Complex sentence: A very small machine, with a 6-bit wide "mill" (arithmetic unit), it was still compatible with the other machines, a 24-bit operation would be performed by the processor as four 6-bit operations.
Split into simpler sentences: A very small machine with a 6-bit wide "mill" (arithmetic unit). For compatability with the other machines a 24-bit operation was performed by the processor as four 6-bit operations.