[TUHS] who invented the link register

[Clem Cole]

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."*

ᐧ

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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