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The Lounge What is on Your Mind? Can assembly language be dispensed with ? Post 302661095 by bakunin on Sunday 24th of June 2012 06:54:52 PM
Old 06-24-2012
I started my career back in the late seventies and on IBM mainframes there was either COBOL or assembler - with the 2.5 KB real memory my first system (already outdated at that time) had the choice was quite clear: learn assembler and every dirty trick there was to save some bits here, some cycles there ....

Later, in the eighties, i wrote software for DSPs and embedded systems, mainly the DSPs of Analog Devices and the Motorola 56k-series. Again, it was assembler to the fullest extent: when you write the internal software for a telephone switchboard and the company wants to sell a few millions the only thing that counts is: how many cycles did you use for that task? The lower cycles, the lower the clock rates could be, which means you can buy lower graded CPUs, which cost exponentially less which means a BIG difference in customer price. If you could save one arithmetic unit or one register, the custom-built processors could be built without these parts which made them cheaper too.

We used to beat optimizing compilers by a very large margin. OK, we couldn't decipher our own code three months after "tape-out" (the tasks got simulated on an ICE and finally the tape with the processor specification was sent to the provider of the DSPs to be built on order), but it was expected to write the next version from scratch anyway.

One of my first jobs on a PC was to write a DOS device driver for a specialized file system for the real-time storage of acoustic data. This was the time of MFM-disks and the data rate for a usual 16-bit sampling in stereo frequency (44.1 kHz) is just a tad below the bandwidth of the ST-506 interface. My driver had to use every ounce of bandwidth it could get just keep up. Mind you, we had one of the brand-new hot 16 MHz NEAT 286, an extremely fast system!

Sure, these are extremes. But ones uses assembly language for the same reason we used to use assembly throughout computing history: for its unparalleled speed and for the sheer control it gives you over the system and its hardware. C is a language nicely suited for rapid prototyping, but i'm still quite confident to be able to beat any C-compiler in terms of speed of execution with hand-crafted assembler code. Even with all those nifty optimizations switched on.

This is nothing to say against C - in fact i like C. But to really understand what C is about you have to have experienced the problems solved/avoided by its use. You have to have been at the very bottom at least once to appreciate being on top. And, who knows, you might even start to like being at the source of things.

bakunin
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PAPI_epc(3)							       PAPI							       PAPI_epc(3)

NAME
PAPI_epc - Simplified call to get arbitrary events per cycle, real and processor time. SYNOPSIS
Detailed Description C Interface: #include <papi.h> int PAPI_epc( int event, float *rtime, float *ptime, long long *ref, long long *core, long long *evt, float *epc ); Parameters: event event code to be measured (0 defaults to PAPI_TOT_INS) *rtime total realtime since the first call *ptime total process time since the first call *ref incremental reference clock cycles since the last call *core incremental core clock cycles since the last call *evt total events since the first call *epc incremental events per cycle since the last call Return values: PAPI_EINVAL The counters were already started by something other than PAPI_epc(). PAPI_ENOEVNT One of the requested events does not exist. PAPI_ENOMEM Insufficient memory to complete the operation. The first call to PAPI_epc() will initialize the PAPI High Level interface, set up the counters to monitor the user specified event, PAPI_TOT_CYC, and PAPI_REF_CYC (if it exists) and start the counters. Subsequent calls will read the counters and return total real time, total process time, total event counts since the start of the measurement and the core and reference cycle count and EPC rate since the latest call to PAPI_epc(). A call to PAPI_stop_counters() will stop the counters from running and then calls such as PAPI_start_counters() or other rate calls can safely be used. PAPI_epc can provide a more detailed look at algorithm efficiency in light of clock variability in modern cpus. MFLOPS is no longer an adequate description of peak performance if clock rates can arbitrarily speed up or slow down. By allowing a user specified event and reporting reference cycles, core cycles and real time, PAPI_epc provides the information to compute an accurate effective clock rate, and an accurate measure of computational throughput. See Also: PAPI_flips() PAPI_flops() PAPI_ipc() PAPI_stop_counters() Author Generated automatically by Doxygen for PAPI from the source code. Version 5.2.0.0 Tue Jun 17 2014 PAPI_epc(3)
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