Added random number generator via a modified LFSR

map

@@ -4,53 +4,55 @@ Bank #0 (HOME):
   SECTION: $0050-$0052 ($0003 bytes)
   SECTION: $0058-$0058 ($0001 bytes)
   SECTION: $0060-$0060 ($0001 bytes)
-  SECTION: $0100-$0D36 ($0C37 bytes)
-           $0A89 = timer_interrupt
+  SECTION: $0100-$0D66 ($0C67 bytes)
+           $0AB9 = timer_interrupt
            $0952 = init
-           $0D1E = setup_dma
-           $0CFD = stoplcd
+           $0D4E = setup_dma
+           $0D2D = stoplcd
            $0150 = TileData
-           $0C61 = clear_bg
-           $0C73 = splash_text
-           $0CF3 = splash_text_end
-           $0C0E = draw_snake
-           $0AA4 = read_joypad
-           $0B23 = move_snake
-           $0A16 = main
-           $0B10 = draw_gameover
-           $0A80 = subtract_16bit
-           $0AA2 = timer_interrupt_end
-           $0ACB = read_joypad_up_return
-           $0ABD = read_joypad_right_return
-           $0AD2 = read_joypad_down_return
-           $0AC4 = read_joypad_left_return
-           $0CF3 = gameover_text
-           $0CFD = gameover_text_end
-           $0C07 = move_snake_die
-           $0B73 = move_snake_write
-           $0B86 = move_snake_continue
-           $0A3B = tail_collision_scan
-           $0C0C = move_snake_end
-           $0BD6 = move_snake_cut_tail_continue
-           $0C51 = convert_xy_to_screen_addr
-           $0C32 = draw_snake_continue
-           $0D0E = multiply_to_hl
-           $0D2B = dma_copy
-           $0D37 = dma_copy_end
+           $0C91 = clear_bg
+           $0CA3 = splash_text
+           $0D23 = splash_text_end
+           $0C3E = draw_snake
+           $0A47 = lfsr_generate
+           $0AD4 = read_joypad
+           $0B53 = move_snake
+           $0A1F = main
+           $0B40 = draw_gameover
+           $0AB0 = subtract_16bit
+           $0AD2 = timer_interrupt_end
+           $0AFB = read_joypad_up_return
+           $0AED = read_joypad_right_return
+           $0B02 = read_joypad_down_return
+           $0AF4 = read_joypad_left_return
+           $0D23 = gameover_text
+           $0D2D = gameover_text_end
+           $0C37 = move_snake_die
+           $0BA3 = move_snake_write
+           $0BB6 = move_snake_continue
+           $0A6B = tail_collision_scan
+           $0C3C = move_snake_end
+           $0C06 = move_snake_cut_tail_continue
+           $0C81 = convert_xy_to_screen_addr
+           $0C62 = draw_snake_continue
+           $0D3E = multiply_to_hl
+           $0D5B = dma_copy
+           $0D67 = dma_copy_end
   SECTION: $0061-$00B6 ($0056 bytes)
            $007B = mem_CopyMono
            $0061 = mem_Set
            $00A1 = mem_CopyVRAM
            $008B = mem_SetVRAM
            $006D = mem_Copy
-    SLACK: $336A bytes
+    SLACK: $333A bytes
 
 BSS:
-  SECTION: $C000-$C179 ($017A bytes)
+  SECTION: $C000-$C17A ($017B bytes)
            $C000 = variables_start
-           $C17A = variables_end
+           $C17B = variables_end
            $C0A3 = SnakePieces
            $C16B = SnakePiecesEnd
+           $C175 = LFSRSeed
            $C0A0 = GameState
            $C16D = SnakeTail
            $C16B = SnakeHead
@@ -60,7 +62,7 @@ BSS:
            $C174 = TimerTicks
            $C16F = SnakeDirection
            $C170 = SnakeNextDirection
-    SLACK: $1E86 bytes
+    SLACK: $1E85 bytes
 
