Data Systems Design DSD-880 8" floppy and hard drive?

From: Fred N. van Kempen <waltje_at_pdp11.nl>
Date: Wed Jul 23 03:29:00 2003

On Wed, 23 Jul 2003 rogersda_at_cox.net wrote:

> Actually, for device I/O, it doesn't matter whether
> one is using 16, 18, or 22 bit addressing. The PDP-11
> I/O page is defined as from 28KW-32KW, which is entirely
> within the range of 16bit addressing.
No, because the MMU maps this space from the upper 4KW in
*virtual* space (the visible 64KB space the CPU can address)
to the upper 4KW in *physical* space, meaning:

16b: 28-32K
18b: 124-128K
22b: 2044-2048K

Depending on how the OS maps its ring-0 address space, this
might not matter directly to the software. However, devices
doing DMA need to know the *physical* address of the buffer
in memory, not the virtual one :)

So.. DMA devices are usually the cause of trubs with 16/18b
addressing in 22b systems. Try installing an RK11 in an
11/44 or 11/70 and do NOT do anything special :)

(yes, that's why they came up with the Unibus map...)

--f
Received on Wed Jul 23 2003 - 03:29:00 BST