Re: 5.25" PROM MisMatch Between II+ & IIgs

["Bryan Parkoff" <BParkoff@satx.rr.com>]

David,

    I think that you don't understand my saying.  You will need to compare
BIT on both Apple II+ and Apple IIgs.  Look at my code below.

2000: JSR FC58
2003: LDX #03
2005: STX 3C
2007: LDX #06
2009: BIT 3C
200B: BEQ 2012
200D: LDY #C1
200F: STY 0400
2012: LDY #C2
2014: STY 0401
2017: RTS

    Hex 03 shows 011 and Hex 06 shows 110.  Bit's Hex 02 shows 010.  This
answer is correct because second bit shows 1.  BIT should tell BEQ not to
branch because it is nonzero bit.  On Apple II+, it reports "AB", but on
Apple IIgs, it reports "B".  Apple II+ is to be correct.  Apple IIgs should
not execute BEQ.  Please compare my code above on your real Apple II+ and
Apple IIgs.  You will notice the difference.
    It may prove that Apple IIgs gives the wrong value.


--
Yours Truly,

Bryan Parkoff
BParkoff@satx.rr.com
"David Empson" <dempson@actrix.gen.nz> wrote in message
1fqnmoj.lgym7t11hbjgaN%dempson@actrix.gen.nz">news:1fqnmoj.lgym7t11hbjgaN%dempson@actrix.gen.nz...
> Bryan Parkoff <BParkoff@satx.rr.com> wrote:
>
> >     I find an interesting disasembly that is not doing correct.  For
> > example: BEQ should be even and BNE should be odd.  Look at my code
below.
> > I have copied PROM from Apple II+ to Apple IIgs for testing.
> >
> > C600: A2 20        LDX #20
> > C602: A0 00        LDY #00
> > C604: A2 03        LDX #03
> > C606: 86 3C        STX 3C
> > C608: 8A           TXA
> > C609: 0A           ASL
> > C60A: 24 3C        BIT 3C
> > C60C: F0 10        BEQ C61E {+10}
> > C60E: 05 3C        ORA 3C
> >
> >     On Apple II+, 6 is even.  BEQ should state, "6 is equal to even".
BEQ
> > should report TRUE, but Apple II+ reports false.  Apple IIgs reports
true.
> > C60E is not supposed to be executed on Apple II+, but Apple IIgs did
execute
> > C61E.
>
> After the ASL, the accumulator contains #$06, while location $3C
> contains #$03 (from the STX at C606).
>
> The BIT instruction does a logical AND operation between the accumulator
> and the specified memory location.  It discards the result of the AND
> (the accumulator is not modified) but sets the Z flag to indicate
> whether the result of the AND was zero.  (It also copies bits 6 and 7 of
> the memory location into the V and N flags.)
>
> In this case, we are ANDing #$06 with #$03.  They have a bit in common
> (#$02), so the result is not zero, the BEQ instruction should NOT
> branch, and the code should continue executing from C60E.
>
> >     Please compare my example on your both real Apple II+ and Apple
IIgs.
> > It is possible that emulator is not doing correct for manipulating odd
and
> > even like using to test PARITY.
>
> The BIT instruction has nothing to do with testing for odd or even
> values, unless the accumulator happens to contain #$01.  (The 6502
> doesn't have any kind of special support for parity, unlike the 8080,
> Z80, 8088 etc.)
>
> --
> David Empson
> dempson@actrix.gen.nz