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

For subtracting this is generated

  5488  01B8  0218        SUB dacValue_LSB,adcValue_LSB  ;dacValue = dacValue - adcValue
        01B9  00B6
  5489  01BA  0703        SUBB dacValue_MSB,/C          
        01BB  00F7
  5490  01BC  0219        SUB dacValue_MSB,adcValue_MSB 

But again underflow may be missed.

Alternative code from sxlist.com
  mov W, adcValue_LSB
	  sub dacValue_LSB,w
  mov w,adcValue_MSB
  sc
  movsz w,++adcValue_MSB
  sub dacValue_MSB,w ;  dest=dest-source, with valid carry (although the Z flag is not valid).

regards peter

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