[TUHS] who invented the link register

Marc Donner marc.donner at gmail.com
Wed Oct 26 06:05:41 AEST 2022

There was a guy at IBM research who had a patent framed on his wall that he
claimed was for indirect addressing.  Sadly, I don't remember his name.
I'll ask Peter Capek if he remembers who it was.

On the other hand, I'm now reading a book called "Turing's Cathedral" that
goes into considerable length talking about what is essentially indirect
addressing in the context of the evolution of the Von Neumann model and its
mindthegapdialogs.com/home <https://www.mindthegapdialogs.com/home>

On Tue, Oct 25, 2022 at 2:27 PM Clem Cole <clemc at ccc.com> wrote:

> I agree that sounds pretty conclusive. I knew Wheeler had used his JUMP
> with EDSAC, I had been wondering if Wilkes had something in his machine
> (EDSAC II) - sounds like it was proposed. But I would not be surprised if
> the idea was Wilkes, but Whirlwind implemented it.   They all talked to
> each other.
> With apologies to Tom Lehrer ...
> *"And then I write*
> *By morning, night,*
> *And afternoon,*
> *And pretty soon*
> *My name in Dnepropetrovsk is cursed, When he finds out I publish first."*
> On Tue, Oct 25, 2022 at 1:01 PM Lawrence Stewart <stewart at serissa.com>
> wrote:
>> I’ve just spent a fun hour looking at the old Whirlwind documents.  I
>> think I agree with Angelo.
>> The 1947 block diagrams and time-pulse charts show that the original “SP”
>> (subprogram) instruction transferred the low 11 bits of the instruction
>> directly to the program counter.  They do not show the old program counter
>> being saved in the AR register, nor is there yet the “TA” (transfer
>> address) instruction to save the AR register to memory.
>> Evidently both these new features, which together provide a branch and
>> link function were likely described in memo M-647, which is not scanned
>> anywhere I can find.  It is called “Some new orders for WWI"
>> There was already logic for the program counter to drive the bus, and
>> logic to capture the bus into the AR register, so the modification to SP to
>> save the old program counter was likely pretty easy: drive the bus from the
>> program counter, and capture it in AR, just by adding some new diodes to
>> the sequencer.
>> Adding the Transfer Address instruction was likely also pretty easy,
>> since there was a way for the AR register to drive the bus.
>> With the new SP and TA, one would use SP to call a subroutine, and the
>> first instruction of any subroutine would be TA to save the return address
>> into the final location of the subroutine.  (TA only modified the low 11
>> bits of the 16 bit location)
>> Before these instructions, a subroutine call would require one additional
>> memory location, to hold the return address for each point of call, and one
>> additional instruction, one to load the return address into the accumulator
>> and one to store it into the code at the end of the subroutine. (The latter
>> could be the first instruction of the subroutine.)
>> Originally I thought that maybe David Wheeler invented the Link register,
>> since he’s often credited with inventing the subroutine, but it looks like
>> the particular thing he did was the idea of the “Wheeler Jump” where code
>> explicitly stores the return address into the instruction at the end of the
>> subroutine.  That idea was used in Whirlwind as well.  EDSAC I did not have
>> link, but it was proposed for EDSAC II.  Whirlwind was likely first to
>> implement.
>> > On 2022, Oct 25, at 4:35 AM, Angelo Papenhoff <aap at papnet.eu> wrote:
>> >
>> > On 25/10/22, Angelo Papenhoff wrote:
>> >> Might be earlier than this, I just happen to know the Whirlwind
>> somewhat
>> >> well. It's late 40s machine, so you probably won't find anything *much*
>> >> older.
>> >
>> > Addendum: the original report from 1947 does not describe this behaviour
>> > yet. The change came in oct. 1948. M-668 mentions it and refers to
>> M-647,
>> > which however is not available online.
>> > So the concept of saving the resturn address in another register is at
>> > least as old as oct. 1948, but again I wouldn't be surprised if some
>> > even slightly earlier computer had it too.
>> >
>> > aap
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