pokecrystal-board/home/random.asm

Random: ; 2f8c
; A simple hardware-based random number generator (RNG).

; Two random numbers are generated by adding and subtracting
; the divider to the respective values every time it's called.

; The divider is a register that increments at a rate of 16384Hz.
; For comparison, the Game Boy operates at a clock speed of 4.2MHz.

; Additionally, an equivalent function is executed in VBlank.

; This leaves a with the value in hRandomSub.

	push bc

	ld a, [rDIV]
	ld b, a
	ld a, [hRandomAdd]
	adc b
	ld [hRandomAdd], a

	ld a, [rDIV]
	ld b, a
	ld a, [hRandomSub]
	sbc b
	ld [hRandomSub], a

	pop bc
	ret
; 2f9f

BattleRandom: ; 2f9f
; _BattleRandom lives in another bank.

; It handles all RNG calls in the battle engine, allowing
; link battles to remain in sync using a shared PRNG.

	ld a, [hROMBank]
	push af
	ld a, BANK(_BattleRandom)
	rst Bankswitch

	call _BattleRandom

	ld [$cfb6], a
	pop af
	rst Bankswitch
	ld a, [$cfb6]
	ret
; 2fb1


Function2fb1: ; 2fb1
	push bc
	ld c, a
	xor a
	sub c
.asm_2fb5
	sub c
	jr nc, .asm_2fb5
	add c
	ld b, a
	push bc
.asm_2fbb
	call Random
	ld a, [hRandomAdd]
	ld c, a
	add b
	jr c, .asm_2fbb
	ld a, c
	pop bc
	call SimpleDivide
	pop bc
	ret
; 2fcb