- what is the perception of setting the decimal flag, does anyone use this mode? I'm not sure why it was included in the architecture.

Decimal mode is good for point-of-sale and vending machines (also for games scores, but the 6502 was designed for embedded compute and industrial control.)

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In the later CMOS versions of the 6502 family (which came along *long* after the Atari consoles), the Decimal flag was explicitly cleared on Reset and upon entering interrupt handlers. That's the behaviour of the latest emulator version.

In the original NMOS versions, the Decimal flag was not altered by the interrupt microcode, which was also used by Reset, so you couldn't rely on the state of D on reset or interrupt entry. Not too much of a problem if you include CLD in your Reset handler and never set it again anywhere else.

Additionally, ADC and SBC in Decimal mode took one cycle extra on CMOS, unlike on the NMOS chips - this was to allow the NZV flags to be updated to correctly reflect the post-decimal-adjusted result; these flags were often wrong on NMOS (but predictably so, and this is one reliable way to distinguish NMOS from CMOS). Apparently this was considered a sufficiently important bug to fix in hardware.