[HECnet] DU11 vs. DUV11

[John Wilson]

From: "Brian Schenkenberger, VAXman-" <system at TMESIS.COM>

OK, I'll bite.   Why is moving a character in the deferred location in
R5 to the stack and then, from the stack to the address in R4 faster 
than just going from the deferred R5 location to the R4 address?

I don't know the exact answer, but the I/O page in a PDT-11 is emulated by
an 8085A and it's always super slow.   I don't know why DATOB would be any
worse than DATO but if DEC thought it was, I'm sure it's true (it shouldn't
have to be a read-modify-write but some PDP-11 models do gratuitous extra
cycles so it may well be).   So this isn't actually a Q vs. U difference, it's
a PDT vs. real bus difference, but the LSI-11 conditionals will catch PDTs.

Anyway, thanks Johnny!   Good to know that DU and DUV are 2 for the price of 1.
(And it hadn't even clicked that the thing in a PDT is emulating a DUV, so I
guess it's 3 for the price of 1!)

John Wilson
D Bit