The Wheeler Jump is a type of
subroutine call
In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times.
Callable units provide a p ...
methodology that was used on some early computers that lacked hardware support for saving the return address. The concept was developed by
David Wheeler while working on the pioneering
EDSAC
The Electronic Delay Storage Automatic Calculator (EDSAC) was an early British computer. Inspired by John von Neumann's seminal ''First Draft of a Report on the EDVAC'', the machine was constructed by Maurice Wilkes and his team at the Universit ...
machine in the 1950s.
EDSAC had not been built with subroutines in mind, and lacked a suitable
processor register
A processor register is a quickly accessible location available to a computer's processor. Registers usually consist of a small amount of fast storage, although some registers have specific hardware functions, and may be read-only or write-onl ...
or a
hardware stack that 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 the
accumulator and 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 expressing
recursion
Recursion occurs when the definition of a concept or process depends on a simpler or previous version of itself. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in m ...
.
The addition of new registers for this sort of duty was a key design goal of
EDSAC 2.
Example
This example demonstrates the technique using a pseudo-
assembler language
In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence bet ...
for 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
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History of computing in the United Kingdom
Subroutines