[Lazarus] How to program with time in milliseconds?

[Mark Morgan Lloyd]

Michael Schnell wrote:
> On 05/12/2014 10:37 PM, Henry Vermaak wrote:
>> On linux, glibc (and others) will route gettimeofday() (and 
>> clock_gettime() for certain clock IDs) via vDSO and no syscall will be 
>> called, so it's very fast. I don't think the fpc rtl does this, though? 
> 
> I checked with X86 32 Bit Linux: you are right.
> 
> e.g. http://x86.renejeschke.de/html/file_module_x86_id_277.html
> 
> talks about the ASM instruction "RDPMC":

That's hardly surprising, since the various processor ID etc. registers 
were available in real mode.

>> When in protected or virtual 8086 mode, the performance-monitoring 
>> counters enabled (PCE) flag in register CR4 restricts the use of the 
>> RDPMC instruction as follows. When the PCE flag is set, the RDPMC 
>> instruction can be executed at any privilege level; when the flag is 
>> clear, the instruction can only be executed at privilege level 0.
> 
> Hence, I don't know if accessing the " performance-monitoring counter" 
> this is possible / viable in the fpc RTL.

There's an obvious pitfall there: a poor implementation might /think/ 
that it's directly accessing the counters when in actual fact it's being 
virtualised or fixed up by a signal handler. There's debate elsewhere 
about inadvertently relying on the OS to fix up e.g. alignment errors, 
with a vast performance hit being suggested for e.g. Solaris if poor 
coding practice forces the OS to straighten things out.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

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