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posted by cmn32480 on Wednesday May 23 2018, @06:47PM   Printer-friendly
from the your-computer-is-not-a-fast-PDP-11 dept.

Very interesting article at the IEEE ACM by David Chisnall.

In the wake of the recent Meltdown and Spectre vulnerabilities, it's worth spending some time looking at root causes. Both of these vulnerabilities involved processors speculatively executing instructions past some kind of access check and allowing the attacker to observe the results via a side channel. The features that led to these vulnerabilities, along with several others, were added to let C programmers continue to believe they were programming in a low-level language, when this hasn't been the case for decades.


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  • (Score: 2, Interesting) by pTamok on Thursday May 24 2018, @10:28AM

    by pTamok (3042) on Thursday May 24 2018, @10:28AM (#683490)

    The more things change, the more they stay the same.

    I used to know a technical genius who specialised in VAX/VMS programming for oil exploration companies. His job was to write programs that analysed the geophysical data coming back from the geological survey teams - basically, huge amounts of data listening to the echoes after precise explosions (or at least, loud noises). Time was money, so he was paid to optimise the programming, so he did not only program in VAX assembler, but also re-programmed the microcode of the cpus to get better performance for these highly specific tasks. The DEC VAX 11/780 loaded its microcode from a floppy, so it was entirely possible to modify it, and indeed there was even a Pascal complier that targeted the 11/780s microcode [dtic.mil] as its output. DEC provided support tools for people to be able to do this: "User microprogramming". See references here: https://people.cs.clemson.edu/~mark/uprog.html [clemson.edu]

    ...a study of pattern matching on the VAX-11/750 and reported that a microcoded implementation was 14-28 times faster in execution performance than hand-coded assembly language or a compiled C program [Lar82]. In 1978, Bell, Mudge, and McNamara estimated microprogramming productivity at 700 microinstructions per man year [Bel78]. This meant that microprogramming was five to ten times more expensive than conventional programming of same task, but it was estimated that microprogramming was usually ten times faster in performance.

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