TweetFollow Us on Twitter

Oct 95 Tips
Volume Number:11
Issue Number:10
Column Tag:Tips & Tidbits

Tips & Tidbits

By Steve Sisak, Contributing Editor

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

TIP OF THE MONTH

Spotting the Elusive Ram Disk

Here is a handy function for performing a dynamic check for the existence of a RAM disk as created by Apple’s Memory control panel. I use it in some of my programs when speed is paramount. Since the user can remove the RAM disk at any time by turning it off from the Memory control panel, and can rename it at any time, the code here must be called before using the RAM disk. In other words, don’t check just once during your application startup and assume thereafter that it will still be there, check before each access if possible. Program defensively...

- Greg Poole


HasRamDisk.h
#pragma once

#ifdef __cplusplus
 extern "C" {
#endif
 extern Boolean  HasRamDisk( FSSpecPtr ramDiskSpec );
#ifdef __cplusplus
 }
#endif

HasRamDisk.c
/******************************************************************************
    HasRamDisk.c

    A dynamic check for the existence of a RAM disk as created by Apple's
    Memory control panel. Since the user can remove the RAM disk at any time
    by turning it off from the Memory control panel, and can rename it at any
    time, the code here needs to be called before assuming the existence of
    a RAM disk. In other words, don't check just once during your application
    startup and assume thereafter that it will still be there, check 
    before each access. Program defensively...
    
    history:

    modified:  xx/xx/xx  who are you? what did you do?
    created:   08/10/94         greg poole

    Greg Poole
    Vital Images, Inc.
    505 N. 4th Street
    Fairfield, IA 52556
    (515) 472-7726
    email: greg@vitalimages.com

 ******************************************************************************/

#include <string.h>
#include "HasRamDisk.h"

// this structure is based on the DRVR definition in MPWTypes.r
//
struct DRVRresourceRec
{
    // description of drvrFlags
    //
    //    struct
    //    {
    //           unsigned hiUnused     : 1;    // unused
    //           unsigned needLock     : 1;   // lock drvr in memory
    //           unsigned needTime     : 1;   // for periodic action
    //           unsigned needGoodbye  : 1;    // call before heap reinit
    //           unsigned statusEnable : 1;      // responds to status
    //           unsigned ctlEnable    : 1;           // responds to control
    //           unsigned writeEnable  : 1;  // responds to write
    //           unsigned readEnable   : 1;  // responds to read
    //           unsigned loUnused     : 8;    // low byte of drvrFlags word unused
    //    } drvrFlags;
 
 short  drvrFlags; // flags as defined above      
 unsigned short  driverDelay; // driver delay (ticks)
 short  deskAccEventMask; // desk acc event mask
 short  driverMenuID;// driver menu ID

 unsigned short  offsetOpen;// offset to DRVRRuntime open
 unsigned short  offsetPrime; // offset to DRVRRuntime prime
 unsigned short  offsetControl;// offset to DRVRRuntime control
 unsigned short  offsetStatus;// offset to DRVRRuntime status
 unsigned short  offsetClose; // offset to DRVRRuntime close

 Str31  driverName;// driver name
 char   driverCode[1];    // driver code
};
typedef struct DRVRresourceRec DRVRresourceRec;
typedef DRVRresourceRec *DRVRresourcePtr, **DRVRresourceHndl;

// constants
//
const char kDrvrHandleBit = 0x40;  
 // bit 7 of 'DRVR' dCtlFlags signals driver is handle
    // instead of pointer and needs to be locked in memory
const char kRamDiskName[] = "\p.EDisk";// Apple's RAM disk driver name


// pass in an FSSpecPtr to hold a reference to a RAM disk,
// returns TRUE if there is currently a RAM disk, FALSE if not
//
Boolean HasRamDisk( FSSpecPtr ramDiskSpec )
{
 BooleanhasRamDisk = FALSE, isHandle = FALSE;
 short  whichVol = 1;// start with first disk volume
 HVolumeParam    volPB;
 OSErr  theErr = noErr, anErr = noErr;
 DCtlHandle dctlHndl = NULL;
 DRVRresourcePtr drvrPtr = NULL;
 DRVRresourceHndldrvrHndl = NULL;
 Ptr    aPtr = NULL;
 Str31  volName;

