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

i tried to explain it well. i really dint think this was so complex untill this little proof

my algorithm goes like so
if valHi > valLo then
result_cw = valHi - valLo
result_ccw = 512 - (valHi - valLo) = 512 + valLo - valHi

yours states mask = $01
result_cw = valHi - valLo
result_ccw = valLo - valHi // 256 =  [calcultator] ((65536 + valLo - valHi) / 256 - 255) * 256)

((65535 + valLo - valHi) / 256 - 255) * 256 ?= 512 + valLo - valHi
65536 + valLo - valHi - 65280 ?= 512 + valLo - valHi
256 + valLo - valHi) ?= 512 + valLo - valHi
256 != 512

ok so your algy wraps around 255 (8bits)
if we change the mask to $03...
result_ccw = valLo - valHi // 512 = [calculator] ((65536 + valLo - valHi) / 512 - 127) * 512
66536 + valLo - valHi - 65024 = 512 + valLo - valHi
512 == 512 *SUCCESS*

so you can also div by the wrap around value to do circular arithmatic! i honestly did not think it was that simple
many thanks tracy

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