TheWheeler Jumpis a type ofsubroutine callmethodology that was used on some early computers that lacked hardware support for saving the return address. The concept was developed byDavid Wheelerwhile working on the pioneeringEDSACmachine in the 1950s.[1]EDSAC had not been built with subroutines in mind, and lacked a suitableprocessor registeror ahardware stackthat might allow the return address to be easily stored.
Wheeler's solution was a particular way to write the subroutine code. To implement it, the last line of the subroutine was a "jump to this address" instruction, which would normally be followed by a memory location. In a Wheeler subroutine, this address was normally set to a dummy number, say 0.
To call the routine, the address of the caller would be placed in theaccumulatorand then the code would jump to the starting point of the routine. The first instructions in the routine would calculate the return address based on the value in the accumulator, typically the next memory location so an increment will suffice, and then write the result to the dummy address previously set aside. When the routine runs its course it naturally reaches the end of the routine which now says "jump to the return address".
As writing to memory is a slow process compared to register access, this methodology is not particularly fast. It also is not capable of expressingrecursion.[2]The addition of new registers for this sort of duty was a key design goal ofEDSAC 2.
Example
editThis example demonstrates the technique using a pseudo-assembler languagefor a simple byte-oriented accumulator-based machine with a single register, A:
'prepare to call the subroutine 10 COPY PC TO A ' copy the program counter (10) into the accumulator 11 JUMP ' jump to... 12 70 '... location 70 ... many more lines... 70 ADD CONST ' add the following value to the accumulator... 71 3 '... three locations past the original PC value 72 STORE ' store the value in the accumulator to... 73 91 '... the set-aside memory location ... lines that perform the actual subroutine... 90 JUMP ' return to... 91 0 '... which will be replaced by 13
When this code completes, the JUMP instruction in address 90 will naturally return to location 13, the next instruction after the subroutine.
References
edit- ^Hill, Mark (2000).Readings in Computer Architecture.Gulf Professional Publishing. p. 3.ISBN9781558605398.
- ^Prof. David Brailsford, Sean Riley (6 February 2018).The Wheeler Jump.Computerphile; School of Computer Science, University of Nottingham, UK.Retrieved2023-03-30.
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