 do// test each mounted disk volume
 {
 volPB.ioNamePtr = volName;
 volPB.ioVRefNum = 0;// 0 means use ioVolIndex
 volPB.ioVolIndex = whichVol; // use this to determine volume
 
 if ( (theErr=PBHGetVInfoSync( (HParmBlkPtr)&volPB )) == noErr)
 {
 // get this volume's device control entry from the unit table.
    // do not lock the dctlHndl, I spent a couple of days figuring
    // out that locking this handle causes a crash in the CompServer
    // because it is locked at interrupt time...
    //
 if ( (dctlHndl = GetDCtlEntry(volPB.ioVDRefNum)) != NULL )
 {
 // is the device's driver in a handle or a pointer?
    //
 if ((isHandle=(*dctlHndl)->dCtlFlags&kDrvrHandleBit)!= 0)
 {
 drvrHndl = (DRVRresourceHndl) (*dctlHndl)->dCtlDriver;
 drvrPtr = *drvrHndl;
 }
 else
 drvrPtr = (DRVRresourcePtr) (*dctlHndl)->dCtlDriver;

 // get this device's driver, check if it is a RAM disk
    //
 if ( !memcmp( drvrPtr->driverName, kRamDiskName,  
              *kRamDiskName+1 ) )
 {
 // this driver is the RAM disk driver, create an FSSpec to its root dir
    //
 anErr = FSMakeFSSpec( volPB.ioVRefNum, fsRtDirID, 
                                 volName, ramDiskSpec );
 if ( anErr == noErr )
 hasRamDisk = TRUE;
 break;
 }
 }
 }
 whichVol++; // go to next volume
 } 
 while ( theErr != nsvErr );

 return hasRamDisk;
 
} // end HasRamDisk


// define TEST_RAM_DISK for a standalone test
//
#define TEST_RAM_DISK

#if defined( TEST_RAM_DISK )

 // local function prototypes
    //
 static void InitTheMac( void );
 
 static void InitTheMac( void )
 {
 InitGraf( &qd.thePort );
 InitFonts();
 InitWindows();
 InitMenus();
 TEInit();
 InitDialogs( 0L );
 InitCursor();
 MaxApplZone();
 
 } // end InitTheMac

 void main( void )
 {
 FSSpec ramDiskSpec; 
 BooleanhasRamDisk;
 
 InitTheMac();
 hasRamDisk = HasRamDisk( &ramDiskSpec );
 
 } // end main

#endif // TEST_RAM_DISK

Anti-Tip of the Month

Since Greg Poole won the Tip-of-the-Month I thought we could have a little fun and also give him the Anti-Tip-of-the-Month as well for a different submission. (Don’t worry, Greg, you’re getting paid for this too.)

Greg writes:

Here’s a quick and clean way to swap data in place without having to resort to using a temporary memory location:

   short *aPtr, *bPtr;

        *aPtr ^= *bPtr;
        *bPtr ^= *aPtr;
        *aPtr ^= *bPtr;

While this is mathematically cool, lets take a look at the assembly code that it generates and see what’s really happening. First, for comparison, a couple of more pedestrian implementations:

void swap2(short *aPtr, short *bPtr)
{
    short a = *aPtr;    // a version with two temporaries
    short b = *bPtr;

    *aPtr = b;
    *bPtr = a;
}

void swap1(short *aPtr, short *bPtr)
{
    short a = *aPtr;    // a version with one temporary

    *aPtr = *bPtr;
    *bPtr = a;
}

void swap0(short *aPtr, short *bPtr)
{
    *aPtr ^= *bPtr;     // Greg’s tip
    *bPtr ^= *aPtr;
    *aPtr ^= *bPtr;
}

Now, let’s take a look at what the compiler actually generates for these functions. (I’m using CodeWarrior with all optimizations on for these examples.)

Recall that as processsors have gotten faster, memory has not. For instance 1/80ns (the speed on memory in most Macintoshes) = 12.5 MHz. This means that if adjacent instructions have to address memory with no intervening computation, it’s as if the processor has slowed to 12.5MHz.

First the 68K compiler, starting with the two temp case:

Name="swap2"(6)  Size=26
    MOVEA.L   $0004(A7),A1
    MOVEA.L   $0008(A7),A0
    MOVE.W    (A1),D0
    MOVE.W    (A0),D1
    MOVE.W    D1,(A1)
    MOVE.W    D0,(A0)
    RTS

Ignoring the two MOVEA.L’s which set up the address registers and the return, this takes four instructions, all of which touch memory. Notice, however that there are no cases where the result of an instruction is used an an input to the next instruction, meaning that most of the instructions can overlap in the processor pipeline.

