(image courtesy DayStar Digital)
(image courtesy DayStar Digital) |
The Unofficial Turbo 601 SiteMacOS Issues |
Introduction |
Systems 7.5 & 7.5.1 |
Systems 7.5.3 & 7.5.5 |
MacOS 7.6 | ||||||||
MacOS 8 |
The Future ? |
Return to Main Index | |||||||||
24 July - MacOS 8 now has it's own section.
17 June - MacOS 8 doesn't support 030 Macs with PowerPC upgrades--so why have we received reports that indicate that it installs and runs OK on Turbo 601 upgraded machines? I've attempted an explanation which has been added to the MacOS 8 topic. This 'explanation' is still in draft, and I'll refine it once I've had a chance to re-read it a few times.
1 June - Willi Murray has added some comments about the MacOS 7.6.1 update.
27 April - Posted a couple of brief reports about the MacOS 7.6.1 update.
8 April - Willi Murray added some notes about installing MacOS 7.6 on a Mac with a JackHammer installed. You might have problems if you try to install MacOS 7.6 over Systems 7.5 or 7.5.1.
30 March - Jubel Chen sent a report about MacOS 7.6 installation problems.
20 Feb - Thomas Schwab reports that MacOS 7.6 is working fine on his machine.
16 Feb - Systems 7.5.3 and 7.5.5 topic has been severely edited and summarised. Now that MacOS 7.6 is available we're trying to draw a line under Systems 7.5.3 and 7.5.5. The Systems 7.5.3 and 7.5.5 topic will probably be edited a few times over the next week or so to get it saying what we want it to say. We would welcome suggestions about the presentation and content of this topic if anyone has any suggestions.
3 Feb - Sigvard Svensson has installed MacOS 7.6 on his IIvx/Turbo 601 and reports that he might be experiencing the floppy drive problem that has affected IIvx, IIvi and Performa 600s since System 7.5.3.
2 Feb - Bronson has sent an early report about MacOS 7.6.
This section covers general issues relating to MacOS versions 7.5, 7.5.1, 7.5.3, 7.5.5 and 7.6. Specific problems with applications, control panels and extensions are described in the Eccentricities, Bugs & Fixes section of this web site.
When the Turbo 601 was released System 7.5 was current. In fact, System 7.5 is the earliest System that the Turbo 601 can be used with. The Turbo 601 will not run under System 7.1.
Shortly after System 7.5 Update 1.0 (System 7.5.1) was released DayStar updated the Turbo 601 Control Panel and Turbo 601 System Enabler to fully support the new System. This is the latest System that DayStar support with the Turbo 601, ie:
DayStar do not, at present, support any System later that 7.5.1 running on a Turbo 601.
If you contact DayStar Technical Support and report a problem with Systems 7.5.3 or 7.5.5 they will most likely advise you to downgrade to System 7.5.1 to ensure compatibility.
Reports from people running Systems 7.5.3 or 7.5.5 are, at best, mixed. Many people find that the new Systems are perfectly stable on their machines, while others experience all sorts of problems. It is not clear if either DayStar or Apple are doing anything to resolve the problems that many users are experiencing with Systems 7.5.3 and 7.5.5. It is probably fair to say to say that this is the most pressing issue for many Turbo 601 owners at present.
MacOS 7.6 works well on Turbo 601 upgraded Macs. It seems to be a fairly safe bet if you are considering whether or not to purchase it.
An early report has come in from someone who was able to try an alpha version of Tempo (MacOS 8) on a Turbo 601 upgraded Mac. Early indications are MacOS 8 seems to work OK with the Turbo 601 - there's life in the old dog yet!
There isn't really much point in anyone running System 7.5. System Update 1.0 which takes the System to 7.5.1 is a worthwhile upgrade and fixes a number of possible problems.
System 7.5.1 is the latest System that DayStar support on the Turbo 601, and there's no doubt that this is a very stable combination. In fact if stability is critical to you, then this is the System you should use.
Willi Murray has a Mac IIvi/100MHz Turbo 601 with 68M RAM, Radius Thunder/24 GT graphics accelerator, a recently installed FWB JackHammer SCSI accelerator and a Sonic Systems 10 base-T ethernet card. He writes:
This set-up runs very quickly and smoothly with System 7.5.1 and everything works very well. Crashes are very,very rare and all the main applications I use (PageMaker, PhotoShop, Illustrator, PageMill, Acrobat, ClarisWorks, OmniPage, & Agfa's scanning software FotoLook) are proving to be extremely stable. It is worth noting, however, that I use very few third-party extensions or control panels - I don't use any of the XXX Doubler or Now utilities for example - and I reckon this contributes a lot to the overall stability of my machine.
