mirror of https://github.com/falsovsky/z80.git
Optimizations and documenting code
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@ -6,6 +6,9 @@ tv_flag EQU $5c3c ; TV flags
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last_k EQU $5c08 ; Last pressed key
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clr_screen EQU $0daf ; ROM routine to clear the screen
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; http://www.z80.info/pseudo-random.txt
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; seed - http://www.wearmouth.demon.co.uk/sys/frames.htm
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; Screen is 256x192
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; Star Structure
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@ -21,17 +24,18 @@ start
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ld (tv_flag), a
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push bc
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call clear_screen
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call initStars
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call clear_screen ; Clear the screen
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call initStars ; Initialize the number of stars defined in MAX_STARS
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main_start
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ld hl, STARS
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ld hl, STARS ; Points to X
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ld c, MAX_STARS
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main
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inc hl ; Y
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inc hl ; Speed
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inc hl ; PrevX
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; CLEAR THE LAST POSITION
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ld de, $3 ; Skip to PrevX
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adc hl, de
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ld a, (hl)
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ld d, a ; Save PrevX to D
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inc hl ; PrevY
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@ -39,40 +43,38 @@ main
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ld a, (hl)
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ld e, a ; Save PrevY to E
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push hl
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push bc
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push hl
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call get_screen_address
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; Video RAM address for those X,Y is now in HL and the bit needed
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; to be set in that address value is in A
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call clear_pixel ; Uses those values clears the pixel
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pop bc
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pop hl
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dec hl ; PrevX
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dec hl ; Speed
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dec hl ; Y
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dec hl ; X
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ld bc, $4 ; Go back to X
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sbc hl, bc
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pop bc
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; WRITES THE PIXEL
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ld a, (hl) ; HL points to X
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ld d, a ; Save X to D
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inc hl
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inc hl ; Y
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ld a, (hl) ; HL now points to Y
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ld a, (hl)
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ld e, a ; Save Y to E
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push hl
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push bc
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push hl
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call get_screen_address
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; Video RAM address for those X,Y is now in HL and the bit needed
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; to be set in that address value is in A
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call write_pixel ; Uses those values and writes the pixel
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pop bc
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pop hl
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inc hl ; Speed
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inc hl ; PrevX
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inc hl ; PrevY
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inc hl ; Next star
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ld bc, $4 ; Jump 4 positions - Next star
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adc hl, bc
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pop bc
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dec c ; Decrement counter
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jr nz, main ; Repeat if not zero
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@ -84,45 +86,44 @@ main
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ret
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PROC
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; Initialize stars X and Y with "random" values
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initStars
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push bc
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ld d, MAX_STARS
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ld hl, STARS
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ld d, MAX_STARS ; Number of stars to process
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ld hl, STARS ; HL points to X of first start
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initStars_loop
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push hl
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call getRandomX
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call getRandomX ; Get a random value for X | 1 - 250 | TODO: 255?
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pop hl
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ld (hl), a
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inc hl ; Y
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ld (hl), a ; Set X to random value
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inc hl ; points to Y
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push hl
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call getRandomY
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call getRandomY ; Get a random value for Y | 1 - 191 | TODO : allow 0
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pop hl
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ld (hl), a ; Set Y to random value
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ld (hl), a
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inc hl ; Speed
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inc hl ; points to Speed
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push hl
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call getRandomSpeed
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call getRandomSpeed ; Get a random value for Speed | 1 - 4
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pop hl
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ld (hl), a ; Set Speed to random value
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ld (hl), a
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ld bc, $3 ; Jump to next star
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adc hl, bc ; Skip PrevX and PrevY
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inc hl ; PrevX
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inc hl ; PrevY
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inc hl ; Next one
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dec d
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jr nz, initStars_loop
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dec d ; Decrement counter
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jr nz, initStars_loop ; If not zero, do it again
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pop bc
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ret
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ENDP
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PROC
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; Gets a value a from a list of pre-calculated values
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; Returns to begin after 0 is found | TODO: change this
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getRandomX
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push hl
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getRandomX_loop
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@ -141,6 +142,8 @@ getRandomX_reset
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ENDP
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PROC
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; Gets a value a from a list of pre-calculated values
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; Returns to begin after 0 is found | TODO: change this
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getRandomY
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push hl
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getRandomY_loop
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@ -159,6 +162,8 @@ getRandomY_reset
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ENDP
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PROC
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; Gets a value a from a list of pre-calculated values
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; Returns to begin after 0 is found | TODO: change this
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getRandomSpeed
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push hl
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getRandomSpeed_loop
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@ -183,68 +188,71 @@ increment_x
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ld hl, STARS
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ld c, MAX_STARS
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increment_x_loop
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push de
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; First lets copy current position to previous position
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ld d, (hl) ; Save current X to D
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inc hl ; Y
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inc hl ; points to Y
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ld e, (hl) ; Save current Y to E
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inc hl ; Speed
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inc hl ; PrevX
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ld (hl), d
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inc hl ; points to Speed
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inc hl ; points to PrevX
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ld (hl), d ; Save X
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inc hl ; PrevY
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ld (hl), e
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dec hl ; PrevX
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dec hl ; Speed
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dec hl ; Y
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dec hl ; X
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pop de
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ld (hl), e ; Save Y
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ld de, $4
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sbc hl, de ; Go back to X
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ld a, (hl)
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ld a, (hl) ; Is X at $FF - end of screen
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cp $ff
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jr z, increment_x_zero
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jr z, increment_x_zero ; Yes, lets reset
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inc hl ; Skip to Y
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inc hl ; Skip to Speed
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; Increments X position by speed value
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; X = X + Y
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inc hl ; points to Y
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inc hl ; points to Speed
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ld b, (hl) ; Read speed to B
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dec hl ; Back to Y
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dec hl ; Back to X
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add a, b ; Add speed to X
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add a, b ; X = X + Speed
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jr c, increment_x_zero ; If carry is set, it passed $ff, lets reset
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jr c, increment_x_zero
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increment_x_update
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ld (hl), a
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inc hl ; Y
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inc hl ; Speed
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inc hl ; PrevX
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inc hl ; PrevY
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inc hl ; Next
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dec c
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jr nz, increment_x_loop
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; Saves to X the value in A
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ld (hl), a ; Save X with the value in A
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ld de, $5 ; Skip 5 bytes to the next star
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adc hl, de
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dec c ; Decrement counter
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jr nz, increment_x_loop ; If not zero, do it again
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pop bc
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ret
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increment_x_zero
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; Sets X to 0 and Y and Speed to random values
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push bc
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inc hl ; Set to Y position
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inc hl ; point to Y
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push hl
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call getRandomY
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call getRandomY ; Get a random Y value
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pop hl
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ld (hl), a ; Set Y = getRandomY
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inc hl ; Set to speed position
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inc hl ; point to speed
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push hl
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call getRandomSpeed
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call getRandomSpeed ; Get a random Speed value
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pop hl
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ld (hl), a ; Set Speed = getRandomSpeed
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dec hl
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dec hl ; Get back to X position
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ld de, $2
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sbc hl, de ; Get back to X position
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ld a, $0 ; X = 0
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pop bc
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