rand: Add an example with PRNG

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 438 insertions, 0 deletions

diff --git a/rand/rand.s b/rand/rand.s
new file mode 100644
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+;;;
+;; Showcase different strategies for Random Number Generation (RNG).
+;;
+;; Refer to the README.md file for documentation. Here I have left comments
+;; whenever there's something "new" if you are coming from the `basics/`
+;; directory.
+
+.segment "HEADER"
+    .byte 'N', 'E', 'S', $1A
+    .byte $02, $01
+    .byte $00, $00
+
+.segment "VECTORS"
+    .addr nmi, reset, irq
+
+.segment "CHARS"
+.incbin "../assets/alphanum.chr"
+
+.segment "CODE"
+
+.include "../shared/ppu.s"
+.include "../shared/clear.s"
+.include "../shared/joypad.s"
+
+;; Different algorithms.
+.include "linear.s"
+.include "precalc.s"
+
+;; Number of algorithms for this game.
+ALGORITHM_SIZE = $02
+
+;; See Vars::zp_button_timer.
+KEY_TIMER = 15
+
+;; Variables used by this game.
+.scope Vars
+    ;; 0: "Press start" state; 1: "number generation" state.
+    zp_state = $30
+
+    ;; Timer for key presses. A button press will only be considered if the
+    ;; button timer is zero. Whenever that's the case, the code will reset the
+    ;; timer to this new value, and decrement it whenever a new press is found.
+    ;; Whenever we reach back to zero, then the button press will be considered
+    ;; again. All of this is because whenever the player presses a button, it
+    ;; actually presses it for more than one frame, and things can go crazy from
+    ;; this fact.
+    zp_button_timer = $31
+
+    ;; Algorithm that has been selected.
+    zp_algorithm = $32
+
+    ;; The random seed for this game. As described in the README.md file, this
+    ;; is actually a frame counter for the "Press start" state, and whenever the
+    ;; player hits "Start", it will be paused. That is, our random seed is
+    ;; simply the number of frames that the player took to press "Start" at the
+    ;; beginning.
+    ;;
+    ;; NOTE: it's not going to be initialized to get a more random feeling on
+    ;; real hardware from unknown RAM state.
+    zp_seed = $33
+
+    ;; The number to be displayed.
+    zp_number = $34
+.endscope
+
+;; Main function, this takes care of reading input, changing the state, and
+;; calling the relevant algorithm to get new numbers.
+.proc main
+    ;; Initialize all variables (except Vars::zp_seed as explained above).
+    lda #0
+    sta Vars::zp_state
+    sta Vars::zp_button_timer
+    sta Vars::zp_algorithm
+    sta Vars::zp_number
+
+    ;; Clear both screens. Yes, over the top, but it gets the job done.
+    CLEAR_SCREENS $20, $28
+
+    ;; Initialize palettes and show the "Press start" message.
+    jsr init_palettes
+    jsr show_init_screen
+
+    cli
+    lda #%10001000
+    sta $2000                   ; PPUCTRL
+    lda #%00011110
+    sta $2001                   ; PPUMASK
+
+@main_game_loop:
+    ;; Should we actually read the joypad? This is handled via the button timer
+    ;; as explained above.
+    lda Vars::zp_button_timer
+    beq @check_joypad
+    dec Vars::zp_button_timer
+    jmp @end
+
+@check_joypad:
+    ;; Yes! Then read the joypad.
+    READ_JOYPAD1
+
+    ;; What's the current game state?
+    lda Vars::zp_state
+    bne @check_change_algorithm
+
+    ;; "Press start" state. If the player is not pressing "Start", ignore
+    ;; everything and go to the end.
+    lda Joypad::zp_buttons1
+    and #Joypad::BUTTON_START
+    beq @end
+
+    ;; Reset the button timer.
+    lda #KEY_TIMER
+    sta Vars::zp_button_timer
+
+    ;; The random seed has a proper value and we can use that as a first random
+    ;; number.
+    lda Vars::zp_seed
+    sta Vars::zp_number
+
+    ;; The 'linear' algorithm actually disregards any parameters and needs a
+    ;; 16-bit register. Let's initialize this register with the current seed.