Next with one temp:

Name="swap1"(4)  Size=24
    MOVEA.L   $0004(A7),A1
    MOVEA.L   $0008(A7),A0
    MOVE.W    (A1),D0
    MOVE.W    (A0),(A1)
    MOVE.W    D0,(A0)
    RTS

Here we have three instructions, all accessing memory and all can overlap. This is smaller than the example above. Whether it is faster depends on the relative timing of the MOVE.W (A0),(A1) instruction. (If anyone wants to time this, I’ll print the results.)

Now Greg’s ‘tip’:

Name="swap0"(1)  Size=30
    MOVEA.L   $0004(A7),A1
    MOVEA.L   $0008(A7),A0
    MOVE.W    (A0),D0
    EOR.W     D0,(A1)
    MOVE.W    (A1),D0
    EOR.W     D0,(A0)
    MOVE.W    (A0),D0
    EOR.W     D0,(A1)
    RTS

This generates six instructions, all of which touch memory. Furthermore three of these are read-modify-write cycles, which are slower that a read or write and each instruction depends on the result of the instructon directly before it, meaning it won’t overlap in the pipeline, making this both the largest and slowest implementation of the three.

Now lets look at the PowerPC code:

Name=".swap2"(6)  Size=20
    lha      r0,0(r3)
    lha      r5,0(r4)
    sth      r5,0(r3)
    sth      r0,0(r4)
    blr

Name=".swap1"(4)  Size=20
    lha      r5,0(r3)
    lha      r0,0(r4)
    sth      r0,0(r3)
    sth      r5,0(r4)
    blr

Note that both of the versions with temporaries generated the same code (4 instructions, all touching memory but pipelineable). This is because RISC processors typically don’t have memory to memory operations; instead, they must move data to a register before operating on it.

Now our ‘tip’:

Name=".swap0"(1)  Size=52
    lha      r5,0(r4)
    lha      r0,0(r3)
    xor      r0,r0,r5
    sth      r0,0(r3)
    lha      r5,0(r3)
    lha      r0,0(r4)
    xor      r0,r0,r5
    sth      r0,0(r4)
    lha      r4,0(r4)
    lha      r0,0(r3)
    xor      r0,r0,r4
    sth      r0,0(r3)
    blr

This implementation is by far the largest and slowest, generating 12 instructions, including 6 memory accesses. Furthermore there are 2 pipeline stalls. Clearly this implementation is the largest and slowest of all.

The moral of the story is: don’t get tricky. C programmers often try to minimize the number of lines of C in their program without consideration for what the compiler will generate. When in doubt, write clear code and give the optimizer a chance to maximize performance. Look at the compiler output. Your code will be easier to debug and probably faster too.

’Till next time,

- Steve

 
AAPL
$105.22
Apple Inc.
+0.39
MSFT
$46.13
Microsoft Corpora
+1.11
GOOG
$539.78
Google Inc.
-4.20

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Server 4.0 - For OS X 10.10 Yosemit...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
TotalFinder 1.6.12 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
BusyCal 2.6.3 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. BusyCal's unique... Read more
calibre 2.7 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Skitch 2.7.3 - Take screenshots, annotat...
With Skitch, taking, annotating, and sharing screenshots or images is as fun as it is simple.Communicate and collaborate with images using Skitch and its intuitive, engaging drawing and annotating... Read more
Delicious Library 3.3.2 - Import, browse...
Delicious Library allows you to import, browse, and share all your books, movies, music, and video games with Delicious Library. Run your very own library from your home or office using our... Read more
Art Text 2.4.8 - Create high quality hea...
Art Text is an OS X application for creating high quality textual graphics, headings, logos, icons, Web site elements, and buttons. Thanks to multi-layer support, creating complex graphics is no... Read more
Live Interior 3D Pro 2.9.6 - Powerful an...
Live Interior 3D Pro is a powerful yet very intuitive interior designing application. View Video Tutorials It has every feature of Live Interior 3D Standard, plus some exclusive ones: Create multi... Read more
The Hit List 1.1.7 - Advanced reminder a...
The Hit List manages the daily chaos of your modern life. It's easy to learn - it's as easy as making lists. And it's powerful enough to let you plan, then forget, then act when the time is right.... Read more
jAlbum Pro 12.2.4 - Organize your digita...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more