Bronson Trevor had very few problems with System 7.5.1 when he was using it, in spite of using a lot of extensions.
It is generally good advice, nonetheless, to keep your control panels and extensions to a minimum when running a Mac with a Turbo 601 installed.
DayStar have officially acknowledged one problem with System 7.5.3 which affects only Mac IIvi, IIvx and Performa 600s that have a Turbo 601 installed. System 7.5.3, when installed on these machines, will 'cause the floppy drive to operate improperly'. Willi Murray ran System 7.5.3 on his IIvi/Turbo 601 for a few days to investigate and found:
The problem was easily reproduced and can seemingly occur during any operation involving the floppy drive. I experienced read and write errors, and quite often floppy disk format operations would fail.
I spent some time trying various different combinations of DD and HD disks, but the problems are repeatable. Every time, switching to 7.5.1 cured the problem.
It was most likely to happen when a large number of files were being written or read from floppy disk. Copying single files, or a few small files would normally prove less troublesome than copying large numbers of files - although this is obviously what you would expect if the problem were random in nature.
DayStar have apparently been investigating this problem for some time now, but no information has been published as to what the cause of the problem is or if a fix will be made available at some point in the future.
Under Systems 7.5.3 it seems that many extensions can cause problems with the Turbo 601. Bronson Trevor reckons that the more low-level work an extension or control panel does, the more likely it is to cause a crash or a lock up.
Bronson writes:
I used to use a ton of INITs/CDEVs, and had lots of problems. As a general rule I try to run as few as I can nowadays, and have very few problems. If you use Open Transport, be sure to upgrade to version 1.1.2. DayStar has a FAQ about upgrading to System 7.5.3 called System 7.5.3 Installation Tips and Issues.
Quite frankly however, now that 7.5.5 has been available for some time, 7.5.3 isn't worth bothering about. So let's move on straight away...
The reports we have received about System 7.5.5 have been generally very favourable. All-in-all, with certain caveats, System 7.5.5 will probably work OK with your Turbo 601 upgraded Mac. But you will need to careful about how you configure your machine, and there are certain things that do not work with System 7.5.5 and these will require you to use System 7.5.1.
If you do go ahead and install System 7.5.5 on your Turbo 601 upgraded Mac you do so entirely at your own risk. You will not be able to get help from any official sources. At the risk of repeating ourselves:
DayStar do not, at present, support any System later that 7.5.1 running on a Turbo 601.
Interestingly enough though, Bill Corea got this note from DayStar's Tech Support Manager, Leslie Gwin, regarding compatibility with OS 7.5.5 (and 7.5.3):
As far as Systems 7.5.3 and 7.5.5 we are in contact with Apple regarding the problems we have encountered. Although we are not insuring compatibility, many of our users are successfully running System 7.5.5. It does appear to be more stable than 7.5.3. If all else fails I would recommend reinstalling System 7.5.1 doing a 'Clean Install' 'for any Macintosh'. To do this hold down the Command-Shift-K keys while you select 'Custom', then turn down the "System" triangle and select 'for any Macintosh'.
In other words - "no guarantees, but it seems to work OK"!
I (Willi Murray) experimented for some time with System 7.5.5, but I am now using it permanently with my IIvi/Turbo 601, FWB JackHammer SCSI accelerator, Radius Thunder/GT graphics accelerator, Sonic Solutions 10base-T ethernet card, 68M RAM and various internal and external fixed and removable disk drives. I was worried that there might be problems because of the 3 NuBus cards I have installed, but everything works very well. In fact I run the System and applications from a 2GB AV hard disk attached to the JackHammer without any problems whatsoever. System 7.5.5 does seem slightly faster and more responsive than 7.5.1, presumably because more System and Finder code is 'native'.
The only thing that gave me real cause for concern was when I booted my machine for the first time with System 7.5.5 newly installed. About half-way through the boot-up, when the JackHammer control panel loaded the machine restarted itself for some reason. I was pretty nervous while it was rebooting, but it completed the self-induced 'second' soft reset OK and it's never happened again. Who knows...!
I have found that one of the big benefits of System 7.5.5 is being able to use Open Transport. OT 1.1.2 with OT/PPP 1.0 does give consistently faster data transfer rates. This is most noticeable with FTP transfers (using Fetch) where I now routinely get download speeds in excess of 3.4kbytes/sec compared with a best of about 2.9kbytes/sec using System 7.5.1, MacTCP 2.0.6 and FreePPP 2.5. Both of these figures are with a 33.6kbps connection with my Global Village Teleport Platinum modem.
