diff options
Diffstat (limited to 'home/random.asm')
| -rw-r--r-- | home/random.asm | 51 |
1 files changed, 41 insertions, 10 deletions
diff --git a/home/random.asm b/home/random.asm index 5b2f4933..9d62b9b6 100644 --- a/home/random.asm +++ b/home/random.asm @@ -1,24 +1,46 @@ -Random:: ; 30a2 (0:30a2) +Random:: +; 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 + ldh a, [rDIV] ld b, a - ldh a, [hRandom] + ldh a, [hRandomAdd] adc b - ldh [hRandom], a + ldh [hRandomAdd], a + ldh a, [rDIV] ld b, a ldh a, [hRandomSub] sbc b ldh [hRandomSub], a + pop bc ret BattleRandom:: +; _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. + ldh a, [hROMBank] push af - ld a, BANK(BattleRandom_) + ld a, BANK(_BattleRandom) rst Bankswitch - call BattleRandom_ + + call _BattleRandom + ld [wPredefTemp + 1], a pop af rst Bankswitch @@ -26,24 +48,33 @@ BattleRandom:: ret RandomRange:: +; Return a random number between 0 and a (non-inclusive). + push bc ld c, a + + ; b = $100 % c xor a sub c -.asm_30cb +.mod sub c - jr nc, .asm_30cb + jr nc, .mod add c ld b, a + + ; Get a random number + ; from 0 to $ff - b. push bc -.asm_30d1 +.loop call Random - ldh a, [hRandom] + ldh a, [hRandomAdd] ld c, a add b - jr c, .asm_30d1 + jr c, .loop ld a, c pop bc + call SimpleDivide + pop bc ret |