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
$118.63
Apple Inc.
+2.16
MSFT
$47.59
Microsoft Corpora
-0.39
GOOG
$539.27
Google Inc.
+1.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Carbon Copy Cloner 4.0.3 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
ForeverSave 2.1.3 - Universal auto-save...
ForeverSave auto-saves all documents you're working on while simultaneously doing backup versioning in the background. Lost data can be quickly restored at any time. Losing data, caused by... Read more
Voila 3.8.1 - Capture, annotate, organiz...
Voila is a screen-capture, recording, and annotation tool that is a full-featured replacement for Mac's screen-capture and screen-recording capabilities. It has a large and robust set of editing,... Read more
SyncTwoFolders 2.0.6 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
Duplicate Annihilator 5.1.1 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
Cobook 3.0.7 - Intelligent address book....
Cobook Contacts is an intuitive, engaging address book. Solve the problem of contact management with Cobook Contacts and its simple interface and powerful syncing and integration possibilities.... Read more
StatsBar 1.9 - Monitor system processes...
StatsBar gives you a comprehensive and detailed analysis of the following areas of your Mac: CPU usage Memory usage Disk usage Network and bandwidth usage Battery power and health (MacBooks only)... Read more
Cyberduck 4.6 - FTP and SFTP browser. (F...
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
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Evernote 6.0.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more

Latest Forum Discussions

See All

RuPaul’s Drag Race: Dragopolis 2.0 is C...
RuPaul’s Drag Race: Dragopolis 2.0 is Coming to iOS December 4th Posted by Jessica Fisher on November 24th, 2014 [ permalink ] So Much Drama | Read more »
Crimson Keyboard (Utilities)
Crimson Keyboard 1.0 Device: iOS iPhone Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Introducing a faster way to type. Crimson Keyboard adapts to your writing style and provides smart, personal and contextual... | Read more »
The Sandbox EDU Review
The Sandbox EDU Review By Nadia Oxford on November 24th, 2014 Our Rating: :: COME PLAY IN THE SANDBOX AGAINUniversal App - Designed for iPhone and iPad Like its predecessor, The Sandbox EDU offers lots for players to see, do, and... | Read more »
Taichi Panda Hits iOS in December
Taichi Panda Hits iOS in December Posted by Jessica Fisher on November 24th, 2014 [ permalink ] Snail Games has released the first official game play trailer for  | Read more »
Five Apps to Make Your Thanksgiving Plan...
Thanksgiving is nearly upon us! You know what that means? Eating too much turkey, watching the Big Game, and spending time with family (whether you like it or not). Oh, and that scary Black Friday thing, but we won’t talk about that here. For those... | Read more »
Kingdom Rush Origins HD Review
Kingdom Rush Origins HD Review By Jennifer Allen on November 24th, 2014 Our Rating: :: JUST AS GOOD AS BEFOREiPad Only App - Designed for the iPad It’s more of the same again, but that’s really no bad thing at all.   | Read more »
Skylanders, Show Off How You Fight Kaos...
Skylanders, Show Off How You Fight Kaos to Win Cool Prizes Posted by Jessica Fisher on November 24th, 2014 [ permalink ] iPad Only App - Designed for the iPad | Read more »
Mark of the Dragon – Tips, Tricks, and S...
Calling All Dragon Riders: | Read more »
Playdek has Annouced a Delicious Thanksg...
Playdek has Annouced a Delicious Thanksgiving Sale Posted by Jessica Fisher on November 24th, 2014 [ permalink ] Playdek, makers of | Read more »
Pair Solitaire Review
Pair Solitaire Review By Jennifer Allen on November 24th, 2014 Our Rating: :: ADDICTIVE. SO ADDICTIVEUniversal App - Designed for iPhone and iPad Welcome to your new ‘five more minutes, oh no, why is it dark outside’ addiction.   | Read more »

Price Scanner via MacPrices.net

Jumptuit Launches One-Tap Windows 8.1 iTunes...
Jumptuit has launched Windows 8.1 support for One-Tap iTunes Sync. with which Windows 8.1 users can now easily sync their iTunes libraries with Microsoft OneDrive. Jumptuit provides easy access from... Read more
CEA Study Finds More People Recycling Electro...
A new study by the Consumer Electronics Association (CEA) finds that electronics recycling receives the continued and growing support of consumers. According to the CEA,s Recycling and Reuse Study,... Read more
15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale today for $1749. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of Parallels Desktop... Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the new 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available... Read more
21-inch 1.4GHz iMac on sale for $979, save $1...
B&H Photo has the new 21″ 1.4GHz iMac on sale for $979.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. B&H will also include free copies of Parallels Desktop... Read more
13-inch 1.4GHz/256GB MacBook Air on sale for...
B&H Photo has lowered their price on the 13″ 1.4GHz/256GB MacBook Air to $1059.99 including free shipping plus NY sales tax only. Their price is $140 off MSRP, and it’s the lowest price for this... Read more
Save up to $400 with Apple refurbished 2014 1...
The Apple Store has restocked Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and shipping... Read more
New 13-inch 1.4GHz MacBook Air on sale for $8...
 Adorama has the 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. B&H Photo has the 13″ 1.4GHz/128GB MacBook... Read more
Apple Expected to Reverse Nine-Month Tablet S...
Apple and Samsung combined accounted for 62 percent of the nearly 36 million branded tablets shipped in 3Q 2014, according to early vendor shipment share estimates from market intelligence firm ABI... Read more
Stratos: 30 Percent of US Smartphone Owners t...
Stratos, Inc., creator of the Bluetooth Connected Card Platform, has announced results from its 2014 Holiday Mobile Payments Survey. The consumer survey found that nearly one out of three (30 percent... 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* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple 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.