The disadvantage of OT and OT/PPP is that it uses about 1.5M more RAM than MacTCP/FreePPP.
Bronson used System 7.5.5 for a while with no real problems. (He's onto System 7.6 now!)
David Murray (no relation!) has a Performa 600, 20M RAM, 66MHz Turbo 601 and a Radius PrecisionColor 24X:
I keep system 7.5.5 on one partition and 7.5.1 on another. I boot from the 7.5.5 system nearly all of the time and just use 7.5.1 when I want to use the floppy drive for something important, or need as much RAM as possible for SoftWindows. I keep my extensions to a minimum, more to preserve RAM than anything else. I can't be bothered with RAM/Speed Doubler although both seem to work.
This machine is totally stable with 7.5.5 as long as I am 'sensible' with the floppy drive - it does work most of the time for simple operations involving single files.
I imagine the rare problems I have with this machine are only the same software conflicts that any Mac user will encounter.
If you can afford the disk space to keep a copy of System 7.5.1 on a seperate partition then we would suggest that it is a good idea to do so. It's always useful as a fall-back position if something goes wrong when running System 7.5.5.
There are conflicting reports regarding the floppy drive problem that afflicted IIvi, IIvx and Performa 600 owners with System 7.5.3. Jubel Chen (who has a IIvx) tells us that the problems he had with the floppy drive with System 7.5.3 were cured when he installed System 7.5.5. Willi Murray's early experiences with his IIvi seemed to indicate that the problem was still present with System 7.5.5, and this seems to be confirmed by David Murray's report above.
My (Willi Murray again!) experiences with the floppy drive problem and System 7.5.5 are rather odd, but I'll try my best to explain what happened. I first installed and ran System 7.5.5 before I installed the JackHammer and at that time I did experience the same problems that I had with System 7.5.3 and the floppy drive. When I installed the JackHammer I ran 7.5.1 for a while to get things working properly before plucking up the courage to try 7.5.5. I have found that since I installed the JackHammer I DO NOT get the floppy drive problems I used to get with System 7.5.5! Why this should be I do not know. But I can say, quite categorically, that the floppy drive on my machine now works perfectly OK with System 7.5.5.
I have wondered if this is in some way related to an earlier problem I had with System 7.5.1, SCSI Manager 4.3 and my floppy drive. While DayStar were helping me sort this problem they sent the following bit of information:
...(we have) experienced floppy problems as well with SCSI 4.3 enabled, but with the JackHammer card installed they do not occur...
The important point being that DayStar were confirming that they had seen floppy drive problems in their Turbo 601 upgraded Macs when SCSI Manager 4.3 was enabled but no JackHammer was fitted. SCSI Manager 4.3 is built into the System file with System 7.5.3 and later and it might be case that SCSI Manager 4.3 is running on Turbo 601 upgraded Macs with System 7.5.5 (and 7.5.3) installed even when SCSI Manager 4.3 is disabled in the Turbo 601 Control Panel. If so, perhaps the floppy drive problems are related to what DayStar said above.
This is pretty wild speculation however, and it doesn't explain why DayStar would only expect to see floppy drive problems with IIvi, IIvx and Performa 600 machines and not IIci, IIsi etc. Maybe IIvi, IIvx and Performa 600s are more susceptible in some way.
If anyone has any better ideas about this strangeness I'd be glad to hear them!
That's the upside, now for the downside...
The general advice that Turbo 601 upgraded Macs work best when a minimum of control panels and extensions are used is particularly relevant to System 7.5.5.
RAM Doubler and Speed Doubler can be problematic for example. We have received reports that RAM Doubler 1.6.1 and 1.6.2 work OK with System 7.5.5. RAM Doubler 2 doesn't seem to work at all - it caused Bronson's IIci/Turbo 601 to hang during startup. Don't use things like RAM Doubler, Speed Doubler or the Now Utilities for example with System 7.5.5 unless you really have to.
Any application that uses the Mac's in-built sound hardware will not work properly with System 7.5.5 (or System 7.5.3). There doesn't seem to any work around for this. If you are doing something that requires properly functioning sound then you'll have to use System 7.5.1.
Anything that works on the system at a very low level seems likely to break a Turbo 601, especially when System 7.5.5 is used. Several people have written expressing the contention that a Turbo 601 upgraded machine should behave just like any other 'early' PowerMac. Our experience, however, is that this is not the case and that there are subtle behavioural differences. If anyone knows why this should be please let us know.
Summary
As long as you're careful with your machine configuration then System 7.5.5 will probably work OK.
If you're going to use System 7.5.5 then do install the PPC Interrupt Extension from Apple. This extension fixes some problems with interrupts that can cause crashes on all desktop PPC computers (including older Macintosh computers with PPC accelerator cards).
