In SX Microcontrollers, SX/B Compiler and SX-Key Tool, bean wrote:

Let's say you want to subtract $FF01 from $1000
 

First you would subtract the 1 from zero, and that would cause a underflow (carry clear).

 

Then W would get loaded with $FF +1 (zero), so the final SUB would NOT cause a underflow, but it should.

 

But having the SZ we skip the SUB leaving the carry clear to indicate the underflow and still get the right result.

 

 

I have fixed the typo. Try this version...

 

Bean.

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