[TUHS] Re: who invented the link register

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(a)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(a)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 > >