This routine copies a block of memory, starting at the hi address, and working down to the low address. The base of the source address is specified in the U register, and the base of the destination address is specified in the X register. The A register contains the number of bytes to copy; 0x80 is the maximum value which can be specified.
A-reg = byte count (Move_Mem_a only)
A-reg = byte count minus 1 (Move_Mem_a_1 only)
X-reg points to the destination
U-reg points to the source
A-reg = $FF
B-reg = first byte of source