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.export _randb, _randi, _randl
; .export _rand1in5
; .export _randbit
.importzp sreg, tmp3
.include "atari.inc"
; RANDOM is the POKEY LFSR read address. This appears to be the low 8
; bits of a 17-bit LFSR (Atari calls it a poly counter). Unfortunately,
; a read from this address will never return 0, since LFSR's can't do
; that. If Atari had made the RANDOM register read the higher order bits
; of the LFSR, rather than the bottom 8, this wouldn't be a problem... but
; they didn't, so there's extra code in here to (try to) compensate.
; Might use this at some point:
;_randbit:
; lda RANDOM
; asl
; lda #0
; adc #0
; rts
; unsigned char __fastcall__ randbit(void);
;_randbit:
; ldx #0
;randbit:
; lda RANDOM
; lsr
; and #$01
; rts
; This doesn't give evenly distributed results, it's twice as
; likely to return 2 or 3 than 0, 1, or 4.
; unsigned char __fastcall__ rand1in5(void);
;_rand1in5:
; ldx #0
;rand1in5:
; lda RANDOM
; lsr
; lsr
; and #$03
; adc #0
; rts
; unsigned char __fastcall__ randb(void);
_randb: ; C-callable entry point
ldx #0
randb: ; asm-callable (doesn't trash X reg)
lda RANDOM ; bit 7 of this read ends up as bit 0 of result
sta tmp3
nop ; let the LFSR cook for a bit...
nop
lda RTCLOK+2 ; different amount of cooking depending on whether
and #$01 ; we're on an even or odd numbered TV frame
bne @1
nop
nop
nop
@1:
rol tmp3 ; tmp3 bit 7 now in carry
lda RANDOM
rol ; carry now in bit 0 of A
nop
nop
rts
; unsigned int __fastcall__ randi(void);
; NB cc65's rand() returns a positive signed int, meaning
; 0 to 0x7fff.
_randi:
jsr randb
and #$7f
tax
jsr randb
rts
; unsigned long __fastcall__ randl(void);
; this returns the full range of an unsigned long, 0 to 2**32-1
_randl:
jsr randb
sta sreg
jsr randb
sta sreg+1
jsr randb
tax
jsr randb
rts
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