+    sta Linear::zp_register_lo
+    sta Linear::zp_register_hi
+
+    ;; Move into the next state.
+    inc Vars::zp_state
+
+    jmp @end
+
+@check_change_algorithm:
+    ;; We are in a running state. Check if the player is asking to change the
+    ;; algorithm.
+    lda Joypad::zp_buttons1
+    and #Joypad::BUTTON_SELECT
+    beq @check_a_button
+
+    ;; Reset the button timer.
+    lda #KEY_TIMER
+    sta Vars::zp_button_timer
+
+    ;; The player asked to change the algorithm. Do it now and go generate a new
+    ;; number with that.
+    ldx Vars::zp_algorithm
+    inx
+    cpx #ALGORITHM_SIZE
+    bne @store_algorithm
+    ldx #0
+@store_algorithm:
+    stx Vars::zp_algorithm
+    jmp @next_number
+
+@check_a_button:
+    ;; Is the player asking for a new number? If not go to the end.
+    lda Joypad::zp_buttons1
+    and #Joypad::BUTTON_A
+    beq @end
+
+    ;; Reset the button timer.
+    lda #KEY_TIMER
+    sta Vars::zp_button_timer
+
+@next_number:
+    ;; Setup parameters depending on the algorithm and actually call it.
+    ldx Vars::zp_algorithm
+    bne @precalc
+    jsr linear_feedback_shift_register
+    jmp @store_number
+@precalc:
+    lda Vars::zp_number
+    jsr precalc
+@store_number:
+    sta Vars::zp_number
+
+@end:
+    ;; And wait for the render to happen as it's done in any other example.
+    lda #%10000000
+    ora $20
+    sta $20
+@wait_for_render:
+    bit $20
+    bmi @wait_for_render
+
+    jmp @main_game_loop
+.endproc
+
+;; NMI code is pretty standard. I have added comments for the code which is
+;; specific to this game.
+.proc nmi
+    bit $20
+    bpl @next
+
+    pha
+    txa
+    pha
+    tya
+    pha
+
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+    ;; What's the current game state?
+    lda Vars::zp_state
+    beq @seed_inc
+
+    ;; Running state. Print the current algorithm and number.
+    jsr print_algorithm
+    jsr print_value
+
+    ;; The running state is displayed on the other nametable. Update the PPU
+    ;; control register for this (it's of course stupid to update it every time,
+    ;; but I didn't feel like doing the proper thing of shadowing the PPU
+    ;; control register and update only on changes, etc.).
+    lda #%10001010
+    sta $2000                   ; PPUCTRL
+    bne @after_seed
+
+@seed_inc:
+    ;; "Press start" state: just increase the frame counter which is used as a
+    ;; seed.
+    inc Vars::zp_seed
+
+@after_seed:
+    bit $2002                   ; PPUSTATUS
+    lda #$00
+    sta $2005                   ; PPUSCROLL
+    sta $2005                   ; PPUSCROLL
+
+    lda #%01111111
+    and $20
+    sta $20
+
+    pla
+    tay
+    pla
+    tax
+    pla
+@next:
+    rti
+.endproc
+
+;; Show the "Alg: <algorithm>" message on screen.
+.proc print_algorithm
+    ;; "ALG: "
+    WRITE_PPU_DATA $298B, $1A
+    WRITE_PPU_DATA $298C, $25
+    WRITE_PPU_DATA $298D, $20
+    WRITE_PPU_DATA $298E, $34
+    WRITE_PPU_DATA $298F, $00
+
+    lda Vars::zp_algorithm
+    beq @linear
+
+    ;; "PRECALC"
+    WRITE_PPU_DATA $2990, $29
+    WRITE_PPU_DATA $2991, $2B
+    WRITE_PPU_DATA $2992, $1E
+    WRITE_PPU_DATA $2993, $1C
+    WRITE_PPU_DATA $2994, $1A
+    WRITE_PPU_DATA $2995, $25
+    WRITE_PPU_DATA $2996, $1C
+    rts
+
+    ;; "LINEAR "
+@linear:
+    WRITE_PPU_DATA $2990, $25
+    WRITE_PPU_DATA $2991, $22
+    WRITE_PPU_DATA $2992, $27
+    WRITE_PPU_DATA $2993, $1E
+    WRITE_PPU_DATA $2994, $1A
+    WRITE_PPU_DATA $2995, $2B
+    WRITE_PPU_DATA $2996, $00
+    rts
+.endproc
+
+;; Show the "Val: $<number>" message on screen.
