T11 design WAS - Re: Inside old games machines, was: Re: Simulated CP/M-68K?

Allison ajp166 at bellatlantic.net
Wed Jun 20 17:44:06 CDT 2007

>Subject: Re: T11 design WAS - Re: Inside old games machines,was: Re: Simulated CP/M-68K?
>   From: "Ethan Dicks" <ethan.dicks at gmail.com>
>   Date: Wed, 20 Jun 2007 16:22:25 -0400
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>On 6/20/07, Roger Ivie <rivie at ridgenet.net> wrote:
>> On Wed, 20 Jun 2007, Allison wrote:
>> > It's an easy cpu to interface and use...
>> However, it's not without its obnoxious bits.
>> On the T-11, all writes are performed as read-modify-writes.
>Hmm... was that because of the needs of core memory, or was it just to
>simplify some aspect of its internal design?

It's part of the memory to memory design and the way intructions work.

It's annying as micros go but ALL PDP-11s word that way and the T-11
is a PDP-11 in LSI.

>RMW cycles are typically no problem with main memory, but I've seen
>problems with them in a number of situations involving I/O, typically
>involving "clever" peripherals that take a read of some register as a
>trigger signal, but also with some 68000-family designs where you
>weren't allowed to use instructions in your code which invoked RMW
>cycles because they weren't interruptible (intentionally) and could
>cause interrupt or instruction-restart problems.

Yep the fix is easy.  The read address is dirrent from the write address.
So when the system doe sthe read before write or Read modify write
it only touches the address register for thatread or write as desired.

>At least with the 68000, RMW cycles were rarely done by real-world
>code, and never accidentally.
>With a from-scratch T-11 SBC, one could, I'd expect, select I/O
>peripherals that wouldn't be subject to side-effects (like clearing
>status bits on reads), but that's a lot harder when you have a mandate
>to use a particular vendor's chip and the chip tries to be clever.

Easy, make the read and write different addresses.  Just like PDP-11s do.


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