Guy,
I understand that cylinder command has no particular timing requirements, while 
head command must be effective within microseconds. My doubt is, RAM access on 
high performance port could be fast enough to satisfy also the latter.
In case it couldn't or was not assured, I think the best strategy could be to 
preload only a small block of data for each head, for prompt start on head 
command; enough to manage safely RAM access latency.
Each block also would work as buffer for data of subsequent RAM accesses, until 
whole cylinder had been processed.
This strategy would remove the strict requirement of blockram capacity for the 
Zynq, and given that bigger models cost a lot, it would be a significant spare 
for anybody.
Furthermore,  support for any hypotetical disk with bigger cylinder (not SMD) 
or for tape with very large blocks or "infinite" streams could not be feasible 
with the whole cylinder design. I would prefer to avoid any such limitation, in 
way to possibly reuse the same data transfer modules for any media.

Andrea

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