Also, be sure to visit Ted Landau's excellent MacFixIt Web site and read the Troubleshooting System 7.5.5 report.
It's a good idea to use the Finder Heap Fix utility to increase the Finder's allocated memory to at least 256k. The reasons for this are covered in MacFixIt's Troubleshooting System 7.5.5 report and you can download Finder Heap Fix from the MacFixIt Library.
If you try System 7.5.5 and experience problems which you cannot solve, then realistically the only thing you can do is revert to System 7.5.1. Mark Derbyshire received the following from DayStar (dated 10/10/96):
...As far as Systems 7.5.3 and 7.5.5 we are in contact with Apple regarding the problems we have encountered. I will forward your issues along with some others to the group at Apple who is working on the next version of the system software. At this time we are still recommending downgrading to System 7.5.1 for stability reasons.
Bronson has just installed MacOS 7.6 on his IIci/Turbo 601 and it looks promising:
I recently installed MacOS 7.6 on my IIci and everything is working great! The installer even recognized that the computer was a PPC accelerated Mac and installed Apple's own Processor Upgrade control panel (which can only turn the card on or off). It also comes with a utility that gives vital system information (i.e. type of computer, system version, etc), and the computer is identified as a Power Macintosh Upgrade (or something to that effect). This leads me to suspect that we _will_ be supported with future upgrades of at least the System 7 MacOS - if they were going to abandon PPC upgraded 68k Macintosh computers, I think that it would have been with System 7.6.
I tried out the Processsor Upgrade control panel that is installed with 7.6, and the machine does boot up into 030 mode. Apple's control panel is basically just a simplified version of the DayStar one.
Things seem a bit more responsive (as more of the OS has gone PPC native, I suppose).
It's too early to draw firm conclusions yet, but it does seem that PPC upgraded Macs are better supported with MacOS 7.6 than in the past. One thing seems certain though, and that is that the MacOS 7.6 installer automatically installs a suitable System that will boot in 030 mode and PPC mode on a Turbo 601 upgraded Mac. This should obviate the need for the nauseous "System for any Macintosh" installs that we had to do with System 7.5.x.
Update (3 Feb)
Sigvard Svensson reports that:
I am using a IIvx/Turbo 601/66MHz, 20MB RAM, CD, Ethernet card and some external drives (HD 1280 MB, SyQuest 44 + EZ 135).
Last Friday I installed The US MacOS 7.6 + OT 1.1.2, on a separate HD, adding only the Turbo 601 Enabler, Control Panel and necessary network software. Everything worked fine until I tried to format a DD floppy, which almost immediately returned a dialog telling me something like "Formatting unsuccessful" demanding an OK. Clicking OK created a complete freeze. I concluded the floppy problem still exists, and returned to 7.5.1. Unfortunately I have not yet had the time to get on with testing, but I will.
DayStar have warned about a problem with System 7.5.3 that can cause the floppy drive to operate improperly on IIvx, IIvi and Performa 600s that have a Turbo 601 installed. Willi Murray and others have subsequently found that this problem is also present when System 7.5.5 is installed, so perhaps it's no big surprise that owners of these machines will find that the problem persists with MacOS 7.6. If this is the case, it's disappointing because in all other respects System 7.5.5 seems to work well on these (and other) machines and early reports about System 7.6 seem very positive. Is it really asking too much of DayStar and Apple to sort this out!
Update (20 Feb)
Thomas Schwab sent the following information:
MacOS 7.6 has run well for one week on my accelerated Macintosh IIci. MacOS 7.6 installation was straightforward. I did not need to install the Universal System Folder and am using Apple's MacOS 7.6 Accelerator control panel not Daystar's.
I have had excellent system stability and internet access.
My Macintosh IIci has a Daystar Turbo 601/66MHz, 24 MB of RAM, Quantum 1.2G hard drive, Apple 13" High Res color monitor, Apple Personal Laserwriter NT, Apple External CD, Syquest EZ 135 and a Supra 14.4 FaxModem.
I am running RAM Doubler 2.0 which triples my RAM. Speed Doubler 1.3.1 with the latest patch for System 7.6. I also run the latest version of Aaron. I use Netscape Navigator 3.0, America OnLine 2.7, and subscribe to AT&T Worldnet for web access.
I have Virtual Memory turned off - I have not tried this set up with virtual memory on due to speed slowdowns.
Bronson adds that since installing MacOS 7.6 almost three weeks ago he's had very few problems. Other than occasional crashes involving After Dark, MacOS 7.6 is proving to be very stable on his IIci.
