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

Sound Studio 4.10.0 - Robust audio recor...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
Sparkle Pro 4.0 - Visual website creator...
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
Dropbox 140.4.1951 - Cloud backup and sy...
Dropbox for Mac is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files... Read more
FotoMagico 6.0.5 - Powerful slideshow cr...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Remotix 6.4.2 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features: Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
Microsoft Office 365, 2019 16.57 - Popul...
Microsoft Office 365. The essentials to get it all done. Unmistakably Office, designed for Mac Get started quickly with new, modern versions of Word, Excel, PowerPoint, Outlook and OneNote-... Read more
War Thunder 2.13.0.66 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
RoboForm 9.2.8 - Password manager; syncs...
RoboForm is a password manager that offers one-click login, mobile syncing, easy form filling, and reliable security. Password Manager. RoboForm remembers your passwords so you don't have to! Just... Read more
Adobe Photoshop 23.1.1 - Professional im...
You can download Photoshop for Mac as a part of Creative Cloud for only $20.99/month (or $9.99/month if you have purchased an earlier software version). Adobe Photoshop is a recognized classic of... Read more
VirtualBox 6.1.32 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more

Latest Forum Discussions

See All

‘Ark Legends’ Gives Players a Chance to...
It’s Airpods and Amazon gift cards galore as Melting Games opens pre-registration for Ark Legends. The upcoming mobile RPG is giving away tons of in-game goodies such as gold, energy, iron core, hero summon chest and rare iron core to players who... | Read more »
‘Nickelodeon Extreme Tennis’ Out Now on...
Nickelodeon Extreme Tennis () from Old Skull Games and Nickelodeon is this week’s new Apple Arcade release. Nickelodeon Extreme Tennis features characters from old and new Nickelodeon shows including SpongeBob, TMNT, and many more. The tennis game... | Read more »
SwitchArcade Round-Up: ‘RPGolf Legends’,...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 20th, 2022. In today’s article, we’ve got a massive amount of new releases to check out. We’ve got summaries of all of them, from heaven to hell. We also have the lists of... | Read more »
‘Zed Blade ACA NEOGEO’ Review – Well, It...
SNK’s NEOGEO platform played host to a great many classics, both famous and under-the-radar. The Metal Slug games. The King of Fighters series. Magician Lord. Shock Troopers. Sengoku 3. NEO Turf Masters. Fatal Fury. Samurai Shodown. Twinkle Star... | Read more »
‘Inua – A Story in Ice and Time’ is a Un...
One thing I know about ARTE from their output on mobile over the years is that they love collaborating with really interesting and unique studios to put out really interesting and unique gaming experiences. This is true yet again with the latest... | Read more »
Out Now: ‘Angry Birds Journey’, ‘RPG Dic...
Each and every day new mobile games are hitting the App Store, and so each week we put together a big old list of all the best new releases of the past seven days. Back in the day the App Store would showcase the same games for a week, and then... | Read more »
SwitchArcade Round-Up: ‘Banjo-Kazooie’ C...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 19th, 2022. After a couple of big-pants articles in recent days, today is somewhat of a lighter one. We’ve got a little news to go over, a small handful of new releases to... | Read more »
‘Yu-Gi-Oh! Master Duel’ Is Out Now on PC...
Yu-Gi-Oh! Master Duel was confirmed for all consoles including Switch, PS5, and more in addition to PC and mobile platforms last year. Since then, Yu-Gi-Oh! | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 18th, 2022. We’ve got another batch of reviews today, with our pal Mikhail covering the lamentable Grand Theft Auto: The Trilogy Definitive Edition. Has it been whipped into... | Read more »
‘Total War: Medieval II’ Coming to iOS a...
After a bit of teasing, Feral Interactive just announced that it is bringing Total War: Medieval II to iOS and Android. This will be the developer’s first release since the brilliant Alien Isolation. Check out my review of the iOS version here. | Read more »

