NOCHEAT

;jump to next level
jsr StartLevel

inc LEVEL_NR
jsr BuildScreen

jsr CopyLevelToBackBuffer
jsr DisplayGetReady
.NOCHEAT




And still something new: Element Areas. A new primitive that fills an area with m/n repeats of an element:


!zone LevelElementArea
LevelElementArea
;!byte LD_ELEMENT_AREA,24,16,5,1,EL_SN_BROWN_ROCK

;X pos
iny
lda (ZEROPAGE_POINTER_1),y
sta PARAM1
sta PARAM10

;Y pos
iny
lda (ZEROPAGE_POINTER_1),y
sta PARAM2
;x count
iny
lda (ZEROPAGE_POINTER_1),y
sta PARAM7
sta PARAM9
;y count
iny
lda (ZEROPAGE_POINTER_1),y
sta PARAM8
;type
iny
lda (ZEROPAGE_POINTER_1),y
sta PARAM3
;store y for later
tya
pha

.NextElementRow
jsr DrawLevelElement

dec PARAM7
beq .RowDone

lda PARAM1
clc
adc PARAM4
sta PARAM1
jmp .NextElementRow

.RowDone
lda PARAM2
clc
adc PARAM5
sta PARAM2

lda PARAM9
sta PARAM7

lda PARAM10
sta PARAM1

dec PARAM8
bne .NextElementRow
jmp NextLevelData




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