TweetFollow Us on Twitter

Efficient 68000
Volume Number:8
Issue Number:2
Column Tag:Assembly workshop

Efficient 68000 Programming

If a new CPU speeds up inefficient code, what do you think it will do to efficient code?

By Mike Scanlin, MacTutor Regular Contributing Author

The dew is cold. It is quiet. I hear nothing except for crackling sounds coming from the little fire burning two inches to the left of my keyboard. It wasn’t there a minute ago. Seems that Doo-Dah, the god of efficient programming, is upset with me for typing “Adda.W #10,A0” and just sent me a warning in the form of a lightning bolt. I hate it when he does that. You’d think that after three years in his service, researching which 68000 assembly language instructions are the most efficient ones for any given job, that he would lighten up a little. I guess that’s what makes him a god and me a mere mortal striving for enlightenment through the use of optimal instructions. As I extinguish the fire with a little Mountain Dew, I reflect upon the last three years.

My first lesson in the service of Doo-Dah was that proficiency in assembly language is a desirable skill in programmers so long as performance is a desirable attribute of software. The nay-sayers who depend upon faster and faster CPUs to make their sluggish software run at acceptable speeds don’t realize the underlying relativeness of the universe. If a new CPU will speed up a set of non-optimal instructions by 10%, then it will also speed up a set of optimal instructions by 10%. One should strive to be right on the edge of absolute maximum performance all the time. Users may not notice the difference in a 2K document but when they start working with 20MB documents they will soon be able to separate the optimal software from the non-optimal.

In the months following that lesson, I was given the task of compiling a list of instructions that should only very rarely appear in any program executing on a 68000 (and only then because you’re dealing with either self-modifying code or special hardware that depends on certain types of reads and writes from the processor). They are:

Don't Use Use Save

Move.B #0,Dx Clr.B Dx 8 cycles, 2 bytes

Move.W #0,Dx Clr.W Dx 8 cycles, 2 bytes

Clr.L Dx Moveq #0,Dx 2 cycles

Move.L #0,Dx Moveq #0,Dx 8 cycles, 4 bytes

Move.L #0,Ax Suba.L Ax,Ax 4 cycles, 4 bytes

Move.L #[-128..127],Dx Moveq #[-128..127],Dx 8 cycles, 4 bytes

Move.L #[-128..127],ea Moveq #[-128..127],Dx 4 cycles, 2 bytes

Move.L Dx,ea

Move.L #[128..254],Dx Moveq #[64..127],Dx 4 cycles, 2 bytes

Add Dx,Dx

Move.L #[-256..-130],Dx Moveq #[-128..-65],Dx 0 cycles, 2 bytes

Add.L Dx,Dx

Lea [1..8](Ax),Ax Addq #[1..8],Ax 0 cycles, 2 bytes

Add.W #[9..32767],Ax Lea [9..32767](Ax),Ax 4 cycles

Lea [-8..-1](Ax),Ax Subq #[1..8],Ax 0 cycles, 2 bytes

Sub.W #[9..32767],Ax Lea [-32767..-9](Ax),Ax 4 cycles

Asl.W #1,Dx Add.W Dx,Dx 4 cycles

Asl.L #1,Dx Add.L Dx,Dx 2 cycles

Cmp.x #0,ea Tst.x ea 4-10 cycles, 2 bytes

And.L #$0000FFFF,Dx Swap Dx 4 cycles

Clr.W Dx

Swap Dx

In addition, if you don’t care about the values of the condition codes then the following may be optimized:

Don't Use Use Save

Move.W #nnnn,-(SP) Move.L #ppppnnnn,-(SP) 4 cycles, 2 bytes

Move.W #pppp,-(SP)

