From: utzoo!decvax!duke!bcw Newsgroups: net.unix-wizards Title: Re: DZ-11 vs DH-11 Article-I.D.: duke.2531 Posted: Mon Sep 13 22:06:32 1982 Received: Tue Sep 14 06:15:04 1982 From: Bruce C. Wright @ Duke University Re: DZ-11 vs DH-11 It is entirely reasonable that there are tradeoffs between the DZ-11 and the DH-11. The DH-11 requires the acquisition of things like buffered data paths (on the VAX), and freeing them when done; it also requires some extra overhead for setting up the dma transfer registers. The considerations would look something like: DZ-11 time = per_io_setup_time + length_of_transfer * (character_translation_time + interrupt_time) DH-11 time = per_io_setup_time + length_of_transfer * character_translation_time + interrupt_time + dma_setup_time where: per_io_setup_time is the time to set up the i/o operation (read and check parameters, etc) length_of_transfer is the transfer size in bytes character_translation_time is the time to translate things like tabs and so forth (you will have to scan the string and move the translated characters to some new buffer) interrupt_time is the length of an interrupt dma_setup_time is the time required to set up the dma transfer. If the transfer is short enough (1 or two bytes), the DZ-11 will clearly win since it will not have the dma setup time, and will have the same number of interrupts as the dh-11. If the transfer is long enough, the DH-11 will win because all those interrupts will not happen for the dh. The only question is where the tradeoff point is, which is going to be hardware and operating-system dependent. I have heard that this happens around 10-15 characters, but have not done any timing myself; the claim was by someone with an ax to grind (DEC maybe, or maybe Able), so I'm not sure that the numbers haven't been fiddled. For many systems (especially if they tend to do single-character or only a few-character I/O), the DH-11 will clearly not be worth it. The DMF-32 has both programmed I/O (like the DZ) and dma I/O (like the DH), so the device driver can choose the most efficient method, so this looks like a better choice in the long run (though I think the DMF-32 is still not intelligent enough...). Bruce C. Wright @ Duke University