TweetFollow Us on Twitter

Avoiding traps
Volume Number:2
Issue Number:10
Column Tag:Advanced Macing

Reduce Your Time in the Traps!

By Mike Morton, Senior Software Engineer, Lotus Development Corp., Cambridge, MA

Life in the fast lane

The Macintosh ROM subroutines are called with “trap” instructions, intercepted by dispatching software which interprets the trap and calls the routine. This method is very general, providing compatibility with future ROMs and allowing buggy routines to be replaced.

It's also slow, taking about 45 microseconds for the dispatch process. This article tells you a way to avoid the dispatcher without losing its generality. Since the timing differences are measured in microseconds, there's also a discussion of techniques for measuring the time consumed by a piece of code. Also, a program is included to show the alternate way to call the ROM and how to measure the times used by different methods.

Avoiding traps

When a program executes a trap instruction, the 68000 detects the “error” and transfers control to the trap dispatcher pointed to by the longword at $0028. The dispatching software must, among other things:

• preserve some registers on the stack

• fetch the trap instruction from the code

• decide if the trap is a Toolbox or OS call

• look up the trap number to find whether the routine is in RAM or ROM, and what its address is

• handle the “auto-pop” and “pass A0” bits

• call the routine

• restore registers from the stack

Most of this work can be avoided if you know the routine's address and call it directly, but this is a bad idea for two reasons. First, the address may change in future ROMs. Second, Apple distributes “patches” to ROM routines by changing the dispatch table to call new versions in RAM -- if your program “knows” the address, it'll call the old, buggy ROM routines, ignoring the new RAM-based ones.

There is a balance between hardwiring the address and using the trap dispatcher for every call. The Toolbox “GetTrapAddress” function decodes a trap instruction for you and returns the address of the routine, just as the dispatcher does. You can do this decoding just once in your program, save the address, and repeatedly call it later.

The main reason not to bypass the dispatcher is that it saves a few registers across each call. If you're working in assembler, this is no problem -- just save registers yourself, as needed. In most high-level languages, it also won't be a problem, since the registers lost are typically scratch registers: D1, D2, and A2.

Fig. 1 Our TrapTime Utility shows the difference!

A high-level example

First, let's look at the normal way of calling a Toolbox routine: the simple “SetPt” procedure, which sets the coordinates of a Quickdraw “point”. The following example and the timing program are in TML Pascal; they should be easy to convert to other languages.

Most programs include the Quickdraw unit, which declares “setPt” with

procedure SetPt(VAR pt: point; h, v: integer); INLINE $A880;

When you call the routine with the statement

 setPt (myPt, x, y); { set the point }

it pushes the parameters on the stack and executes the instruction $A880 to trap to the dispatcher, which calls the routine. If you want to skip the cost of repeatedly decoding the trap, you can do it once like this:

 var setPtAddr:longint; { addr of setPt }
  
 setPtAddr := getTrapAddress ($A880);

To call this address, declare a new routine like SetPt, but which produces different in-line 68000 code:

procedure mySetPt
 (VAR pt: point; h, v: integer;
 addr: longint);
 INLINE $205F, $4E90;

Note the extra parameter to this routine: the address of the routine to be called. The instructions given in hex after the “INLINE” do a JSR to that address. The result is nearly the same as executing a trap, but faster.

Calling with this interface is almost like a normal call; pass the address as a parameter:

 mySetPt (myPt, x, y, setPtAddr);

This can be used for most Toolbox calls - just declare your own routine (choose any name) with the same parameters plus the address parameter, and include the exact same “INLINE” code after it. Don't forget to initialize the address with GetTrapAddress before calling, or awful things will happen.

Other high-level languages

You should be able to use this method with almost any language which allows you to insert assembler code in your high-level program. Some languages may have trouble calling the ROM directly -- for instance, many C compilers pass parameters differently than ROM routines do. Some C compilers allow you to choose the method of parameter passing; this will allow you to dispense with assembler altogether and just call the routine through a pointer (ask your nearest C guru how to do this).

More straightforward approaches

This approach assumes that “SetPt” is too slow. If you actually need Toolbox operations to be faster, consider writing the code yourself. You can write a procedure or function to assign two integers to the coordinates of a point -- or just do the assignment yourself. For a simple operation, this approach is preferable to spending lots of effort avoiding the trap dispatcher. (The “K.I.S.S.” rule applies here: “Keep It Simple, Stupid.”)

