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).

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Yasu 3.0.3 - System maintenance app; per...
Yasu was created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks quickly within a... Read more
MYStuff Pro 2.0.26 - $39.99
MYStuff Pro is the most flexible way to create detail-rich inventories for your home or small business. Add items to MYStuff by dragging and dropping existing information, uploading new images, or... Read more
MarsEdit 3.7.8 - Quick and convenient bl...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
Apple iBooks Author 2.5 - Create and pub...
Apple iBooks Author helps you create and publish amazing Multi-Touch books for iPad. Now anyone can create stunning iBooks textbooks, cookbooks, history books, picture books, and more for iPad. All... Read more
TunnelBear 3.0.1 - Subscription-based pr...
TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
ExpanDrive 5.4.0 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Opera 40.0.2308.62 - High-performance We...
Opera is a fast and secure browser trusted by millions of users. With the intuitive interface, Speed Dial and visual bookmarks for organizing favorite sites, news feature with fresh, relevant content... Read more
Apple iOS 10.0.2 - The latest version of...
iOS 10 is the biggest release of iOS ever. A massive update to Messages brings the power of the App Store to your conversations and makes messaging more personal than ever. Find your route with... Read more
Herald 7.0 - Notification plugin for Mai...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), 4.0.8 (for OS X 10.9), 5.0.2 (for OS X 10.10), and 6.0.3 (for OS X 10.11) are no longer supported by the developer. Herald is a... Read more
Postbox 5.0.2 - Powerful and flexible em...
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

How to build and care for your team in D...
Before you hit the trail and become a dog sledding legend, there’s actually a fair bit of prep work to be done. In Dog Sled Saga, you’re not only racing, you’re also building and caring for a team of furry friends. There’s a lot to consider—... | Read more »
How to win every race in Dog Sled Saga
If I had to guess, I’d say Dog Sled Saga is the most adorable racing game on the App Store right now. It’s a dog sled racing sim full of adorable, loyal puppies. Just look at those fluffy little tails wagging. Behind that cute, pixelated facade is... | Read more »
Let the war games commence in Gunship Ba...
Buzz Lightyear famously said, “This isn’t flying, this is falling – with style!” In the case of Gunship Battle: Second War, though, this really is flying - with style! The flight simulator app from Joycity puts you in control of 20 faithfully... | Read more »
How to get a high score in Fired Up
Fired Up is Noodlecake Games’ high score chasing, firefighting adventure. You take control of a wayward firefighter who propels himself up the side of a highrise with blasts of water. Sound silly? It is. It’s also pretty difficult. You can’t... | Read more »
NBA 2K17 (Games)
NBA 2K17 1.0 Device: iOS iPhone Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Following the record-breaking launch of NBA 2K16, the NBA 2K franchise continues to stake its claim as the most authentic sports video... | Read more »
Dog Sled Saga (Games)
Dog Sled Saga 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: A game by Dan + Lisa As a rookie musher, foster a dogsledding team whose skills will grow if they're treated right. Week by... | Read more »
60 Seconds! Atomic Adventure (Games)
60 Seconds! Atomic Adventure 1.2 Device: iOS Universal Category: Games Price: $2.99, Version: 1.2 (iTunes) Description: 60 Seconds! is a dark comedy atomic adventure of scavenge and survival. Collect supplies and rescue your family... | Read more »
Tons of Bullets! (Games)
Tons of Bullets! 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Tons of BulletsA retro 2D pixel platformer infused with tons of bullets and tons of features!Fight as Kenji the Ninja and... | Read more »
Sorcery! 4 (Games)
Sorcery! 4 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: *** PLEASE NOTE: Sorcery! 4 requires a minimum of iPad 3 or iPhone 4 *** An epic adventure through a cursed citadel of monsters,... | Read more »
Building the perfect Animation Throwdown...
Animation Throwdown is a casual card game full of plenty of inside jokes from some of your favorite cartoons. It’s accessible as far as card games go, but that’s not to say that it hands your victories to you easily. There’s quite a bit that goes... | Read more »

Price Scanner via MacPrices.net

Duplicate Sweeper Free On Mac App Store For O...
To celebrate the launch of Apple’s latest macOS Sierra, Stafford, United Kingdom based Wide Angle Software has announced that its duplicate file finder software, Duplicate Sweeper, is now available... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina Apple MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1174.99 $125 off MSRP - 13... Read more
Evidence Surfaces Pointing To New A10X Chip F...
Citing a job description for a Project Lead position at Apple’s Austin, Texas engineering labs, Motley Fool’s Ashraf Eassa deduces that development is progressing well on Apple’s next-generation in-... Read more
Check Print’R for macOS Allows Anyone to Easi...
Delaware-based Match Software has announced the release and immediate availability of Check Print’R 3.21, an important update to their easy-to-use check printing application for macOS. Check Print’R... Read more
Apple refurbished 11-inch MacBook Airs availa...
Apple has Certified Refurbished 11″ MacBook Airs (the latest models), available for up to $170 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
Apple refurbished 15-inch Retina MacBook Pros...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2... Read more
Major Version 5 Release Of Free myTuner Radio...
AppGeneration Software has announced the release of myTuner Radio 5.0.1, an important update to their live radio app for iOS and Android platforms. With myTuner Radio, you can listen to more than 30,... Read more
TheAppStore Searches And Filters iOS And macO...
Boston based InchWest has announced the official launch of TheAppStore, a free web based service that lets users search iOS and Mac apps on the web without iTunes or the App store. TheAppStore... Read more
2TB Time Capsule on sale for $199, save $100
Amazon has 2TB Apple Time Capsules on sale for $199 including free shipping. Their price is $100 off MSRP. Choose Amazon as the seller when making your purchase, rather than a third party seller. Read more
12-inch 1.2GHz Rose Gold Retina MacBook on sa...
Amazon.com has the 2016 12″ 1.2GHz Rose Gold Retina MacBook on sale for $1254.02 including free shipping. Their price is $345 off MSRP, and it’s the lowest price available for this model from any... Read more

Jobs Board

*Apple* Retail - Multiple Positions- Manhatt...
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
Sr. *Apple* Mac Engineer - Net2Source Inc....
…staffing, training and technology. We have following position open with our client. Sr. Apple Mac Engineer6+ Months CTH Start date : 19th Sept Travelling Job If 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
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.