mirror of
https://github.com/deater/dos33fsprogs.git
synced 2024-11-20 03:35:24 +00:00
61 lines
1.2 KiB
ArmAsm
61 lines
1.2 KiB
ArmAsm
; 16-bit 6502 Random Number Generator
|
|
|
|
; Linear feedback shift register PRNG by White Flame
|
|
; http://codebase64.org/doku.php?id=base:small_fast_16-bit_prng
|
|
|
|
; The Apple II KEYIN routine increments this field
|
|
; while waiting for keypress
|
|
|
|
;SEEDL = $4E
|
|
;SEEDH = $4F
|
|
|
|
XOR_MAGIC = $7657 ; "vW"
|
|
|
|
;=============================
|
|
; random16
|
|
;=============================
|
|
; takes:
|
|
; not 0, cc = 5+ = 27
|
|
; not 0, cs = 5+12+19 = 36
|
|
; $0000 = 5+7+19 = 31
|
|
; $8000 = 5+6+14 = 25
|
|
; $XX00 = 5+6+7+19 = 37
|
|
random16:
|
|
|
|
lda SEEDL ; 3
|
|
beq lowZero ; $0000 and $8000 are special values ; 2
|
|
|
|
asl SEEDL ; Do a normal shift ; 5
|
|
lda SEEDH ; 3
|
|
rol ; 2
|
|
bcc noEor ; 2
|
|
|
|
doEor:
|
|
; high byte is in A
|
|
|
|
|
|
eor #>XOR_MAGIC ; 2
|
|
sta SEEDH ; 3
|
|
lda SEEDL ; 3
|
|
eor #<XOR_MAGIC ; 2
|
|
sta SEEDL ; 3
|
|
rts ; 6
|
|
|
|
lowZero:
|
|
; 1
|
|
lda SEEDH ; 3
|
|
beq doEor ; High byte is also zero ; 3
|
|
; so apply the EOR
|
|
; -1
|
|
; wasn't zero, check for $8000
|
|
asl ; 2
|
|
beq noEor ; if $00 is left after the shift ; 2
|
|
; then it was $80
|
|
bcs doEor ; else, do the EOR based on the carry ; 3
|
|
|
|
noEor:
|
|
; 1
|
|
sta SEEDH ; 3
|
|
|
|
rts ; 6
|