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
$ 
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple Pro Video Formats 2.0.2 - Updates...
Apple Pro Video Formats brings updates to Apple's professional-level codes for Final Cut Pro X, Motion 5, and Compressor 4. Version 2.0.2: Includes support for the following professional video codecs... Read more
Apple Final Cut Pro X 10.2.2 - Professio...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more
Apple Compressor 4.2.1 - Adds power and...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Powerful... Read more
Apple Motion 5.2.2 - Create and customiz...
Apple Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time... Read more
A Better Finder Rename 10.00b1 - File, p...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
CrossOver 14.1.6 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Printopia 2.1.14 - 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
Google Drive 1.24 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more
Chromium 45.0.2454.85 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 45.0.2454.85: Note: Does not contain the "... Read more
OmniFocus 2.2.5 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more

Zen Brush 2 (Entertainment)
Zen Brush 2 1.0 Device: iOS Universal Category: Entertainment Price: $2.99, Version: 1.0 (iTunes) Description: Zen Brush 2 is a drawing app focused on the strong yet beautiful feel of the East Asian ink brush. | Read more »
NetNewsWire (News)
NetNewsWire 4.0.0 Device: iOS iPhone Category: News Price: $3.99, Version: 4.0.0 (iTunes) Description: Follow the Web NetNewsWire 4, completely written from the ground up for iPhone. NetNewsWire is the best way to keep up with the... | Read more »
Huzzah! Farming Simulator 16 Supports Cl...
As though it weren't difficult enough to resist the siren call of Farming Simulator 16, now it's been updated with cloud saves - so you can jump between devices without missing any of that precious crop-harvesting time. [Read more] | Read more »
Don't Starve: Pocket Edition Finall...
I've made no effort to hide my enjoyment of Don't Starve: Pocket Edition, but I will admit I was a tiny bit disappointed that it was only available for the iPad. Well nuts to that, because Klei Entertainment has made it universal! [Read more] | Read more »
Goat Simulator Wasn't Enough? Then...
If you just didn't get enough goat in Goat Simulator - or if you've been wanting to play a simulated MMO as a microwave - then you're in luck! Goat Simulator MMO Simulator is out now, and it's a game about simulated goats in simulated MMOs. Plus... | Read more »
You Can Play Madfinger Games' Unkil...
Madfinger Games - probably best known for the Dead Trigger series - has officially launched their newest zombie shooter (that isn't called Dead Trigger), named Unkilled. [Read more] | Read more »
KORG iELECTRIBE for iPhone (Music)
KORG iELECTRIBE for iPhone 1.0.1 Device: iOS iPhone Category: Music Price: $9.99, Version: 1.0.1 (iTunes) Description: ** 50% OFF Special Launch Sale - For a Limited Time **The ELECTRIBE reborn in an even smaller form A full-fledged... | Read more »
Toca Life: City Just Got a Bunch of New...
Toca Life: City is Toca Boca's most popular app (number 1 in 47 different countries, apparently), and it's just had an update that adds a bunch of new content. [Read more] | Read more »
My Country 3D is More About Cities than...
My Country 3D is an upcoming city builder from Game Insight that looks pretty decent - although the name seems a tad out of whack for a city builder. Ah well, it is what it is. [Read more] | Read more »
I am Bread (Games)
I am Bread 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ‘I am Bread’ is the latest quirky adventure from the creators of 'Surgeon Simulator', Bossa Studios. This isn't the best thing... | Read more »

Price Scanner via MacPrices.net

Near-Office Input Functionality Virtually Any...
Today Logitech introduced the Logitech K380 Multi-Device Bluetooth Keyboard and the Logitech M535 Bluetooth Mouse, giving users the freedom to work on any device, most anywhere. According to... Read more
College Student Deals: Additional $100 off Ma...
Take an additional $100 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through September 4, 2015. Anyone with a valid .EDU email address can take... Read more
2.8GHz Mac mini available for $988, includes...
Adorama has the 2.8GHz Mac mini available for $988, $11 off MSRP, including a free copy of Apple’s 3-Year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax in NY & NJ... Read more
Will You Buy An iPad Pro? – The ‘Book Mystiqu...
It looks like we may not have to wait much longer to see what finally materializes as a new, larger-panel iPad (Pro/Plus?) Usually reliable Apple product prognosticator KGI Securities analyst Ming-... Read more
eFileCabinet Announces SMB Document Managemen...
Electronic document management (EDM) eFileCabinet, Inc., a hosted solutions provider for small to medium businesses, has announced that its SecureDrawer and eFileCabinet Online services will be... Read more
WaterField Designs Unveils American-Made, All...
San Francisco’s WaterField Designs today unveiled their all-leather Cozmo 2.0 — an elegant attach laptop bag with carefully-designed features to suit any business environment. The Cozmo 2.0 is... Read more
Apple’s 2015 Back to School promotion: Free B...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
128GB MacBook Airs on sale for $100 off MSRP,...
B&H Photo has 11″ & 13″ MacBook Airs with 128GB SSDs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799.99, $100 off MSRP... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 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
Content Partner Engineer - *Apple* TV - App...
**Job Summary** The Apple TV team is looking for an experienced engineer with a passion for delivering first in class home entertainment solutions. The candidate will Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
*Apple* Retail - Multiple Customer Support P...
Job Description: Customer Support Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.