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

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

WhiteCap 6.7.1 - Visual plug-in for iTun...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
DiskMaker X 6.0 rc5 - Make a bootable OS...
DiskMaker X (was Lion DiskMaker) helps you to build a bootable drive from the official OS X installer app (the one you download from the Mac App Store). It detects the OS X Install program with... Read more
Lyn 1.9 - Lightweight image browser and...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
MacFamilyTree 8.2.1 - Create and explore...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
LaunchBar 6.9 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
LaunchBar 6.9 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
MacFamilyTree 8.2.1 - Create and explore...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
Spotify 1.0.57.474. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
VOX 2.8.26 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
beaTunes 5.0.1 - Organize your music col...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more

Latest Forum Discussions

See All

This War of Mine gets a new ending and m...
This War of Mine just got a big new update, featuring free DLC that adds a new ending to the game, among other exciting changes. The update is celebrating the game's two-year release anniversary. Apart from the new ending, which will be quite... | Read more »
Summon eight new heroes in Fire Emblem H...
Nintendo keeps coming at us with Fire Emblem Heroes updates, and it doesn't look like that trend is stopping anytime soon. The folks behind the game have just announced the new War of the Clerics Voting Gauntlet, expected to start next Tuesday. [... | Read more »
The best deals on the App Store this wee...
iOS publishers are pulling out all the stops this week -- there's a huge number of seriously great games at discounted prices this week. Let's not waste any time and get right down to business. [Read more] | Read more »
The House of da Vinci (Games)
The House of da Vinci 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enter The House of Da Vinci, a new must-try 3D puzzle adventure game. Solve mechanical puzzles, discover hidden... | Read more »
Solve the disappearance of history’s gre...
Blue Brain Games invites you to indulge in an immersive hands-on 3D puzzle adventure in similar vein to The Room series, with its debut release The House of Da Vinci. Set during the historic period of the Italian Renaissance (when Leonardo himself... | Read more »
Age of Rivals (Games)
Age of Rivals 3.3 Device: iOS Universal Category: Games Price: $.99, Version: 3.3 (iTunes) Description: Deep civilization-building strategy in a fast-paced card game! | Read more »
Panthera Frontier (Games)
Panthera Frontier 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Angry Birds Evolution beginner's gu...
Angry Birds changes things up a fair bit in its latest iteration, Angry Birds Evolution. The familiar sling-shot physics mechanics are still there, but the game now features team-based gameplay, RPG elements, and a new top-down view. With all of... | Read more »
Sega Forever is for the retro game fans
Sega is launching a new retro games service titled Sega Forever, in a move that's sure to delight games enthusiasts with a bit of nostalgia. Sega's releasing five classic games for free. The titles include Sonic the Hedgehog, Phantasy Star II,... | Read more »
The Little Acre (Games)
The Little Acre 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

ABBYY TextGrabber 6 for iOS Implements Instan...
ABBYY has announced the release of TextGrabber 6.0.0, an important feature update to the company’s productivity app developed for iOS and Android devices. TextGrabber 6.0 now offers Real-Time... Read more
vPhone, First Smartphone That Can’t Be Lost,...
Austin, Texas based Hypori has introduced the vPhone, a virtual smartphone that affords every business user the benefits of separate work and personal phones, conveniently delivered on a single... Read more
Save this weekend with 2016 refurbished MacBo...
Apple has dropped prices on Certified Refurbished 2016 15″ and 13″ MacBook Pros by as much as $590 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: -... Read more
New 27-inch 3.4GHz iMac on sale for $1699, sa...
MacMall has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping. Their price is $100 off MSRP. Read more
Clearance 2016 MacBook Pros available for up...
B&H Photo has clearance 2016 13″ and 15″ MacBook Pros in stock today and on sale for up to $400 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 15″ 2.7GHz... Read more
Apple Ranks 9th In comScore Top 50 U.S. Digit...
comScore, Inc. has released its monthly ranking of U.S. online activity at the top digital media properties for May 2017 based on data from comScore Media Metrix Multi-Platform. * Entity has... Read more
10.5-inch iPad Pros available for up to $20 o...
B&H Photo has the new 2017 10.5″ iPad Pros available for up to $20 off MSRP including free shipping plus NY & NJ sales tax only: - 64GB iPad Pro WiFi: $649 - 256GB iPad Pro WiFi: $749 - 512GB... Read more
Three Off-The-Beaten-Track iOS Apps That Dese...
One of the great things about using iPads and iPhones is the vast selection of apps available for most anything you want or need to do. The three outlined in this article have been in my core app... Read more
Apple No. 1 Spot In Gartner Top 100 Vendors i...
Gartner, Inc. has unveiled the top global 100 vendors in IT in 2016 based on their revenue across IT (excluding communication services) and component market segments. In the Gartner Global Top 100:... Read more
Clearance iMacs available for up to $300 off...
B&H Photo has clearance 21″ and 27″ Apple iMacs available starting at $949, each including free shipping plus NY & NJ sales tax only: - 27″ 3.3GHz iMac 5K: $1999 $300 off original MSRP - 27″... Read more

Jobs Board

*Apple* News Product Marketing Mgr., Publish...
…organizational consensus on strategy and vision for publisher tools, authoring, and Apple News Format.Carries this strategy and vision across the organization to Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject matter Read more
Lead *Apple* Solutions Consultant - Apple I...
…integrity, and trust.Success Metrics/Key Performance Indicators:Quantitative* Year over Year growth in Apple Product and Beyond the Box sales in the assigned Point of Read more
*Apple* Solutions Consultant till v%u00E5r...
…ethics, integrity, and trust.Success Metrics/Key Performance Indicators:QuantitativeYear over Year growth in Apple Product and Beyond the Box sales in the assigned Point Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.