Latest Forum Discussions

See All

Rami Ismail Opens Up distribute​() for D...
Rami Ismail Opens Up distribute​() for Developers Posted by Jessica Fisher on October 24th, 2014 [ permalink ] Rami Ismail, Chief Executive of Business and Development at indie game studio | Read more »
Great Hitman GO Goes on Sale and Gets Ne...
Great Hitman GO Goes on Sale and Gets New Update – Say That Three Times Fast Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
Rival Stars Basketball Review
Rival Stars Basketball Review By Jennifer Allen on October 24th, 2014 Our Rating: :: RESTRICTIVE BUT FUNUniversal App - Designed for iPhone and iPad Rival Stars Basketball is a fun mixture of basketball and card collecting but its... | Read more »
Rubicon Development Makes Over a Dozen o...
Rubicon Development Makes Over a Dozen of Their Games Free For This Weekend Only Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
I Am Dolphin Review
I Am Dolphin Review By Jennifer Allen on October 24th, 2014 Our Rating: :: NEARLY FIN-TASTICUniversal App - Designed for iPhone and iPad Swim around and eat nearly everything that moves in I Am Dolphin, a fun Ecco-ish kind of game... | Read more »
nPlayer looks to be the ultimate choice...
Developed by Newin Inc, nPlayer may seem like your standard video player – but is aiming to be the best in its field by providing high quality video play performance and support for a huge number of video formats and codecs. User reviews include... | Read more »
Fighting Fantasy: Caverns of the Snow Wi...
Fighting Fantasy: Caverns of the Snow Witch Review By Jennifer Allen on October 24th, 2014 Our Rating: :: CLASSY STORYTELLINGUniversal App - Designed for iPhone and iPad Fighting Fantasy: Caverns of the Snow Witch is a sterling... | Read more »
A Few Days Left (Games)
A Few Days Left 1.01 Device: iOS Universal Category: Games Price: $3.99, Version: 1.01 (iTunes) Description: Screenshots are in compliance to App Store's 4+ age rating! Please see App Preview for real game play! **Important: Make... | Read more »
Toca Boo (Education)
Toca Boo 1.0.2 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.2 (iTunes) Description: BOO! Did I scare you!? My name is Bonnie and my family loves to spook! Do you want to scare them back? Follow me and I'll... | Read more »
Intuon (Games)
Intuon 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Join the battle with your intuition in a new hardcore game Intuon! How well do you trust your intuition? Can you find a needle in a... | Read more »

Price Scanner via MacPrices.net

Weekend sale: 13-inch 128GB MacBook Air for $...
Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
Nimbus Note Cross=Platform Notes Utility
Nimbus Note will make sure you never forget or lose your valuable data again. Create and edit notes, save web pages, screenshots and any other type of data – and share it all with your friends and... Read more
NewerTech’s Snuglet Makes MagSafe 2 Power Con...
NewerTech has introduced the Snuglet, a precision-manufactured ring designed to sit inside your MagSafe 2 connector port, providing a more snug fit to prevent your power cable from unintentional... Read more
Apple Planning To Sacrifice Gross Margins To...
Digitimes Research’s Jim Hsiao says its analysts believe Apple is planning to sacrifice its gross margins to save its tablet business, which has recently fallen into decline. They project that Apple’... Read more
Who’s On Now? – First Instant-Connect Search...
It’s nighttime and your car has broken down on the side of the highway. You need a tow truck right away, so you open an app on your iPhone, search for the closest tow truck and send an instant... Read more
13-inch 2.5GHz MacBook Pro on sale for $949,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $949.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $150 off MSRP. Price is... Read more
Save up to $125 on Retina MacBook Pros
B&H Photo has the new 2014 13″ and 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
Textilus New Word, Notes and PDF Processor fo...
Textilus is new word-crunching, notes, and PDF processor designed exclusively for the iPad. I haven’t had time to thoroughly check it out yet, but it looks great and early reviews are positive.... Read more
WD My Passport Pro Bus-Powered Thunderbolt RA...
WD’s My Passport Pro RAID solution is powered by an integrated Thunderbolt cable for true portability and speeds as high as 233 MB/s. HighlightsOverviewSpecifications Transfer, Back Up And Edit In... Read more

Jobs Board

*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.