TweetFollow Us on Twitter

Volume Number:3
Issue Number:9
Column Tag:Technical Note

ShiftMod Patch

By Mike Scanlin, San Diego, CA

Last week we were talking with an IBM freak about the differences between our Macs and his footrest, that is, PC. One of his complaints about the Mac was that you get upper case letters when both the shift key and caps lock key are down at the same time. He wanted to be able to type a lower case letter if the caps lock key was down by pressing the shift key. Then he could type sentences like “i WISH i HAD A mACINTOSH” without having to hold down the shift key the whole time. Well, Mac fans, fear not. Now the Mac can do this too. Say hello to Mr. ShiftMod.

All that is really needed is a tail patch to _GetNextEvent that checks if a keyDown or autoKey event occured while the caps lock and shift keys were held down. The only problem with implementing it is that because _GetNextEvent is a stack based trap (it gets its parameters from the stack) the patch requires a bit of self modifying code to get it to set things up correctly. While this example is trivial, the technique used here can be used to patch any stack based trap routine.

A tail patch is something that first calls the original trap and then does some post-processing before returning to the caller. If the original trap is not stack based, we can do a JSR OriginalTrap at the beginning of our patch, do our post-processing, and end with an RTS. However, if the original trap is stack based it will expect the stack to look a certain way when it gets called. By doing a JSR to it, we put a return address on top of the existing stack which, in effect, shifts all of the parameters by 4 bytes (relative to the top of the stack). The routine being called does not know about the extra return address and will use the wrong bytes as parameters.

The code listed here is an application, but it can be easily (via ResEdit) be made into an INIT resource and pasted into your system file. It installs itself in the system heap and hangs around as long as you don’t turn your Mac off. This code is useful if you want to install any event processing stuff that is global for all applications. For instance, you could call _Eject every time you detect a diskEvt and count how many times someone tries to insert a disk in a minute. Or you could do something semi-useful like adding a keyclick after keyDowns (how about a mouseclick on mouseDowns?) for debugging.

Now how about an IBM hacker writing a routine to make his machine return upper case letters when both the shift key and caps lock key are down? Is it even possible?

/* shiftMod.c    2 July 1987
 * by Mike Scanlin and Andy Voelker
 * This program installs a tail patch on _GetNextEvent so
 * that the shift key toggles between upper and lower case
 * letters if the caps lock key is down.
 * No toggle is done if the option and/or command key was
 * held down.

#define whatOFFSET(EventRecord,what)
#define message  OFFSET(EventRecord,message)
#define modifiersOFFSET(EventRecord,modifiers)

#define GetNextEvent 0xA970
#define JMP 0x4EF9
#define memFullErr -108

 asm  {
 move.l D3,-(SP)
/* set up the JMP instruction at the end of the patch */
 lea    @patchExit,A0
 move #JMP,(A0)

/* get the old trap address */
 move #GetNextEvent,D0
/* set up the JMP instruction that calls the original trap */
 lea    @origTrap,A1
 move #JMP,(A1)+
 move.l A0,(A1)

/* get some space in the system heap */
 lea    @last,A0
 lea    @first,A1
 suba.l A1,A0
 move.l A0,D0
 move.l D0,D3  /* save for _BlockMove */
 cmpi #memFullErr,D0
 beq.s  @noPatch
 move.l A0,-(SP) /* save for _BlockMove */

/* set the trap address to the space we just got in the
 * system heap
 move #GetNextEvent,D0
/* now move it into place */
 lea    @first,A0
 move.l (SP)+,A1
 move.l D3,D0
@noPatchmove.l (SP)+,D3

/* Here’s the new _GetNextEvent.  It calls the existing
 * _GetNextEvent and then checks if a keyDown or autoKey
 * event is being reported.

 /* save original return address */
@first  lea @exitAddress,A0
 move.l (SP)+,(A0)

 /* save ptr to event record */
 lea    @eventPtr,A0
 move.l (SP),(A0)

 /* return to our routine */
 pea    @tailPatch

@origTrap nop    /* JMP to original trap */
@tailPatchlea    @eventPtr,A0
 move.l (A0),A0

 /* Is it a keyDown or autoKey? */
 move what(A0),D0
 cmpi #keyDown,D0
 beq.s  @isKeyDown
 cmpi #autoKey,D0
 bne.s  @patchExit

@isKeyDownmove modifiers(A0),D0
 andi   #shiftKey+alphaLock+optionKey
 eori   #shiftKey+alphaLock,D0
 bne.s  @patchExit

 move.l message(A0),D0
 cmpi.b #’A’,D0
 bmi.s  @patchExit
 cmpi.b #’Z’,D0
 bgt.s  @patchExit
 addi.b #’a’-’A’,D0
 move.l D0,message(A0)

 /* return to original caller via long JMP */

@eventPtr dc.l 0 /* storage for event record ptr */


Community Search:
MacTech Search:

