1401 "memory-mapped I/O" [was RE: Happy Birthday VAX 11/780 (influence of)]

Rich Alderson RichA at vulcan.com
Wed Oct 27 16:41:21 CDT 2010


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

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