27 Oct
2010
27 Oct
'10
5:41 p.m.
From: Fred Cisin
Sent: Wednesday, October 27, 2010 2:21 PM
>> I do believe the PDP-11's use of
memory-mapped I/O was original - at least I
>> can't think of any earlier examples. -- Ian
On Wed, 27 Oct 2010, Ethan Dicks wrote:
> It's the earliest one I am aware of, but my knowledge of non-DEC stuff
> before 1970 is admittedly full of gaps.
> I started with the 6502 (in 1977), so I thought memory-mapped I/O was
> "normal". The 1802 was the first processor I used that had I/O
> instructions.
> So I guess since any processor that has gaps in its memory map _could_
> have those gaps filled with memory-mapped I/O, the question is _did_
> any implement it prior to 1969/1970?
My memories from ~1968 are not very clear, and
definitions of
memory-mapped I/O may vary, . . .
Didn't the 1401 sort-of have it? Not fully automatic memory-mapped I/O
such as the TRS80 and PC video RAM, more like the CP/M and PC's PSP DTAs.
I seem to recall there being a command to read a card into a fixed buffer
in memory, and another to write a card from anbother fixed buffer. I
remember "cheating" and using those buffer spaces when I ran short of
space writing short programs on the 1401 emulator on the 1620.
The way 1401 addressing works, you address the high-numbered character and
everything is operated on towards the first lower numbered character with
the Word Mark bit set. Locations 80, 180, and 332 are special: A MOVE
instruction pointed at location 80 as the source reads a card from the 1402,
a MOVE with location 180 as the destination punches a card on the 1402, and
a MOVE with 332 as the destination prints a line on the 1403. Word marks
are set in locations 1, 101, and 201.
I don't know how the disks (or tapes, if the system had them) handled things.
I saw Autocoder as a losing proposition by then.
(Lord, that's dredging up 40+ year old memories. I stuck to the 360/370
family once I graduated high school, until I encountered the DEC-20 eight
years later in grad school.)
Rich Alderson
Vintage Computing Sr. Server Engineer
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104
mailto:RichA at vulcan.com
mailto:RichA at LivingComputerMuseum.org
http://www.PDPplanet.org/
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