Price Scanner via MacPrices.net

In stock and on sale! 16″ 10-Core M1 Pro MacB...
Amazon has new 16″ 10-Core/512GB M1 Pro MacBook Pros in stock today and on sale for $50 off MSRP including free shipping. Their prices are the lowest available for new M1 Pro 16″ MacBook Pro from any... Read more
Deal Alert!: 14″ M1 Pro with 10-Core CPU in s...
Amazon has the new 14″ M1 Pro MacBook Pro with a 10-Core CPU and 16-Core GPU in stock today and on sale for $2299.99 including free shipping. Their price is $200 off Apple’s standard MSRP, and it’s... Read more
Apple has 24-inch M1 iMacs (8-Core CPU/8-Core...
Apple has restocked a wide array of 24-inch M1 iMacs with 8-Core CPUs and 8-Core GPUs in their Certified Refurbished store. Models are available starting at only $1269 and range up to $260 off... Read more
Select 24″ M1 iMacs are on sale for $100 off...
Sales of Apple’s new 24″ M1 iMacs have been rare since its introduction, perhaps due to global supply issues. However, B&H is offering a $100 discount on select 24″ iMacs, and they’re in stock... Read more
M1 Mac minis are back in stock today at Apple...
Apple has M1-powered Mac minis available in their Certified Refurbished section starting at only $589 and up to $140 off MSRP. Each mini comes with Apple’s one-year warranty, and shipping is free: –... Read more
B&H has M1-powered Mac minis on sale for...
B&H Photo has Apple’s Mac minis with M1 Apple Silicon CPUs in stock today and on sale for $50-$100 off MSRP, starting at $649. Free 1-2 shipping is free to many US addresses. Their prices are... Read more
New Amazon sale: Apple’s 13″ M1 MacBook Airs...
Amazon has Apple 13″ M1 MacBook Airs on sale for $100 off MSRP, starting at only $899. Their prices are the lowest available for new MacBook Airs today. Stock may come and go, so check their site... Read more
Get an Apple Watch Series 7 for $50 off MSRP,...
Amazon has Apple Watch Series 7 models on sale for $50 off MSRP including free shipping. Their prices are the lowest available for Apple Watch Series 7 models today: – 41mm Apple Watch Series 7 GPS... Read more
Here are the details of Apple’s 2022 Educatio...
Need a new Apple Mac or iPad for school? Whether you’re a student, teacher, or staff member, you can use your .edu email address when ordering at Apple Education to take up to $400 off the price of a... Read more
Amazon is blowing out 2020 21″ iMacs for only...
Amazon has clearance 2020 21″ iMacs (2.3GHz Dual-Core i5, 8GB RAM, 256GB SSD) on sale right now for $599.99 including free shipping. Original MSRP for this model was $1099. Amazon expects delivery in... Read more

Jobs Board

Registered Nurse (RN) Employee Health PSJH -...
…is calling for a Registered Nurse (RN) Employee Health PSJH to our location in Apple Valley, CA.** We are seeking a Registered Nurse (RN) Employee Health PSJH to be Read more
Systems Administrator - Pearson (United State...
…and troubleshoot Windows operating systems (workstation and server), laptop computers, Apple iPads, Chromebooks and printers** + **Administer and troubleshoot all Read more
IT Assistant Level 1- IT Desktop Support Anal...
…providing tier-1 or better IT help desk support in a large Windows and Apple environment * Experience using IT Service Desk Management Software * Knowledge of IT Read more
Human Resources Business Partner PSJH - Provi...
…**is calling a** **Human Resources Business Partner, PSJH** **to our location in Apple Valley, CA.** **Applicants that meet qualifications will receive a text with Read more
Manager Community Health Investment Programs...
…is calling a Manager Community Health Investment Programs PSJH to our location in Apple Valley, CA.** **Qualified candidates will be invited to do a self-paced video Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.