Archive

Archive for January, 2018

Learning a cpu’s instruction set

January 25th, 2018 No comments

A few years ago I wrote about the possibility of secret instruction sets making a comeback and the minimum information needed to write a code generator. A paper from the sporadic (i.e., they don’t release umpteen slices of the same overall paper), but always interesting, group at Stanford describes a tool that goes a long way to solving the secret instruction set problem; stratified synthesis learns an instruction set, starting from a small set of known instructions.

After feeding in 51 base instructions and 11 templates, 1,795.42 instruction ‘formulas’ were learned (119.42 were 8-bit constant instructions, every variant counted as 1/256 of an instruction); out of a maximum of 3,683 possible instructions (depending on how you count instructions).

As well as discovering ‘new’ instructions, they also discovered bugs in the Intel 64 and IA-32 Architectures Software Developer Manuals. In my compiler writing days, bugs in cpu documentation were a pet hate (they cause huge amounts of time to be wasted).

The initial starting information used is rather large, from the perspective of understanding the instruction set of an unknown cpu. I’m sure others will be working to reduce the necessary startup information needed to obtain useful results. The Intel Management Engine is an obvious candidate for investigation.

Vendors sometimes add support for instructions without publicizing them and sometimes certain bit patterns happen to do something sensible in a particular version of a design because some random pattern of bits happens to do whatever it does without being treated as an illegal opcode. You journey down the rabbit hole starts here.

On a related note, I continue to be amazed that widely used disassemblers fail to correctly handle surprisingly many, documented, x86 opcodes; benchmarks from 2010 and 2016

First use of: software, software engineering and source code

January 16th, 2018 No comments

While reading some software related books/reports/articles written during the 1950s, I suddenly realized that the word ‘software’ was not being used. This set me off looking for the earliest use of various computer terms.

My search process consisted of using pfgrep on my collection of pdfs of documents from the 1950s and 60s, and looking in the index of the few old computer books I still have.

Software: The Oxford English Dictionary (OED) cites an article by John Tukey published in the American Mathematical Monthly during 1958 as the first published use of software: “The ‘software’ comprising … interpretive routines, compilers, and other aspects of automotive programming are at least as important to the modern electronic calculator as its ‘hardware’.”

I have a copy of the second edition of “An Introduction to Automatic Computers” by Ned Chapin, published in 1963, which does a great job of defining the various kinds of software. Earlier editions were published in 1955 and 1957. Did these earlier edition also contain various definitions of software? I cannot find any reasonably prices copies on the second-hand book market. Do any readers have a copy?

Software engineering: The OED cites a 1966 “letter to the ACM membership” by Anthony A. Oettinger, then ACM President: “We must recognize ourselves … as members of an engineering profession, be it hardware engineering or software engineering.”

The June 1965 issue of COMPUTERS and AUTOMATION, in its Roster of organizations in the computer field, has the list of services offered by Abacus Information Management Co.: “systems software engineering”, and by Halbrecht Associates, Inc.: “software engineering”. This pushes the first use of software engineering back by a year.

Source code: The OED cites a 1965 issue of Communications ACM: “The PUFFT source language listing provides a cross reference between the source code and the object code.”

The December 1959 Proceedings of the EASTERN JOINT COMPUTER CONFERENCE contains the article: “SIMCOM – The Simulator Compiler” by Thomas G. Sanborn. On page 140 we have: “The compiler uses this convention to aid in distinguishing between SIMCOM statements and SCAT instructions which may be included in the source code.”

Running pdfgrep over the archive of documents on bitsavers would probably throw up all manners of early users of software related terms.

Computer books your great grandfather might have read

January 12th, 2018 No comments

I have been reading two very different computer books written for a general readership: Giant Brains or Machines that Think, published in 1949 (with a retrospective chapter added in 1961) and LET ERMA DO IT, published in 1956.

‘Giant Brains’ by Edmund Berkeley, was very popular in its day.

Berkeley marvels at a computer performing 5,000 additions per second; performing all the calculations in a week that previously required 500 human computers (i.e., people using mechanical adding machines) working 40 hours per week. His mind staggers at the “calculating circuits being developed” that can perform 100,00 additions a second; “A mechanical brain that can do 10,000 additions a second can very easily finish almost all its work at once.”

The chapter discussing the future, “Machines that think, and what they might do for men”, sees Berkeley struggling for non-mathematical applications; a common problem with all new inventions. Automatic translator and automatic stenographer (typist who transcribe dictation) are listed. There is also a chapter on social control, which is just as applicable today.

This was the first widely read book to promote Shannon‘s idea of using the algebra invented by George Boole to analyze switching circuits symbolically (THE 1940 Masters thesis).

The ‘ERMA’ book paints a very rosy picture of the future with computer automation removing the drudgery that so many jobs require; it is so upbeat. A year later the USSR launched Sputnik and things suddenly looked a lot less rosy.

Added two more books

Cybernetics Or Communication And Control In The Animal And The Machine by Norbert Wiener

The Organization Of Behavior A Neuropsychological Theory by D. O. Hebb

and another

The Preparation of Programs for an Electronic Digital Computer by Wilkes, Wheeler, Gill (the link is to the 1957 second edition, not the 1951 first edition)

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Was a C90, C99, or C11 compiler used?

January 2nd, 2018 2 comments

How can a program figure out whether it has been compiled with a C90, C99 or C11 compiler?

Support for the // style of commenting was added in C99.

Support for Unicode string literals (e.g., U"Hello World") was added in C11.

Putting these together we get the following:

#include <stdio.h>
 
#define M(U) sizeof(U"s"[0])
 
int main(void)
{
    switch(M("")*2 //**/ 2
                          )
       {
       case 1: printf("C90\n"); break;
       case 2: printf("C99\n"); break;
       case 8: printf("C11\n"); break;
       }
 
}
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