Software Updates via MacUpdate

Dash 3.4.0 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
RapidWeaver 7.1.7 - 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
Printopia 2.1.22 - Share Mac printers wi...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
SteerMouse 5.0 - Powerful third-party mo...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
Arq 5.5.1 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
Slack 2.3.0 - Collaborative communicatio...
Slack is a collaborative communication app that simplifies real-time messaging, archiving, and search for modern working teams. Version 2.3.0: Note: Now requires OS X 10.8 or later New The app... Read more
Cocktail 10.1 - General maintenance and...
Cocktail is a general purpose utility for macOS that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Firefox 49.0.2 - Fast, safe Web browser.
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
Art Text 3.1 - $49.99
Art Text is graphic design software to create stunning illustrations, such as badges, flyers, logos, social headers and icons, text mockups, website graphics and buttons, picture captions, word art,... Read more
AirRadar 3.1.9 - $9.95
With AirRadar, scanning for wireless networks is now easier and more personalized! It allows you to scan for open networks and tag them as favourites or filter them out. View detailed network... Read more

Latest Forum Discussions

See All

Oh...Sir! The Insult Simulator (Games)
Oh...Sir! The Insult Simulator 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
WitchSpring2 (Games)
WitchSpring2 1.27 Device: iOS Universal Category: Games Price: $3.99, Version: 1.27 (iTunes) Description: This is the story of Luna, the Moonlight Witch as she sets out into the world. This is a sequel to Witch Spring. Witch Spring 2... | Read more »
4 popular apps getting a Halloween makeo...
'Tis the season for all things spooky. So much, so, in fact, that even apps are getting into the spirt of things, dressing up in costume and spreading jack o' lanterns all about the place. These updates bring frightening new character skins, scary... | Read more »
Pokémon GO celebrates Halloween with can...
The folks behind Pokémon GO have some exciting things planned for their Halloween celebration, the first in-game event since it launched back in July. Starting October 26 and ending on November 1, trainers will be running into large numbers of... | Read more »
Best Fiends Forever Guide: How to collec...
The fiendship in Seriously's hit Best Fiends has been upgraded this time around in Best Fiends Forever. It’s a fast-paced clicker with lots of color and style--kind of reminiscent of a ‘90s animal mascot game like Crash Bandicoot. The game... | Read more »
5 apps for the budding mixologist
Creating your own cocktails is something of an art form, requiring a knack for unique tastes and devising interesting combinations. It's easy to get started right in your own kitchen, though, even if you're a complete beginner. Try using one of... | Read more »
5 mobile strategy games to try when you...
Strategy enthusiasts everywhere are celebrating the release of Civilization VI this week, and so far everyone seems pretty satisfied with the first full release in the series since 2010. The series has always been about ultra-addictive gameplay... | Read more »
Popclaire talk to us about why The Virus...
Humanity has succumbed to a virus that’s spread throughout the world. Now the dead have risen with a hunger for human flesh, and all that remain are a few survivors. One of those survivors has just called you for help. That’s the plot in POPCLAIRE’... | Read more »
Oceans & Empires preview build sets...
Hugely ambitious sea battler Oceans & Empires is available to play in preview form now on Google Play - but download it quickly, as it’s setting sail away in just a few days. [Read more] | Read more »
Rusty Lake: Roots (Games)
Rusty Lake: Roots 1.1.4 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.4 (iTunes) Description: James Vanderboom's life drastically changes when he plants a special seed in the garden of the house he has inherited.... | Read more »

Price Scanner via

Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more
64GB Apple TV on sale for $159, save $40
Best Buy has the 64GB Apple TV on sale for $40 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store price may... Read more
EyeQue Introduces iOS And Android Based Advan...
Affordable vision technologies developers EyeQue have announced what they claim to be the world’s most advanced intelligent vision solution, pitched as enabling anyone, anywhere to easily and... Read more
Smartwatch Market Tanks, Declining 51.6% in 2...
The worldwide smartwatch market experienced a round of growing pains in the third quarter of 2016 (3Q16), resulting in a year-over-year decline in shipment volumes. According to data from the... Read more
CAZE announces Ultra Thin Glass Screen Protec...
Hong Kong based CAZE has announced its first ultra thin glass screen protector, the Glazz Pro for iPhone 7/7 Plus. Glazz Pro is made from chemically reinforced glass with an anti-fingerprint... Read more
11-inch MacBook Airs on sale for up to $120 o...
Newegg has 11″ MacBook Airs on sale for up to $120 off MSRP. Shipping is free: - 11″ 1.6GHz/128GB MacBook Air: $799.99 $100 off MSRP - 11″ 1.6GHz/256GB MacBook Air: $979 $120 off MSRP Read more
Up to $300 off Macs, $20 off iPads with Apple...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Apple’s Thursday “Hello Again” Event A Largel...
KGI Securities analyst Ming-Chi Kuo, who has a strong record of Apple hardware prediction accuracy, forecasts in a new note to investors released late last week that a long-overdue redo of the... Read more
12-inch Retina MacBooks on sale for $100 off...
Amazon has 2016 12″ Apple Retina MacBooks on sale for $100 off MSRP. Shipping is free: - 12″ 1.1GHz Silver Retina MacBook: $1199.99 $100 off MSRP - 12″ 1.1GHz Gold Retina MacBook: $1199.99 $100 off... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more

Jobs Board

*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* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 52812872 Houston, Texas, United States Posted: Oct. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Solutions Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 52812906 Houston, Texas, United States Posted: Oct. 18, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC is an Read more
*Apple* Retail - Multiple Positions- Towson,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Software Engineering Intern: Integration / QA...
Job Summary Apple is currently seeking enthusiastic interns who can work full-time for a minimum of 12-weeks between Fall 2015 and Summer 2016. Our software Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.