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

FoldersSynchronizer 4.2.1 - Synchronize...
FoldersSynchronizer is a popular and useful utility that synchronizes and backs-up files, folders, disks and boot disks. On each session you can apply special options like Timers, Multiple Folders,... Read more
Simon 4.0.2 - Monitor changes and crashe...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
Cocktail 8.1.2 - General maintenance and...
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
Cyberduck 4.6.4 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Herald 5.0.2 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
DEVONthink Pro 2.8.3 - Knowledge base, i...
Save 10% with our exclusive coupon code: MACUPDATE10 DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research... Read more
Boom 2 1.0.1 - System-wide pro audio app...
Boom 2 is a system-wide volume booster and equalizer app that is designed especially for OS X 10.10 Yosemite. It comes with a smart interface, self-calibrates itself according to your Mac, offers... Read more
Apple Security Update 2015-001 - For OS...
Apple Security Update 2015-001 is recommended for all users and improves the security of OS X. For detailed information about the security content of this update, please visit: http://support.apple.... Read more
Drive Genius 4.0.1 - Powerful system uti...
Drive Genius 4 gives you faster performance from your Mac while also protecting it. The award-winning and improved DrivePulse feature alerts you to hard drive issues before they become major problems... Read more
Yosemite Cache Cleaner 9.0.3 - Clear cac...
Yosemite Cache Cleaner is an award-winning general purpose tool for OS X. YCC makes system maintenance simple with an easy point-and-click interface to many OS X functions. Novice and expert users... Read more

Unlock Sunshine Girl in Ironkill with th...
Unlock Sunshine Girl in Ironkill with this special 148Apps code Posted by Rob Rich on January 29th, 2015 [ permalink ] Robo-fighter Ironkill has been out on iOS a | Read more »
Crossroad Zombies Review
Crossroad Zombies Review By Jordan Minor on January 29th, 2015 Our Rating: :: CROSSWALKING DEADiPad Only App - Designed for the iPad Crossroad Zombies is a rough draft of a cool genre mash-up.   | Read more »
Blood Brothers 2 – Tips, Cheats, and Str...
War is hell: Is it the kind of hell you want to check out? Read our Blood Brothers 2 review to find out! Blood Brothers 2, DeNA’s follow-up to the original Blood Brothers, is an intriguing card collecting / role-playing / strategy hybrid. There’s... | Read more »
Blood Brothers 2 Review
Blood Brothers 2 Review By Nadia Oxford on January 29th, 2015 Our Rating: :: AN AGGRAVATING RELATIVEUniversal App - Designed for iPhone and iPad Blood Brothers 2 is built on a simple, solid foundation, but its free-to-play system... | Read more »
I AM BREAD, the Toast of the Town, is Ro...
Have you ever dreamt of being deliciously gluten-y? Do you feel passionate about Rye and Wheat? The guys at Bossa Studios do and that is why they are bringing I AM BREAD to iOS soon. The loafy app will feature all the new content that is being... | Read more »
Fahrenheit: Indigo Prophecy Remastered R...
Fahrenheit: Indigo Prophecy Remastered Review By Jennifer Allen on January 29th, 2015 Our Rating: Universal App - Designed for iPhone and iPad 10 years after its initial release and Fahrenheit: Indigo Prophecy still feels like a... | Read more »
Evolve: Hunters Quest is a Match-3 That...
Evolve: Hunters Quest is a Match-3 That Has You Hunting Up Some Big Game. Posted by Jessica Fisher on January 29th, 2015 [ permalink ] | Read more »
Destructamundo Review
Destructamundo Review By Jordan Minor on January 29th, 2015 Our Rating: :: WRECKING CREWUniversal App - Designed for iPhone and iPad Destructamundo blows up real good.   | Read more »
The Sandbox Gets Exclusive Update, Adds...
The Sandbox Gets Exclusive Update, Adds Invincible Characters Campagin Posted by Tre Lawrence on January 29th, 2015 [ permalink ] | Read more »
Mean Girls: The Game Review
Mean Girls: The Game Review By Jordan Minor on January 29th, 2015 Our Rating: :: FETCHUniversal App - Designed for iPhone and iPad A Mean Girls tower defense game makes way more sense than you’d think.   | Read more »

Price Scanner via MacPrices.net

Back in stock: Refurbished iPod nanos for $99...
The Apple Store has Apple Certified Refurbished 16GB iPod nanos available for $99 including free shipping and Apple’s standard one-year warranty. That’s $50 off the cost of new nanos. Most colors are... Read more
New Good Management Suite Simplifies Enterpri...
Good Technology has announced the availability of the Good Management Suite, a comprehensive cross-platform solution for organizations getting started with mobile business initiatives. Built on the... Read more
15-inch 2.0GHz Retina MacBook Pro (refurbishe...
The Apple Store has Apple Certified Refurbished previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1489 including free shipping plus Apple’s standard one-year warranty. Their price is... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon has the 15″ 2.5GHz Retina MacBook Pro on sale for $2319.99 including free shipping. Their price is $180 off MSRP, and it’s the lowest price available for this model from any reseller. Read more
iPad Turns Five – The ”Book Mystique
Five years ago this week, the late Steve Jobs took the stage at an Apple special press event to unveil the first Apple iPad — a slab-shaped, hand-holdable device that was to shake the personal... Read more
Sale! 15-inch Retina MacBook Pros for up to $...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for $150 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1889.99... Read more
Apple refurbished Mac minis for up to $150 of...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Apple Releases New Tranche Of OS Updates For...
Apple has released incremental bugfix and security updates for its most recent Mac and iOS versions. OS X Yosemite 10.10.2 Update Apple says this update includes the following improvements: Resolves... Read more
Samsung To Be Main Supplier Of A9 Chips For i...
Korea’s Maeil Business Newspaper reports that Samsung has beat out its chipmaking rivals, Taiwan’s Qualcomm and TSMC, as supplier of chipsets for the next generations of both its own Galaxy S6 and... Read more
Save up to $340 on iMacs with Apple Certified...
The Apple Store has Apple Certified Refurbished iMacs available for up to $340 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

Jobs Board

*Apple* Systems Engineer - Pre Sales, Educat...
…is responsible for proactively providing technical expertise to drive sales of Apple solutions into assigned accounts. The SE architects, validates, and assists in Read more
*Apple* Solutions Consultant - Retail Sales...
As an ambassador of the Apple brand, the ASC is accountable for driving sales performance by: Connecting with customers. Discovering customers' needs and values. Showing Read more
*Apple* Solutions Consultant - Retail Sales...
As an ambassador of the Apple brand, the ASC is accountable for driving sales performance by: Connecting with customers. Discovering customers' needs and values. Showing Read more
*Apple* Tree Children's Center is hirin...
Apple Tree Learning Center is Hiring Preschool Teachers! Phoenix Childrens Academy is a national leader in the early childhood education industry and we are currently Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.