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

to fix test for an overflow condition, on overflow add "wrap around value" to result

rotosub_9bit
        cjb	value1, result1, r9_no_overflow	; if v > r then overflow
        cjb	value0, result0, r9_no_overflow 
						; on overflow r = r + 512 - v
	or	result1, #$02			; r = r + 512
r9_no_overflow
	mov	w, value0			; r = r - v
        sub	result0, w
	mov	w, value1
	sc
	movsz	w, ++value1
	sub	result1, w
	ret

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