Move.L #$0000nnnn,-(SP) Pea $nnnn 4 cycles, 2 bytes

Move.B #255,Dx St Dx 2 cycles, 2 bytes

Move.L #$00nn0000,Dx Moveq #[0..127],Dx 4 cycles, 2 bytes

Swap Dx

Movem (SP)+,Dx Move (SP)+,Dx 4 cycles

Ext.L Dx

Movem.L Dx,-(SP) Move.L Dx,-(SP) 4 cycles, 2 bytes

Movem.L (SP)+,Dx Move.L (SP)+,Dx 8 cycles, 2 bytes

Movem.L (SP)+,<2 regs> Move.L (SP)+,<reg 1> 4 cycles

Move.L (SP)+,<reg 2>

Note that pushing 2 regs or popping 3 with Movem.L is equivalent in cycles to doing it with multiple Move.L’s, but popping 3 regs with Move.L’s costs you two extra bytes. An easy rule to remember is to always use Movem.L whenever you’re dealing with 3 or more registers.

There are other optimizations you can make with minimal assumptions. For instance, if you are making room for a function result then don’t use Clr:

Don't UseUseSave
Clr.W -(SP)Subq #2,SP6 cycles
_Random _Random
Clr.L -(SP)Subq #4,SP14 cycles
_FrontWindow _FrontWindow

If you’re trying to set, clear, or change one of the low 16 bits of a data register and you don’t need to test it first, then don’t use these:

Don't UseUseSave
Bset #n,DxOr.W #mask,Dx4 cycles
Bclr #n,DxAnd.W #mask,Dx4 cycles
Bchg #n,DxEor.W #mask,Dx4 cycles

You should use registers wherever possible, not memory (because memory is much slower to access). If you need to test for a NIL handle or pointer, for instance, do this:

Don't UseUseSave
Move.L A0,-(SP)Move.L A0,D016 cycles, 2 bytes
Addq #4,SPBeq.S ItsNil
Beq.S ItsNil

Use the “quick” operations wherever you can. Many times you can reverse the order of two instructions to use a Moveq (since Moveq handles bigger numbers than Addq/Subq):

Don't UseUseSave
Move.L D0,D1Moveq #10,D16 cycles, 4 bytes
Add.L #10,D1Add.L D0,D1

Also, use two Addq’s or Subq’s when dealing with longs in the range of 9..16:

Don't UseUseSave
Addi.L #10,D0Addq.L #2,D04 cycles, 2 bytes
Addq.L #8,D0

The following three optimizations will reduce the size of your program but at the expense of a few cycles. This is good for user interface code, but you probably don’t want to use these optimizations in tight loops where speed is important:

Don't UseUseSave
Move.B #0,-(SP)Clr.B -(SP)-2 cycles, 2 bytes
Move.W #0,-(SP)Clr.W -(SP)-2 cycles, 2 bytes
Move.L #0,-(SP)Clr.L -(SP)-2 cycles, 4 bytes

Most of the optimizations from here onward are only applicable in some cases. Many times you can use a slightly different version of the exact code given here to get an optimization that works well for your particular set of circumstances. These optimizations don’t always have the same set of side effects or overflow/underflow conditions that the original code has, so use them with caution.

Shifting left by 2 bits (to multiply by 4) should be avoided if you’re coding for speed:

Don't UseUseSave
Asl.W #2,DxAdd.W Dx,Dx2 cycles, -2 bytes
Add.W Dx,Dx

Use bytes for booleans instead of bits. They’re faster to access (and less code in some cases). If you have many booleans, though, bits may be the way to go because of reduced memory requirements (of the data, that is, not the code).

Don't UseUseSave
Btst #1,myBools(A6)Tst.B aBool(A6)4 cycles, 2 bytes
Btst #1,D0Tst.B D06 cycles, 2 bytes

Avoid the use of multiply and divide instructions like the plague. Use shifts and adds for immediate operands or loops of adds and subtracts for variable operands. For instance, to multiply by 14 you could do this:

Don't UseUseSave
Mulu #14,D0Add D0,D0many cycles, -4 bytes
Move D0,D1
Lsl #3,D0
Sub D1,D0

