[HECnet] DU11 vs. DUV11

[Johnny Billquist]

On 2013-01-06 03:33, John Wilson wrote:
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!)

Just read through the PDT-11/150 manual, and the interrupt fiddling as actually documented. See page 5-19. :-) (Although they don't say *why* you need to jump through that hoop.)

	Johnny

-- 
Johnny Billquist                                   || "I'm on a bus
                                                                  ||   on a psychedelic trip
email: bqt at softjar.se                         ||   Reading murder books
pdp is alive!                                         ||   tryin' to stay hip" - B. Idol