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

Latest Forum Discussions

See All

Top Mobile Game Discounts
Every day, we pick out a curated list of the best mobile discounts on the App Store and post them here. This list won't be comprehensive, but it every game on it is recommended. Feel free to check out the coverage we did on them in the links... | Read more »
Price of Glory unleashes its 1.4 Alpha u...
As much as we all probably dislike Maths as a subject, we do have to hand it to geometry for giving us the good old Hexgrid, home of some of the best strategy games. One such example, Price of Glory, has dropped its 1.4 Alpha update, stocked full... | Read more »
The SLC 2025 kicks off this month to cro...
Ever since the Solo Leveling: Arise Championship 2025 was announced, I have been looking forward to it. The promotional clip they released a month or two back showed crowds going absolutely nuts for the previous competitions, so imagine the... | Read more »
Dive into some early Magicpunk fun as Cr...
Excellent news for fans of steampunk and magic; the Precursor Test for Magicpunk MMORPG Crystal of Atlan opens today. This rather fancy way of saying beta test will remain open until March 5th and is available for PC - boo - and Android devices -... | Read more »
Prepare to get your mind melted as Evang...
If you are a fan of sci-fi shooters and incredibly weird, mind-bending anime series, then you are in for a treat, as Goddess of Victory: Nikke is gearing up for its second collaboration with Evangelion. We were also treated to an upcoming... | Read more »
Square Enix gives with one hand and slap...
We have something of a mixed bag coming over from Square Enix HQ today. Two of their mobile games are revelling in life with new events keeping them alive, whilst another has been thrown onto the ever-growing discard pile Square is building. I... | Read more »
Let the world burn as you have some fest...
It is time to leave the world burning once again as you take a much-needed break from that whole “hero” lark and enjoy some celebrations in Genshin Impact. Version 5.4, Moonlight Amidst Dreams, will see you in Inazuma to attend the Mikawa Flower... | Read more »
Full Moon Over the Abyssal Sea lands on...
Aether Gazer has announced its latest major update, and it is one of the loveliest event names I have ever heard. Full Moon Over the Abyssal Sea is an amazing name, and it comes loaded with two side stories, a new S-grade Modifier, and some fancy... | Read more »
Open your own eatery for all the forest...
Very important question; when you read the title Zoo Restaurant, do you also immediately think of running a restaurant in which you cook Zoo animals as the course? I will just assume yes. Anyway, come June 23rd we will all be able to start up our... | Read more »
Crystal of Atlan opens registration for...
Nuverse was prominently featured in the last month for all the wrong reasons with the USA TikTok debacle, but now it is putting all that behind it and preparing for the Crystal of Atlan beta test. Taking place between February 18th and March 5th,... | Read more »

Price Scanner via MacPrices.net

AT&T is offering a 65% discount on the ne...
AT&T is offering the new iPhone 16e for up to 65% off their monthly finance fee with 36-months of service. No trade-in is required. Discount is applied via monthly bill credits over the 36 month... Read more
Use this code to get a free iPhone 13 at Visi...
For a limited time, use code SWEETDEAL to get a free 128GB iPhone 13 Visible, Verizon’s low-cost wireless cell service, Visible. Deal is valid when you purchase the Visible+ annual plan. Free... Read more
M4 Mac minis on sale for $50-$80 off MSRP at...
B&H Photo has M4 Mac minis in stock and on sale right now for $50 to $80 off Apple’s MSRP, each including free 1-2 day shipping to most US addresses: – M4 Mac mini (16GB/256GB): $549, $50 off... Read more
Buy an iPhone 16 at Boost Mobile and get one...
Boost Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering one year of free Unlimited service with the purchase of any iPhone 16. Purchase the iPhone at standard MSRP, and then choose... Read more
Get an iPhone 15 for only $299 at Boost Mobil...
Boost Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering the 128GB iPhone 15 for $299.99 including service with their Unlimited Premium plan (50GB of premium data, $60/month), or $20... Read more
Unreal Mobile is offering $100 off any new iP...
Unreal Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering a $100 discount on any new iPhone with service. This includes new iPhone 16 models as well as iPhone 15, 14, 13, and SE... Read more
Apple drops prices on clearance iPhone 14 mod...
With today’s introduction of the new iPhone 16e, Apple has discontinued the iPhone 14, 14 Pro, and SE. In response, Apple has dropped prices on unlocked, Certified Refurbished, iPhone 14 models to a... Read more
B&H has 16-inch M4 Max MacBook Pros on sa...
B&H Photo is offering a $360-$410 discount on new 16-inch MacBook Pros with M4 Max CPUs right now. B&H offers free 1-2 day shipping to most US addresses: – 16″ M4 Max MacBook Pro (36GB/1TB/... Read more
Amazon is offering a $100 discount on the M4...
Amazon has the M4 Pro Mac mini discounted $100 off MSRP right now. Shipping is free. Their price is the lowest currently available for this popular mini: – Mac mini M4 Pro (24GB/512GB): $1299, $100... Read more
B&H continues to offer $150-$220 discount...
B&H Photo has 14-inch M4 MacBook Pros on sale for $150-$220 off MSRP. B&H offers free 1-2 day shipping to most US addresses: – 14″ M4 MacBook Pro (16GB/512GB): $1449, $150 off MSRP – 14″ M4... Read more

Jobs Board

All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.