Speed improvements: hard data

Let's get quantitative. Consider four ways to assign to a point:

• the usual trap

• calling the ROM directly with INLINE

• calling your own procedure

• doing the assignment in-line

I wrote all four in Lisa Pascal and found these times on a Mac, and on a Lisa running MacWorks:

Table: Time to assign to a point

(all times in microseconds)

Mac Lisa/MacWorks

Normal “SetPt” trap 67.7 84.9

Pre-decoded call 22.8 25.6

Roll-your-own 34.5 35.2

Assign in-line 4.8 4.8

Writing your own procedure is slower than using the trap routine's address! The ROM is so fast, compared to compiled Pascal, that it's worth the slightly more complicated call. Part of the speed is because the ROM is tightly-coded; part is because the Mac's video refresh slows down code in RAM.

The fastest method is to forget about writing a procedure and do the assignment normally. This is fourteen times faster than using traps to call the ROM! (There's something to be said for the do-it-yourself approach.)

I tried running the program on a Mac Plus, since its ROM dispatch table has been expanded for faster trap calls. The time for a normal trap is 58.9 microseconds, instead of 67.7 microseconds. All the other times are nearly the same.

Speed improvements: summary

First, all this isn't worthwhile for most traps. If you want to speed up disk I/O, resource operations, etc., the microseconds saved at trap time are dwarfed by the amount of time for a disk transfer or to search a large resource. This trick is appropriate only in some situations.

Second, some routines are best done by hand in simple code in your program. ROM tools such as “SetPt” exist for your convenience, not because they're hard to code. If you find they're taking too much time, change them to a few lines of your own code.

But suppose you're trying to draw lines at top speed with repeated “LineTo” calls? Or use one of the simple bit manipulators in a loop? You may find that you can't easily write it yourself, but you can save 45 microseconds by calling into the ROM using a previously determined address. My estimate is that if a trap takes between 200 and 800 microseconds, you should consider skipping the dispatcher.

The timing program

The program “traptime” found the times given in the table. It has four procedures to time methods, and a “getbasetime” procedure to find the overhead of a loop with no calls. You can write a similar program using the same design in nearly any language.

Note that the program prints its results in ticks (60ths of a second) and doesn't compute the time for a loop iteration; I did the conversions to microseconds-per-iteration by hand, rather than trying to get Pascal to do fractional arithmetic.

Timing methods

Unfortunately, doing accurate timings is fraught with problems. This program tries to avoid these. Some points on timings:

• Repeat your measurements to help detect “random” factors. Small discrepancies should be averaged; large ones should be found and removed.

• Be careful when comparing routines: the four timing routines (and the “overhead” routine) are identical except for one section. Keeping this parallel structure makes your program a controlled experiment, helping you time only the differences between procedures.

• Vary the loop size; make sure that your time per iteration converges as your loop gets bigger.

• When waiting for the program, don't move the mouse or fiddle with the keyboard. This causes interrupts and affects the timings.

• I suspect you shouldn't have the disk spinning, nor have a debugger active while timing. (In practice, I can't detect any timing differences due to either of these factors.)

In short, timing is a scientific experiment and is easy to ruin by not controlling the environment carefully.

Conclusion

Bypassing the trap dispatcher can be a valuable technique in a limited number of situations, allowing you to cut about 45 microseconds off the time to call the ROM. It has some drawbacks such as losing register contents, and may be hard to implement in some higher-level languages. In addition, many ROM calls take so long that the savings isn't significant.

Whatever technique you're interesting in optimizing and timing, accurate measurement is a matter of a careful, controlled approach.

{ traptime -- A program to time various methods of doing a toolbox trap:
  The usual method, calling a user-written routine to do the work, doing 
the work in-line, and calling the ROM routine directly without going 
through the trap dispatcher. Times for all routines are written on the 
screen in ticks for a given number of calls, then the number of calls 
is varied for improved accuracy.

  Mike Morton, November 1985. Modified for TML Pascal, June 1986. }

program traptime (output);{ "(output)" lets us do writelns }

{$I MemTypes.ipas  }
{$I QuickDraw.ipas } { we use Quickdraw graphics }
{$I OSIntf.ipas }{ and OS definitions }
{$I ToolIntf.ipas }{ and Toolbox calls }

var         { program-wide variables }
  basetime: longint; { constant overhead for the loop }
  loops: longint;         { number of iterations to time }
  start: longint;         { starting tickcount for timing }
  Event:EventRecord; {simple event loop for cmd-3}
  DoIt: Boolean; {getnextevent boolean}
  Finished:Boolean;{event loop terminator}

{ getbasetime -- Find the time for the loop when nothing is done inside 
it.This tells us the overhead which should be subtracted from other timings. 
}

function getbasetime: longint;
var count: longint;        { loop counter }
begin;
  start := tickcount;        { snapshot starting time }
  for count := 1 to loops do        { loop a bunch of times... }
    ;           { ...doing nothing each time }
  getbasetime := tickcount-start;       { calculate elapsed time }
end;            { function "getbasetime" }