+.proc print_value
+    ;; "VAL: $"
+    WRITE_PPU_DATA $29AB, $2F
+    WRITE_PPU_DATA $29AC, $1A
+    WRITE_PPU_DATA $29AD, $25
+    WRITE_PPU_DATA $29AE, $34
+    WRITE_PPU_DATA $29AF, $00
+    WRITE_PPU_DATA $29B0, $35
+
+    ;; Set the high byte on the 'y' register, and the low byte on the 'x'
+    ;; register.
+    lda #$F0
+    and Vars::zp_number
+    lsr
+    lsr
+    lsr
+    lsr
+    clc
+    adc #$10
+    tay
+    lda #$0F
+    and Vars::zp_number
+    clc
+    adc #$10
+    tax
+
+    ;; Display the actual number.
+    bit $2002
+    lda #$29
+    sta $2006
+    lda #$B1
+    sta $2006
+    sty $2007
+    stx $2007
+
+    rts
+.endproc
+
+;; Show the "Press start" message.
+.proc show_init_screen
+    ;; "PRESS"
+    WRITE_PPU_DATA $21AB, $29
+    WRITE_PPU_DATA $21AC, $2B
+    WRITE_PPU_DATA $21AD, $1E
+    WRITE_PPU_DATA $21AE, $2C
+    WRITE_PPU_DATA $21AF, $2C
+
+    ;; "START"
+    WRITE_PPU_DATA $21B1, $2C
+    WRITE_PPU_DATA $21B2, $2D
+    WRITE_PPU_DATA $21B3, $1A
+    WRITE_PPU_DATA $21B4, $2B
+    WRITE_PPU_DATA $21B5, $2D
+
+    rts
+.endproc
+
+;; Initialize palettes. A bit over the top since only two colors are used.
+.proc init_palettes
+    lda #$3F
+    sta $2006                   ; PPUADDR
+    lda #$00
+    sta $2006                   ; PPUADDR
+
+    ldx #0
+@load_palettes_loop:
+    lda palettes, x
+    sta $2007                   ; PPUDATA
+    inx
+    cpx #$20
+    bne @load_palettes_loop
+    rts
+palettes:
+    DEFAULT_COLOR = $0F
+
+    ;; Background
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+
+    ;; Foreground
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+    .byte DEFAULT_COLOR, $20, $FF, $FF
+
+    rts
+.endproc
+
+;;;
+;; NOTE: down below just boilerplate. Nothing special from the `basics/`
+;; examples.
+
+.proc reset
+    sei
+    cld
+
+    ldx #$40
+    stx $4017                   ; APU Frame Counter
+
+    ldx #$FF
+    txs
+
+    inx
+    stx $2000                   ; PPUCTRL
+    stx $2001                   ; PPUMASK
+    stx $4010                   ; APU DMC
+
+    bit $2002                   ; PPUSTATUS
+@vblankwait1:
+    bit $2002                   ; PPUSTATUS
+    bpl @vblankwait1
+
+    ldx #0
+    lda #0
+@ram_reset_loop:
+    sta $000, x
+    sta $100, x
+    sta $300, x
+    sta $400, x
+    sta $500, x
+    sta $600, x
+    sta $700, x
+    inx
+    bne @ram_reset_loop         ; if x overflows back to #00, then we are done.
+
+    lda #$EF
+@sprite_reset_loop:
+    sta $200, x
+    inx
+    bne @sprite_reset_loop
+
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+@vblankwait2:
+    bit $2002                   ; PPUSTATUS
+    bpl @vblankwait2
+
+    lda #$3F
+    sta $2006                   ; PPUADDR
+    lda #$00
+    sta $2006                   ; PPUADDR
+
+    lda #$0F
+    ldx #$20
+@palettes_reset_loop:
+    sta $2007                   ; PPUDATA
+    dex
+    bne @palettes_reset_loop
+
+    jmp main
+.endproc
+
+.proc irq
+    rti
+.endproc
+