need to specify:
virtual channel (2 bits)
slice (2 bits)
address (21 bits)

how to do reads/writes?
how to handle PGP nacks/timeouts?
how to do read-modify-writes?
bad block protocol?
how to avoid junk down the link?
response to PGP errors?
response to fifo-full and almost-fifo-full?
how to use virtual channels?
when do we reset PGP? (consistent policy with ryan)
how to give reads priority?
interface to PIC? (how to learn DMA addresses)

Ryan commands:
- reset
- board ID
- firmware version
- remote PGP counters
- environmental info (temps, voltages, PHY info)

Leonid commands:
- flash attributes
- fmc attributes
- counters
- erase block
- get blocks ("code blocks")
- set page