 VRAM:
   EMPTY

snake.asm

@@ -47,6 +47,8 @@ NumberOfFoods:		ds 1		; number of food objects that exist (as sprites)
 
 TimerTicks:		ds 1		; count the number of ticks in the timer (to slow down the movement speed)
 
+LFSRSeed:		ds 1		; seed value for our lfsr
+
 DEBUG_BLANK_DONT_TOUCH: ds 5		; this gets allocated to zeroes, makes it easier to see our ram block in the debugger
 variables_end:
 
@@ -146,6 +148,11 @@ init_variables:
 
 ; this code displays the splash screen
 splash:
+	
+	; turn on the timer, we need to use its value for our LFSR seed
+	ld a, TACF_START | TACF_262KHZ		; make it super fast to stop people gaming it
+	ld [rTAC], a
+
 	; we are going to copy across the background tiles from the splash_text label
 	; using: mem_copyVRAM (hl->pSource, de->pDest, bc->byte count)
 	ld hl, splash_text
@@ -169,6 +176,10 @@ splash_wait_for_press:
 	cp 1
 	jr z, splash_wait_for_press	; if the a key has not been pressed, the instruction above will set the zero flag and we will jump back	
 
+	; save our LFSR seed
+	ld a, [rTIMA]			; counter value into a
+	ld [LFSRSeed], a		; save it!
+
 ;------------------------------------------------------------------------
 
 ; ok, time to play the game!
@@ -218,15 +229,16 @@ init_snake:
 	ld a, 8				; piece 2, y
 	ld [hl+], a
 
+	; draw our initial snake
+	call draw_snake
+
 	; now we start the timer interrupts (ll. 828-852)
+	; start the timer, we need this for our LIFR
 	ld a, TACF_START | TACF_4KHZ	; turn on, set to 4khz (timer will interrupt every [255 * 1/4000] = 63.75ms)
 	ld [rTAC], a
 
 	ld a, IEF_VBLANK | IEF_TIMER | IEF_HILO	; enable vblank, timer, joypad interrupts
 	ld [rIE], a			; save the register
-	
-	; draw our initial snake
-	call draw_snake
 
 ;;;;;;;;;;;;;;;;;; DEBUG (make the snake grow by default)
 	ld a, 10
@@ -239,6 +251,8 @@ main:
 ;	nop				; a bug in the cpu means that the instruction after a halt might get skipped
 	ei
 
+	call lfsr_generate		; so we get nice random numbers
+
 	ld a, [GameState]
 	cp 2				; does GameState == 2? (the gameover state)
 	jr z, gameover			; show a nice message
@@ -265,6 +279,47 @@ gameover:
 ;	jr main
 
 ;------------------------------------------------------------------------
+;--------------- 
+; lfsr_generate
+;	generate a random number and put it in LFSRSeed
+;	see http://en.wikipedia.org/wiki/Linear_feedback_shift_register for info on how this works
+;	uses taps at 8, 6, 5, 4 
+;---------------
+lfsr_generate:
+	ld a, [LFSRSeed]		; load up the seed
+	ld e, a				; e will store our seed value
+
+lfsr_generate_loop:
+	ld d, a
+
+	rr d				; roll four times
+	rr d
+	rr d
+	rr d
+	xor d				; xor it
+
+	rr d				; five rolls in total
+	xor d
+
+	rr d				; up to 6 rolls
+	xor d				; xor it in
+
+	rr d
+	rr d				; 8 rolls, this is our fourth and last tap
+	xor d
+	
+	; not really sure if secure crypto allows me to do this, but frankly if you would like to game my system then go for it
+	ld b, a
+	ld a, [rDIV]			; source of some more random
+	xor b				; add it in
+
+	cp e
+	jr z, lfsr_generate_loop	; make sure it is different
+
+	; save it
+	ld [LFSRSeed], a
+
+	ret
 
 ;--------------- 
 ; tail_collision_scan