If you have a variable source operand, but you know that it is typically small (and positive, for this example), then use a loop instead of a multiply instruction. This works really well in the case of a call to FixMul if you know one of the operands is a small integer -- you can avoid the trap overhead and the routine itself by using a loop similar to this one (in fact, the FixMul routine itself checks if either parameter is 1.0 before doing any real work):

Don't UseUseSave
Mulu D1,D0Move D0,D2many cycles, -8 bytes
Neg D2
@1 Add D0,D2
Subq #1,D1
Bne.S @1

Likewise, for division, use a subtract loop if you know that the quotient isn’t going to be huge (and if the destination fits in 16 bits):

Don't UseUseSave
Divu D1,D0Moveq #0,D2many cycles, -10 bytes
Cmp D1,D0
Bra.S @2
@1 Addq #1,D2
Sub D1,D0
@2 Bhi.S @1

Don’t use Bsr/Rts in tight loops where speed is important. Put the return address in an unused address register instead.

Don't UseUseSave
Bsr MyProcLea @1,A08 cycles, -4 bytes
;<blah>Bra MyProc
@1 ;<blah>
MyProc:MyProc:
;<blah blah>;<blah blah>
RtsJmp (A0)

You can eliminate a complete Bsr/Rts pair (or equivalent above) if the Bsr is the last instruction before an Rts by changing the Bsr to a Bra:

Don't UseUseSave
Bsr MyProcBra MyProc24 cycles, 2 bytes
Rts

Don’t use BlockMove for moves of 80 bytes or less where you know the source and destination don’t overlap. The trap overhead and preflighting that BlockMove does make it inefficient for such small moves. Use this loop instead (assuming Dx > 0 on entry):

Don't UseUseSave
_BlockMoveSubq #1,Dxmany cycles, -6 bytes
@1 Move.B (A0)+,(A1)+
Dbra Dx,@1

I base this conclusion on time trials done on a Mac IIci with a cache card. The actual results were (for several thousand iterations):

Figure 1: How fast do blocks move?

I did the same tests on a Mac SE and found that it was only beneficial to call BlockMove on that machine for moves of 130 bytes or more. However, since you should optimize for the lowest common denominator across all machines, you should only use the Dbra loop for non-overlapping moves of 80 bytes or less.

Be warned, though: on the Quadras, BlockMove has been modified to flush the 040 caches because of the possibility that you (or the memory manager) are BlockMoving executable code. So don’t use the above loop for moving small amounts of code (like you might do in some INIT installation code). Apple did this for compatibility reasons with existing non-040 aware applications running in 040 copy-back mode (high performance mode). However, because of this, your non-code BlockMoves are unnecessarily clearing the caches, too. I don’t know if it’s worth it to write a dedicated BlockMove for non-code moves, but it seems like it’s worth doing and then timing to see if there’s a difference.

Unroll loops. At the expense of a few extra bytes you can make any tight loop run faster. This is because short branch instructions that are not taken are faster than those that are taken. Here’s an even faster version of the above loop:

;1

 Subq #1,Dx
 @1 Move.B (A0)+,(A1)+
 Subq #1,Dx
 Bcs.S @2
 Move.B (A0)+,(A1)+
 Subq #1,Dx
 Bcs.S @2
 Move.B (A0)+,(A1)+
 Dbra Dx,@1
 @2

Beware when using the above trick, though, because it doesn’t work for long branches. In that case, a taken branch is faster than a branch not taken.

Preserving pointers into relocatable blocks across code that moves memory: If you need to lock a handle because you’re going to call a routine that moves memory but the handle (and the dereferenced handle) isn’t a parameter to that routine, then you can usually avoid locking the handle with a trick (which has the desirable side effect of reducing memory fragmentation). Assume the handle is in A3 and the pointer into the middle of the block is in A2. All you really have to do is save/restore the offset into the block; you don’t care if the block moves or not:

Don't UseUseSave
Move.L A3,A0Sub.L (A3),A2many cycles, 4 bytes
_HLock
;<move memory> ;<move memory>
Move.L A3,A0Add.L (A3),A2
_HUnlock

