23 Jul
2003
23 Jul
'03
8:18 p.m.
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.
Actually, the I/O page is defined as the highest 8kb (4kw) in memory. For
a 16-bit machine, that is indeed 28kw-32kw. But it is not correct for an
18-bit or 22-bit machine.
The best way to think of it is simply 8kb which is discontiguous with, and
above, the available memory. Access to it is triggered by use of '1' in
the high-order 3 places of a sixteen bit address.
Of course if you have MMGT turned on, then these bits simply select the
appropriate page, which may or may not be the I/O page.
Megan Gentry
Former RT-11 Developer
+--------------------------------+-------------------------------------+
| Megan Gentry, EMT/B, PP-ASEL | email: mbg at world.std.com |
| Member of Technical Staff | megan at savaje.com |
| SavaJe Technologies, Inc. | (s/ at /@/) |
| 100 Apollo Drive | URL: http://world.std.com/~mbg/ |
| Chelmsford, MA 01460 | "pdp-11 programmer - some assembler |
| (978) 256 6521 (DEC '77-'98) | required." - mbg KB1FCA |
+--------------------------------+-------------------------------------+
Formerly with DEC/Compaq/HP