Port A is also used as the “slave port”. Bits 2-4 of SPCR control whether it’s all input, all output, a slave port or used for auxiliary I/O (an 8-bit data bus with a portion of port B used as address lines and a pin on port E used as a chip select).
Thanks Tom. I do not find the digi documentation straight forward at all. It took me 2 days to find the SPCR reference. The 4300 documentation Section 5.2.1 Digital I/O is a couple of paragraphs with no mention of the 4000 uP documentation; very frustrating.