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UBC CS TR-80-09 Summary

Why Is a Goto Like a Dynamic Vector in the BCPL-Slim Computing System, November 1980 Harvey Abramson

The Slim computer is a new virtual machine which can be used in the translation and porting of the BCPL compiler, and eventually, in the porting of an operating system written in BCPL. For the purposes of this paper, the Slim computer is a stack machine with a single accumulator and a register which points to the top of the stack. The procedures LEVEL and LONGJUMP, traditionally used to implement transfers of control across BCPL procedures, and which are usually written in the assembler language of a host machine, cannot be used with this architecture. In developing procedures to implement \underline{all} transfers of control, we show how these essential procedures --- though highly dependant on the slim architecture --- can be written portably in BCPL, and discover an interesting connection between implementing jumps and dynamic vectors (by means of Aptovec) in the BCPL-Slim computing system. Some parameters of portability in rapping an abstract machine to host machines are identified, and it is shown how to maintain the portability of the above mentioned procedures in the face of various mapping problems. Finally, we are led to a comment on the design of BCPL to the effect that \underline{goto}'s are an unnecessary feature of the language.

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