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Yuri
 Post subject: MOS vs WDC in BCD Mode
PostPosted: Sat May 04, 2024 5:14 am 
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So I'm working to convert the assembly level functions for the Kowalski emulator into C; that way it can be made portable across platforms.

One thing I've noticed is it seems that it has different BCD behavior for if it's in "Basic 6502" mode (I presume original MOS) and 6501/65C02

It appears that the negative and zero status flags are computed differently depending on which you use.


As I have no access to a MOS 6502 (or a 6501 for that matter) I was curious if anyone could confirm/deny if these variations do in fact have differences in how they calculate these flags.


Trying to make sure I get this conversion as close to accurate as I can get. Thanks!
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barrym95838
PostPosted: Sat May 04, 2024 6:20 am 
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I highly recommend Bruce's BCD article in the tutorials section.

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BigDumbDinosaur
PostPosted: Sat May 04, 2024 7:14 am 
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Yuri wrote:
So I'm working to convert the assembly level functions for the Kowalski emulator into C; that way it can be made portable across platforms.

Pedantic note: the Kowalski package is a simulator...it doesn’t emulate any hardware

Quote:
One thing I've noticed is it seems that it has different BCD behavior for if it's in "Basic 6502" mode (I presume original MOS) and 6501/65C02

The reason for that is only carry is valid when the NMOS 6502 is performing BCD addition and subtraction.  The negative (n) and zero (z) flags are meaningless, as they reflect the result of the underlying binary operation that occurs before decimal correction is applied.

On the other hand, the 65C02’s n and z correctly reflect the BCD result.  This is also true with the 65C816, whether in emulation or native mode.

The overflow (v) flag is an outlier in BCD arithmetic, as the 6502 family can only do unsigned BCD addition and subtraction.  Hence v is effectively meaningless in BCD mode.

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