MacFormat 1997 October/macformat-055.iso/mac/Shareware Plus/Developers/VideoToolbox/VideoToolboxSources/Seconds.c

C/C++ Source or Header | 1997-05-30 | 4.7 KB | 140 lines | [TEXT/CWIE]

/*
Seconds.c

    s=Seconds();
returns the number of seconds (as a double, with 20 µs or better
accuracy) since the computer started up.

If possible, we use the new UpTime routine, which uses the PPC's
internal clock to get an accurate time with 20 nanosecond or better
precision, but it's only available on PCI PowerMacs. Otherwise we use
the Microseconds routine, which has 20 microseconds precision, and is
part of Mac OS 7.

UpTime offers stable timing even when interrupts are blocked (e.g. by
calls to cscSetEntries). It is my unconfirmed impression that
Microseconds advances at a reduced rate when we're repeatedly calling
cscSetEntries on my PowerMac 7500.

Microseconds() is documented in "Inside Mac: OS Utilities".

UpTime is documented in "Designing PCI Cards and Drivers for Power
Macintosh Computers".

UpTime and Microseconds are briefly described in Martin Minow (1996)
Timing on the Macintosh, develop 26.
<http://devworld.apple.com/dev/techsupport/develop/issue26/minow.html>

The shared library DriverServicesLib must be weak linked with this
project, when compiling for ppc; it's not needed when compiling for 68k.

4/28/97 The latest word from Apple is "The UpTime call will not be
implemented on earlier PowerPCs under Tempo (Mac OS 8). That was the
plan under Maxwell, but is no longer being considered."

HISTORY:
5/8/96    dgp    Wrote the MEX routine Secs.c.
                dhb    Changed name to GetSecs, cosmetic editing.
                dgp Use UpTime if available.
6/6/96    dgp    Omit DriverServicesLib calls when compiling for 68K.
1/26/97    dgp minor changes to allow successful 68K compile.
1/26/97    dgp extracted most of the code from GetSecs.c to create this new VideoToolbox
                        Seconds.c, to make this good timebase more widely available.
2/9/97    dgp    corrected function name in error message.
3/19/97    dgp    minor tuning of the floating point arithmetic
*/
#include "VideoToolbox.h"

#if GENERATINGPOWERPC
    #if UNIVERSAL_HEADERS>1
        #ifndef __CODEFRAGMENTS__
            #include <CodeFragments.h>
        #endif
    #endif
#endif

typedef UnsignedWide AbsoluteTime;    // Types.h
typedef UnsignedWide Nanoseconds;    // DriverServices.h
extern AbsoluteTime UpTime(void);    // DriverServices.h
extern Nanoseconds AbsoluteToNanoseconds(AbsoluteTime absoluteTime);// DriverServices.h

double Seconds(void)
{
    static Boolean firstTime=1,upTimeAvailable=0;
    double s;

    if(firstTime){
        long version;

        // Make sure Microseconds trap is available.
        Gestalt(gestaltSystemVersion,&version);
        if(version<0x700)PrintfExit("%s: your System is too old; we need at least System 7.\n",__FILE__);

        // Is UpTime available?
        // The UpTime glue is in the DriverServicesLib shared library, which is
        // weak-linked, so we must load it before using it. If we fail,
        // we mustn't try to access the library's exports.
        upTimeAvailable=IsDriverServicesLibAvailable();
        firstTime=0;
    }
    if(upTimeAvailable && GENERATINGPOWERPC){
        #if GENERATINGPOWERPC    // we don't want to deal with shared libraries in THINK C
            AbsoluteTime elapsedTime;
            Nanoseconds elapsedNanoseconds;   // an UnsignedWide integer

            elapsedTime=UpTime();
            elapsedNanoseconds=AbsoluteToNanoseconds(elapsedTime);
// NOTE: 4294967296.0 == (double)0x10000*0x10000
            s=elapsedNanoseconds.lo+4294967296.0*elapsedNanoseconds.hi;
            s/=1e9;
        #endif
    }else{
        UnsignedWide microTicks;

        Microseconds(µTicks);
        s=microTicks.lo+4294967296.0*microTicks.hi;
        s*=1e-6;
    }
    return s;
}

Boolean IsDriverServicesLibAvailable(void)
{
    long error;
    Boolean isAvailable;

    if(1){
        // This extra test isn't strictly necessary.
        // However, calling Gestalt is fast, whereas calling GetSharedLibrary requires
        // a disk access, so we only call GetSharedLibrary if we expect it to succeed.
        Boolean pci;
        long value,version;
        #define gestaltNameRegistryVersion 'nreg'    // support old Gestalt.h header

        error=Gestalt(gestaltNameRegistryVersion,&value);
        pci=!error;    // are there PCI slots?
        Gestalt(gestaltSystemVersion,&version);
        isAvailable=(pci || version>=0x800);    // according to Apple's 1995 docs.
    }
    #if GENERATINGPOWERPC && CFMSYSTEMCALLS
        if(isAvailable){
            // The DriverServicesLib shared library is
            // weak-linked, so we must load it before using it. If we fail,
            // we mustn't try to access the library's exports.
            CFragConnectionID connID;
            Ptr mainAddr;
            Str255 errorMessage;
            long value;

            error=Gestalt(gestaltCFMAttr,&value);    // Code Fragment Manager?
            if(!error)error=GetSharedLibrary((ConstStr63Param)"\pDriverServicesLib"
                ,kAnyCFragArch,kFindCFrag,&connID,&mainAddr,errorMessage);
            isAvailable=!error;
            if(error)printf("WARNING: IsDriverServicesLibAvailable: GetSharedLibrary error=%ld, %s\n",(long)error,p2cstr(errorMessage));
        }
    #else
        isAvailable=0;
    #endif
    return isAvailable;
}