{ usualtime -- Find the time used to call the ROM the usual way.  This, 
and all timing routines, should look as much as possible like "getbasetime". 
}

function usualtime: longint;
var
  count: longint;        { loop counter }
  pt: point;        { point to assign to }
  x, y: integer;         { coordinates to assign to the point }
begin;
  start := tickcount;        { snapshot starting time }
  for count := 1 to loops do        { this time, inside the loop... }
    setpt (pt, x, y);        { ...we do the ROM call }
  usualtime := tickcount-start;          { calculate elapsed time }
end;            { function "usualtime" }


{ setmypt -- This isn't a timing function like the others; it's a replacement 
for the ROM's "setpt" routine, to see how fast we can do it ourselves. 
}
procedure setmypt (VAR pt: point; x, y: integer);
begin;
  pt.h := x; pt.v := y; { assign to the coordinates; easy! }
end;    { procedure "setmypt" }

{ myowntime -- Time assignment using our own procedure. }

function myowntime: longint;
var
  count: longint;        { loop counter }
  pt: point;        { point to assign to }
  x, y: integer;         { coordinates to assign to point }
begin;
  start := tickcount;        { snapshot starting time }
  for count := 1 to loops do        { this time, inside the loop... }
    setmypt (pt, x, y);           { ...we call our own routine }
  myowntime := tickcount-start;          { calculate elapsed time }
end;            { function myowntime }

{ inlintime -- The most straightforward way: we do the assignment in 
the loop. }

function inlintime: longint;
var
  count: longint;        { loop counter }
  pt: point;        { point to assign to }
  x, y: integer;         { coordinates to assign to point }
begin;
  start := tickcount;        { snapshot starting time }
  for count := 1 to loops do        { this time, inside the loop... }
    begin; pt.h := x; pt.v := y; end;   { ...we do assignment here }
  inlintime := tickcount-start;          { calculate elapsed time }
end;            { function inlintime }

{ setptx -- This is another replacement for "setpt".  It takes an extra 
parameter, the previously determined address of "setpt", and calls that 
address, leaving the other parameters for "setpt".  Unfortunately, TMLPascal 
doesn't mimic Lisa Pascal closely enough to allow us to generate more 
than one word of code in a single declaration.  So we have two procedures 
-- these MUST always be used together!  TML says their 2.0
 release of the compiler will be Lisa-compatible on this score, so this 
unsightly workaround won't be needed any more. }

procedure setptx1 (var pt: point; h, v: integer; addr: longint);
      INLINE   $205F; { MOVE.L   (A7)+,A0  
 ; pop routine's address into A0  }
procedure setptx2;
      INLINE   $4E90;{ JSR(A0);  and call that address }

{ gettrtime -- The last and most complicated way of calling the routine. 
 We use the trap address to call it directly. }

function gettrtime: longint;
var
  addr: longint;         { actual address of "setpt" }
  count: longint;        { loop counter }
  pt: point;        { point to assign to }
  x, y: integer;         { coordinates to assign to point }
begin;
  addr := gettrapaddress ($a880);    { find where routine lives }
  start := tickcount;         { snapshot starting time }
  for count := 1 to loops do begin { inside the loop... }
    setptx1 (pt, x, y, addr);          { ...we call on ROM  }
    setptx2;{ (kludge to sneak in 2nd instruction }
  end;
  gettrtime := tickcount-start;              { calculate elapsed time 
}
end;             { function gettrtime }

begin;          { *** main program *** }
  writeln ('If launching from a floppy, wait for it to stop and click 
to begin...');
  while not button do; while button do;      { wait for a click }

  loops := 10000;          { start with a small loop size... }
  while loops <= 1000000 do  { and go through several sizes}
  begin;
    basetime := getbasetime;        { find constant overhead }

    writeln ('number of loops:', loops, '; base time is:', basetime);
    writeln ('time for usual method is..........: ', usualtime - basetime);
    writeln ('time for calling my own routine is: ', myowntime - basetime);
    writeln ('time for doing it in-line is......: ', inlintime - basetime);
    writeln ('time for doing it with gettrapaddr: ', gettrtime - basetime);
    writeln;

    loops := loops * 10;   { loop sizes increase exponentially }
  end;

  flushevents(EveryEvent,0);
   writeln ('click to exit or take snapshot ');
  Repeat
  systemtask;
 DoIt:=GetNextEvent(EveryEvent,Event);
 if DoIt then
 Case Event.what of
  KeyDown: begin end;
  Mousedown: begin Finished:=true; end;
  End;
Until Finished;
end.            { of main program "traptime"  }



!PAS$Xfer

trapspeed
PAS$Library
OSTraps
ToolTraps
$ 
 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X 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

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.