Sorry - cryptic title
OK I need to read in a 32bit (8 Bytes) number from four registers of an external device, then add a 24 bit number (6 Bytes) to it and finally load the resultant back into a device.
The reading and writing bit I can do but how do I achieve the individual byte value addition with the obvious carry AND be efficient?
e.g. FF 1C FF A4
+ 15 F9 00
= FF 32 F8 A4
I know Martin posted something similar to this somewhere but I can't find it

Thanks