From 0de843a9d404d1022ac40ba1e25da0194f367027 Mon Sep 17 00:00:00 2001
From: "B. Watson" <yalhcru@gmail.com>
Date: Fri, 8 Jan 2016 02:10:00 -0500
Subject: fix POKEY random wrappers, I hope

---
 rand.s | 50 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 39 insertions(+), 11 deletions(-)

(limited to 'rand.s')

diff --git a/rand.s b/rand.s
index 1fa7d8d..8b4c502 100644
--- a/rand.s
+++ b/rand.s
@@ -1,26 +1,54 @@
 
- .export _randi, _randl
- .importzp sreg
+ .export _randb, _randi, _randl
+ .importzp sreg, tmp3
 
-RANDOM = 53770 ; POKEY LFSR read address, defined in the Atari OS
+ .include "atari.inc"
 
-; void __fastcall__ randi(void);
+; RANDOM is the POKEY LFSR read address.
+; unfortunately, a read from this address will never
+; return 0, since LFSR's can't do that. So there's
+; extra code in here to (try to) compensate.
+
+; 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 ; 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:
- lda RANDOM
+ jsr randb
  and #$7f
  tax
- lda RANDOM
+ jsr randb
  rts
 
-; void __fastcall__ randl(void);
+; unsigned long __fastcall__ randl(void);
 _randl:
- lda RANDOM
+ jsr randb
  sta sreg
- lda RANDOM
+ jsr randb
  sta sreg+1
- lda RANDOM
- ldx RANDOM
+ jsr randb
+ tax
+ jsr randb
  rts
 
-- 
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