[kstenerud]

SNK were the gods of the 68000. I still remember back in the day getting a bug report on my 68000 emulator:

When playing King of Fighters, the time counter would go down to 0 and then wrap around to 99, effectively preventing the round from ending.

Eventually I tracked it down to the behavior of SBCD (Subtract Binary Coded Decimal): Internally, the chip actually does update the overflow flag reliably (it's marked as undefined in the docs). SNK was checking the V flag and ending the round when it got set.

https://github.com/kstenerud/Musashi/blob/master/m68k_in.c#L...

SBCD was an old throwback instruction that was hardly used anymore, and the register variant took 6 cycles to complete (vs 4 for binary subtraction).

HOWEVER... For displaying the timer counter on-screen, they saved a ton of cycles with this scheme because extracting the digits from a BCD value is a simple shift by 4 bits (6 cycles) rather than a VERY expensive divide (140 cycles).

[kevin_thibedeau]

You don't need division to convert to decimal, though it will still be slower than using BCD operations.

[kstenerud]

Technically no, but they were also always fighting against the ROM size, trying to keep costs down. Every byte helped.

[MobiusHorizons]

Oh? How do you do it? Some kind of lookup table?

[kevin_thibedeau]

https://en.wikipedia.org/wiki/Double_dabble

A software implementation with masks and shifts will beat traditional CISC dividers.