If the end of a routine is executing the same set of instructions two or more times, then you may be able to use this trick to save some bytes (at the expense of a few cycles). If the end of the routine looks like a subroutine, then have it Bsr to itself, like this (this example is drawing a BCD byte in D3):

Don't UseUseSave
Ror #4,D3Ror #4,D3many bytes
Move.B D3,D0Bsr @1
And #$000F,D0Rol #4,D3
Add #'0',D0
Move D0,-(SP)
_DrawChar
Rol #4,D3
Move.B D3,D0@1 Move D3,D0
And #$000F,D0And #$000F,D0
Add #'0',D0Add #'0',D0
Move D0,-(SP)Move D0,-(SP)
_DrawChar _DrawChar
Rts Rts

Use multiple entry points to set common parameters. Suppose you have a routine that takes a boolean value in D0 as an input and suppose you call this routine 20 times with the value of True and 30 times with the value of False. It would save code if you made two entry points that each set D0, and then branched to common code. For instance:

Don't UseUseSave
St D0Bsr MyProcTruemany bytes
Bsr MyProc
Sf D0Bsr MyProcFalse
Bsr MyProc
MyProcTrue:
St D0
Bra.S MyProc
MyProcFalse:
Sf D0
MyProc:MyProc:
;<blah>;<blah>
RtsRts

Clean up the stack with Unlk. If your routine already has a stack frame and you create some temporary data on the stack (in addition to the stack frame) then you don’t always need to remove it when you’re done with it -- the Unlk will clean it up for you. For instance, suppose you make a temporary Rect on the stack. You would normally remove it with Addq #8,SP but if it’s near the end of a function that does an Unlk, then leave the Rect there; it’ll be gone when the Unlk executes.

Well, hopefully Doo-Dah has many more learned disciples now. Don’t forget to sacrifice a copy of FullWrite in his honor at least once a year. That makes him happy.

P.S. If you want even more 68000 optimizations there is an excellent article by Mike Morton in the September, 1986, issue of Byte magazine called “68000 Tricks and Traps” (pgs. 163-172). There are more than half a dozen or so tricks in that article not covered in this article (sorry for not listing them here but I didn’t want to get sued for plagiarism).

 
AAPL
$97.67
Apple Inc.
+1.41
MSFT
$43.63
Microsoft Corpora
+0.89
GOOG
$511.17
Google Inc.
-13.34

MacTech Search:
Community Search:

Software Updates via MacUpdate

MenuMeters 1.8 - CPU, memory, disk, and...
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking... Read more
Coda 2.5 - One-window Web development su...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few... Read more
Arq 4.6.1 - Online backup to Google Driv...
Arq is super-easy online backup for the Mac. Back up to your own Google Drive storage (15GB free storage), your own Amazon Glacier ($.01/GB per month storage) or S3, or any SFTP server. Arq backs up... Read more
Airfoil 4.8.10 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
Apple iMovie 10.0.6 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more
OnyX 2.8.8 - Maintenance and optimizatio...
OnyX is a multifunctional utility for OS X. It allows you to verify the startup disk and the structure of its System files, to run miscellaneous tasks of system maintenance, to configure the hidden... Read more
Parallels Desktop 10.1 - Run Windows app...
Parallels Desktop is simply the world's bestselling, top-rated, and most trusted solution for running Windows applications on your Mac. With Parallels Desktop for Mac, you can seamlessly run both... Read more
Apple Keynote 6.5 - Apple's present...
Apple Keynote makes it simple to create and deliver beautiful presentations. Powerful tools and dazzling effects bring your ideas to life. You can work seamlessly between Mac and iOS devices. And... Read more
OmniPlan 2.3.7 - Robust project manageme...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Flavours 1.1.16 - Create and apply theme...
Flavours is a Mac application that allow users to create, apply and share beautifully designed themes. Classy Give your Mac a gorgeous new look by applying delicious themes! Easy Unleash your... Read more

