[stella] code incompatible with stella emulator

Subject: [stella] code incompatible with stella emulator
From: Jim Crawford <pfister@xxxxxxxxxxxxxx>
Date: Wed, 15 Oct 1997 07:54:54 -0700 (PDT)
i just learned to code for the 2600 a few hours ago and wrote this (or
rather, modified it slightly from howtodrw.s (which, incidentally,
doesn't work under stella 0.7 at all -- it says there's a bad opcode but
rudely refuses to tell me which opcode or memory location))

this seems odd to me... the following code works fine under PCAE, but not
under Stella.  i can't test it on a real atari myself, so i can't tell
which is right :)

can someone tell me what's going on?

(assembled under DASM v2.02)

	processor 6502
	org $f000

vsync   equ $00		;wait for vertical retrace
vblank  equ $01		;vertical blank
wsync	equ $02		;wait for horizontal retrace
colubk	equ $09		;background color

intim	equ $284	;timer read
tim64t	equ $296	;timer write

beginy	equ $80

        sei             ;disable interrupts
        cld             ;clear bcd math bit

        ldx #$ff        ;set stack pointer

        lda #0          ;clear memory
        sta 0,x
        bne clearloop
        jsr vertblank
        inc beginy
        jsr drawscreen
        jmp mainloop

        sta vsync	;wait for vsync
        sta wsync	;wait for 3 scanlines
        sta wsync
        sta wsync
        lda #44		;set timer for 37 blank scanlines.
        sta tim64t	;why 44?  i didn't calculate it :)
        lda intim	;wait for vblank period to end
        bne drawscreen
        sta wsync	;first scanline
        sta vblank	;turn blanking off

        ldx #191	;191 scanlines left...
	ldy beginy	;start of color cycle
        sty colubk      ;update background color
        sta wsync       ;wait for vsync
	dey		;next color
        bne scanloop

        lda #$ff
        sta wsync       ;last one
	sta vblank	;turn blanking on for overscan

	org $f7fc
	.word start	;entry point
	.word start	;int vector

pf / green grapes / xantis
"The PowerPC is something that should be shown to small children when
 they have been very bad." --Linus Torvalds

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