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0 pointsinkyoto7mo ago0 comments

> The only reason why they have chosen "" as prefix in C, which they later regretted, was because it seemed easier to define the expressions "++p" and "*p++" to have the desired order of evaluation.

There has been no shortage of speculation, much of it needlessly elaborate. The reality, however, appears far simpler – the prefix pointer notation had already been present in B and its predecessor, BCPL[0]. It was not invented anew, merely borrowed – or, more accurately, inherited.

The common lore often attributes this syntactic feature to the influence of the PDP-11 ISA. That claim, whilst not entirely baseless, is at best a partial truth. The PDP-11 did support pre-increment and post-increment indirect address manipulation – but notably lacked their symmetrical complements: pre-increment and post-decrement addressing modes[1]. In other words, it exhibited asymmetry – a gap that undermines the argument for direct PDP-11 ISA inheritance, i.e.

  MOV (Rn)+, Rm

  MOV @(Rn)+, Rm

  MOV -(Rn), Rm

  MOV @-(Rn), Rm

existed but not

  MOV +(Rn), Rm

  MOV @+(Rn), Rm

  MOV (Rn)-, Rm

  MOV @(Rn)-, Rm

[0] https://www.thinkage.ca/gcos/expl/b/manu/manu.html#Section6_...

[1] PDP-11 ISA allocates 3 bits for the addressing mode (register / Rn, indirect register (Rn), auto post-increment indirect / (Rn)+ , auto post-increment deferred / @(Rn)+, auto pre-decrement indirect / -(Rn), auto pre-increment deferred / @-(Rn), index / idx(Rn) and index deferred / @idx(Rn) ), and whether it was actually «let's choose these eight modes» or «we also wanted pre-increment and post-decrement but ran out of bits» is a matter of historical debate.

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adrian_b7mo ago

The prefix "*" and the increment/decrement operators have been indeed introduced in the B language (in 1969, before the launch of PDP-11 in 1970, but earlier computers had some autoincrement/autodecrement facilities, though not as complete as in the B language), where "*" has been made prefix for the reason that I have already explained.

The prefix "*" WAS NOT inherited from BCPL, it was purely a B invention due to Ken Thompson.

In BCPL, "*" was actually a postfix operator that was used for array indexing. It was not the operator for indirection.

In CPL, the predecessor of BCPL, there was no indirection operator, because indirection through a pointer was implicit, based on the type of the variable. Instead of an indirection operator, there were different kinds of assignment operators, to enable the assignment of a value to the pointer, instead of assigning to the variable pointed by the pointer, which was the default meaning.

BCPL has made many changes in the syntax of CPL, whose main reason was the necessity of adapting the language to the impoverished character set available on American computers, which lacked many of the characters that had been available in Europe before IBM and a few other US vendors have succeeded to replace the local vendors, also imposing thus the EBCDIC and later the ASCII character sets.

Several of the changes done between BCPL and B had the same kind of reason, i.e. they were needed to transition the language from an older character set to the then new ASCII character set. For instance the use of braces as block delimiters was prompted by their addition into ASCII, as they were not available in the previous character set.

The link that you have provided to a manual of the B language is not useful for historical discussions, as the manual is for a modernized version of B, which contains some features back-ported from C.

There is a manual of the B language dated 1972-01-07, which predates the C language, and which can be found on the Web. Even that version might have already included some changes from the original B language of 1969.

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