From f1c630523239c550dc920eb14ab4652da7ca4343 Mon Sep 17 00:00:00 2001
From: "B. Watson" <urchlay@slackware.uk>
Date: Tue, 25 Oct 2022 02:29:09 -0400
Subject: Fix PAL/NTSC divisor

---
 dla.s | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

(limited to 'dla.s')

diff --git a/dla.s b/dla.s
index a031fe4..94cd4f1 100644
--- a/dla.s
+++ b/dla.s
@@ -239,11 +239,11 @@ main_done:
 
 ; print menu
  ldx #menulen
-blloop:
+menuloop:
  lda menumsg,x
  sta textbuf,x
  dex
- bpl blloop
+ bpl menuloop
 
 ; calculate and print elapsed time in minutes. our only use of
 ; the floating point ROM routines.
@@ -268,28 +268,40 @@ blloop:
  sta FR0
  jsr IFP  ; ...ok, now:
  jsr FADD ; add the high bytes in jiffies, result in FR0 again
+
  ; we now have the 3-byte jiffy count in FR0.
- ; 3600 NTSC jiffies = 1 minute, so divide.
- ldx #<fp3600
- ldy #>fp3600
- jsr FLD1R ; FR1 = 3600.0
- lda PAL ; are we on a PAL Atari?
+ ; 3600 NTSC jiffies or 3000 PAL jiffies = 1 minute, so divide.
+ ; floating point constants:
+ ; 3600.0 is $41,$36,$00,$00,$00,$00
+ ; 3000.0 is $41,$30,$00,$00,$00,$00
+ lda #$41 ; excess-64 base-100 exponent and sign (bit 7 = 0 means positive)
+ sta FR1
+ ldx #$36 ; NTSC
+ lda PAL
  and #$0e
- bne ntsc ; no, use constant as-is
- lda #$30 ; yes, adjust (3000 PAL jiffies/sec)
- sta FR0+1
+ bne ntsc
+ ldx #$30 ; PAL
 ntsc:
+ stx FR1+1
+ lda #0
+ sta FR1+2
+ sta FR1+3
+ sta FR1+4
+ sta FR1+5
  jsr FDIV  ; FR0 = FR0 / FR1
  jsr FASC  ; render as ASCII
 
+ ; Now clean up the output from FASC and copy it to our menu line.
  ; FASC puts its results at LBUFF ($0580, aka linebuf).
  ; Unfortunately it can have a leading zero, and the last
  ; digit has the high bit set (as a terminator). We want only
  ; 6 characters including the decimal point, which could be e.g.
- ; 1.2345  12.345  123.45  1234.5
+ ; 0.1234 (the 0 will show as a space) or 1.2345  12.345  123.45  1234.5
+ ; Really elegant software would print this as minutes and seconds,
+ ; with tenths of seconds, e.g. 1:23.4. Not that worried about it though.
  ldx #$ff
  lda linebuf
- cmp #'0' ; skip the leading zero if present
+ cmp #'0' ; skip the leading zero if present (a space will be seen in its place)
  bne ascloop
  inx
 ascloop:
@@ -346,9 +358,6 @@ hang: jmp hang
 ; TODO: code to save image goes here.
 ;;; End of main()
 
- ; floating point constant 3600.0
-fp3600: .byte $41,$36,$00,$00,$00,$00
-
 ; screen codes for menu
 menumsg:
  .byte $00,$00,$00    ; 3 digits of minutes
-- 
cgit v1.2.3