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

Civilization VI 1.0.6 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
djay Pro 2.0.1 - Transform your Mac into...
djay Pro provides a complete toolkit for performing DJs. Its unique modern interface is built around a sophisticated integration with iTunes and Spotify, giving you instant access to millions of... Read more
Microsoft OneNote 15.41 - Free digital n...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
TechTool Pro 9.6 - Hard drive and system...
TechTool Pro has long been one of the foremost utilities for keeping your Mac running smoothly and efficiently. With the release of version 9, it has become more proficient than ever. TechTool... Read more
Apple iOS 11.2.1 - The latest version of...
iOS 11 sets a new standard for what is already the world’s most advanced mobile operating system. It makes iPhone better than before. It makes iPad more capable than ever. And now it opens up both to... Read more
Things 3.3 - Elegant personal task manag...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
RapidWeaver 7.5.5 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Adobe Animate CC 2018 18.0.1.115 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more

Latest Forum Discussions

See All

Rules of Survival guide - how to boost y...
It's not easy surviving in the "every-man-for-himself" world of Rules of Survival. You'll be facing off against many other players who might be more skilled than you, or are luckier than you. There are a lot of factors weighing against you. With... | Read more »
FEZ Pocket Edition (Games)
FEZ Pocket Edition 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Amazing Katamari Damacy guide - beginner...
Amazing Katamari Damacy brings the bizarro world of the original games to mobile and shifts them into an endless format that's just as addictive as the PlayStation entries. Your goal is still to roll as much random stuff as you possibly can, though... | Read more »
Portal Knights guide - crafting tips and...
In Portal Knights, you're only as strong as the items you have at your disposal. This sandbox adventure is all about crafting and building up the next big thing. Whether you're an avid explorer or collector, crafting will likely play a large part... | Read more »
The best deals on the App Store this wee...
A new week means new discounts on the App Store. This week's deals run the gamut of action-adventure titles, puzzle games, and one of the best narrative adventure series out there. If you're looking to fill out your mobile gaming library on a... | Read more »
What you need to know about Animal Cross...
We hope you've been hard at work on collecting all of those holiday items in Animal Crossing: Pocket Camp, because you're about to get a whole new list of fun things to do as the game receives its first big update sometime soon. There are a lot of... | Read more »
Reigns: Her Majesty guide - how to use e...
Ruling a kingdom isn't easy--doubly so for a queen whose every decision is questioned by the other factions seeking a slice of power. Reigns: Her Majesty builds on the original game's swipey tactics, adding items that you can use to move the story... | Read more »
The best new games we played this week -...
Friday has crept up on us once again, so it's time to honor the best new games we've played over the past few days. This past week was a pretty exciting one, with the debut of lots of beautiful new indies and some familiar faces returning to the... | Read more »
Portal Knights guide- beginner tips and...
Portal Knights is finally making the jump to iOS and Android, and it's already climbing the ranks to become the next big MMO experience on mobile. This sprawling sandbox game will let you pursue any adventure you wish, whether you want to sling... | Read more »
Reigns: Her Majesty guide - how to swipe...
Reigns: Her Majesty is storming the App Store this week, bringing more tinder-esque kingdom building to eager players everywhere. If you've played the original Reigns, you'll know that leading a kingdom is never easy. It's a careful balancing act... | Read more »

Price Scanner via MacPrices.net

Beats Holiday sale at B&H, headphones and...
B&H Photo has Beats by Dr. Dre headphones, earphones, and speakers on sale for up to $80 off MSRP as part of their Holiday sale. Expedited shipping is free, and B&H charges sales tax to NY... Read more
Holiday sale: Apple resellers offer 2017 15″...
MacMall has 15″ MacBook Pros on sale for $220-$300 off MSRP, each including free shipping: – 15″ 2.8GHz MacBook Pro Space Gray (MPTR2LL/A): $2179, $220 off MSRP – 15″ 2.8GHz MacBook Pro Silver (... Read more
Holiday sale: Apple resellers offer 13″ MacBo...
B&H Photo has 13″ MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13-inch 2.3GHz/128GB Space Gray MacBook Pro (... Read more
Apple Watch Series 2, Certified Refurbished,...
Apple has Certified Refurbished Apple Watch Nike+ Series 2s, 42mm Space Gray Aluminum Case with Anthracite/Black Nike Sport Bands, available for $249 (38mm) or $279 (42mm). The 38mm model was out of... Read more
Apple offers Certified Refurbished 2016 12″ R...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $949. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
B&H drops price on 13″ 256GB MacBook Air...
B&H has the 13″ 1.8GHz/256GB Apple MacBook Air (MQD42LL/A) now on sale for $1079 including free shipping plus NY & NJ sales tax only. Their price is $120 off MSRP, and it’s the lowest price... Read more
Holiday sale: 9″ iPads starting at $299, take...
MacMall has 9″ WiFi iPads on sale for $30 off including free shipping: – 9″ 32GB WiFi iPad: $299 – 9″ 128GB WiFi iPad: $399 Read more
Green Monday deal: 15″ 2.8GHz MacBook Pro on...
B&H Photo has the 15″ 2.8GHz Space Gray MacBook Pro on sale for $250 off MSRP for today only as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax for NY... Read more
Green Monday sale: B&H offers 12″ Apple i...
B&H Photo has 12″ iPad Pros on sale for up to $150 off MSRP as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax in NY & NJ only: – 12″ 64GB WiFi iPad... Read more
Holiday deal: 21″ and 27″ Apple iMacs on sale...
MacMall has 2017 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP. Shipping is free: – 21″ 2.3GHz iMac: $999 $100 off MSRP – 21″ 3.0GHz iMac: $1199 $100 off MSRP – 21″ 3.4GHz iMac: $1379 $120... Read more

Jobs Board

QA Automation Engineer, *Apple* Pay - Apple...
# QA Automation Engineer, Apple Pay Job Number: 113202642 Santa Clara Valley, California, United States Posted: 11-Dec-2017 Weekly Hours: 40.00 **Job Summary** At Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, 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
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Information Security - Security Data...
# Apple Information Security - Security Data Analyst Job Number: 113119545 Austin, Texas, United States Posted: 10-Nov-2017 Weekly Hours: 40.00 **Job Summary** This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.