Latest Forum Discussions

See All

My Very Hungry Caterpillar (Education)
My Very Hungry Caterpillar 1.0.0 Device: iOS Universal Category: Education Price: $3.99, Version: 1.0.0 (iTunes) Description: Care for your very own Very Hungry Caterpillar! My Very Hungry Caterpillar will captivate you as he crawls... | Read more »
Dungeon Dick (Games)
Dungeon Dick 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Dungeon Dick is a fantasy adventure where you must discover the wicked plot to destroy the lands . 'Fling' at your foes and land... | Read more »
Here’s How the Apple Watch Could Transfo...
With the Apple Watch’s generic release date of, “early 2015” hovering on the horizon, it’s only a matter of time before gamers begin to ask “What’s in it for us?” The obvious choice would be to place entire games directly on the face of the watch,... | Read more »
Republique Episode 3: Ones & Zeroes...
Republique Episode 3: Ones & Zeroes is Available Now Posted by Rob Rich on October 17th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Loot Raiders Review
Loot Raiders Review By Campbell Bird on October 17th, 2014 Our Rating: :: PRESS BUTTON, GET LOOTUniversal App - Designed for iPhone and iPad This menu-based loot game is suitable to compel those with pretty extreme loot lust.   | Read more »
Angry Birds are Crashing Puzzle & Dr...
Angry Birds are Crashing Puzzle & Dragons Next Week Posted by Rob Rich on October 17th, 2014 [ permalink ] Next week, Puzzle & Dragons is getting a little angry. Because, you know, Angry Birds. It’s a… a pun. I thought… never mind. | Read more »
Incandescence Review
Incandescence Review By Andrew Fisher on October 17th, 2014 Our Rating: :: ALL TAPPED OUTUniversal App - Designed for iPhone and iPad Incandescence promises intense tapping action, but isn’t particularly fun.   | Read more »
Lion Pig (Games)
Lion Pig 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: "Leaving aside the biology of how a pig and a lion might meet, fall in love, and create this curious cross-breed, Lion Pig is a... | Read more »
Zero Lives (Games)
Zero Lives 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Zero Lives is a puzzle game. Simple and elegant at its core but deep and captivating the more you play. Each puzzle will challenge... | Read more »
#SUPERHYPER (Games)
#SUPERHYPER 001 Device: iOS Universal Category: Games Price: $.99, Version: 001 (iTunes) Description: "This is a game that fans of challenging minimalist arcade games, not to mention pixel art, should keep an eye on" - Touch Arcade "... | Read more »

Price Scanner via MacPrices.net

Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more
C Spire to offer iPad Air 2 and iPad mini 3,...
C Spire on Friday announced that it will offer iPad Air 2 and iPad mini 3, both with Wi-Fi + Cellular, on its 4G+ LTE network in the coming weeks. C Spire will offer the new iPads with a range of... Read more
Belkin Announces Full Line of Keyboards and C...
Belkin International has unveiled a new lineup of keyboard cases and accessories for Apple’s newest iPads, featuring three QODE keyboards and a collection of thin, lightweight folios for both the... Read more
Verizon offers new iPad Air 2 preorders for $...
Verizon Wireless is accepting preorders for the new iPad Air 2, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB iPad Air 2 WiFi... Read more
Price drops on refurbished Mac minis, now ava...
The Apple Store has dropped prices on Apple Certified Refurbished previous-generation Mac minis, with models now available starting at $419. Apple’s one-year warranty is included with each mini, and... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
Refurbished 2013 MacBook Pros available for u...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more
WaterField Designs Unveils Outback iPad Slip...
Fresh on the heels of Apple’s announcement Thursday, WaterField Designs unveils its new Outback Slip Case for the iPad Air 2 and iPad mini 3 — a custom-fitted case for the just-announced Apple iPads... Read more
Apple Introduces iPad Air 2 & iPad mini 3...
Apple on Thursday introduced iPad Air 2, slimmed down again to just 6.1 mm, and weighing less than a pound. iPad Air 2 also features an improved Retina display claimed to deliver enhanced contrast... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.