Update (30 March)
Jubel Chen had some problems when he installed MacOS 7.6:
First of all, I ran the "Install MacOS" program. Went through the 4 steps (though I skipped step 2, since I am using FWB's HDT). I actually only selected the options "MacLinkPlus 8.1", "Cyberdog 1.2.1", and "OT/PPP 1.0". I did not do a custom installation. After I finished the installation and rebooted the computer, the Desktop Files were automatically rebuilt.
After running the OT 1.1.2 updater I rebooted and tried several internet applications, such as NCSA Telnet, Microsoft Internet Explorer (2.1, not even the 3s), Netscape 2.02, ClarisEmailer 2.0 Beta x52, the system crashed with a Type 11 error.
The attempt to reset the computer by Cmd-Ctrl-Power resulted in a "black, SadMac" icon. I tried to reboot off the Disktool 2 disk, it was fine. Then, I ran Disk First Aid and it reported no problem. Sounds fine, right? When I rebooted with the Hard Disk as the startup disk, the computer went into SadMac again...<sigh>
I had to boot the computer off my Zip Emergency Startup Disk, and do a "Clean", "Customized for 'ANY' Macintosh" Install. This time, I took out the OpenDoc, Cyberdog, and OpenTransport/PPP. BTW, this time, I didn't even attempt to run the "Install MacOS" program. I ran the installer from the MacOS 7.6 Folder in the Software Installer Folder. This time, the CPU rebooted after installation, rebuilt the desktop files, and went off without a hitch.
I have had very few crashes after that. Looks like I will have to stick with customized installation permenantly.
I (Willi Murray) finally received my MacOS 7.6 CD from Claris UK on 7 April (it took three weeks to arrive after it was ordered). MacOS 7.6 installed without any problems and is working fine. Just a couple of things to watch:
If you have a JackHammer card fitted then you cannot install MacOS 7.6 onto a disk drive attached to the JackHammer. Well, you can actually install it OK, it's just you can't then restart from the disk drive attached to the JackHammer. Turbo 601/JackHammer owners will no doubt be aware that disk drives attached to the JackHammer can be used as startup disks without any problem, but if you do this you cannot use virtual memory. ie the Turbo 601 does not support virtual memory when a startup disk is attached to a JackHammer. So what happens is that MacOS 7.6 gets installed OK on the disk attached to the JackHammer but in the process the installer sets virtual memory on by default. When you restart the Mac it will crash during the reboot because it tries to initialise virtual memory on a disk that doesn't support it.
Once I figured out what was going on I installed MacOS 7.6 on the internal hard disk. After rebooting I turned virtual memory off and reassigned the disk attached to the JackHammer as the Startup Disk. It all worked OK after that. (NB--I use a 2GB AV Quantum disk drive attached to the JackHammer as my startup disk cause it's faster than running from the Mac's internal SCSI bus).
The other thing I noticed is that you might not be able to install MacOS 7.6 if you are currently using System 7.5 or 7.5.1. As mentioned in the Machine Type Oddities topic on the Eccentricities, Bugs and Solutions page when System 7.5.3 or 7.5.5 is installed on a Turbo 601 upgraded Mac the machine returns a Gestalt ID of 104 - which is actually the identification for an LC 575 w\PPC upgrade. The LC575w\PPC upgrade is recognised and supported by the MacOS 7.6 installer and although it isn't actually what our machines are, the System it installs works just fine on Turbo 601 upgraded 68030 Macs.
If you have Systems 7.5 or 7.5.1 installed however, the machine will return a Gestalt ID of 207, 208, 209 or 210 depending on the base model. Machines returning these Gestalt ID's are not supported directly by the MacOS 7.6 installer and it says that it is going to install a System that it not supported! Holding down the ALT key when launching the installer doesn't give you the "Universal System for any Mac" option that the System 7.5 installer did.
The upshot is that I don't actually know if the MacOS 7.6 installer will install a working system on a Turbo 601 upgraded Mac that is running Systems 7.5 or 7.5.1 Under these circumstances it may very well install a "Universal System" - but I just don't know. I took the safe option and installed it over System 7.5.5!!!!
A couple of brief reports about the MacOS 7.6.1 update:
Bronson writes:
MacOS 7.6.1 : I installed the update about a week ago, and have had no problems since. In fact, my machine has been very stable. I seem to get fewer errors than in 7.6, and the incidence of crashes is far less than with 7.5.3 or 7.5.5.
Jubel Chen wrote on 8 April:
The US-English version of the 7.6.1 update was released yesterday afternoon. I have downloaded it any installed it over my computer. Not a single crash in the entire day. Sounds impressive isn't it. I will have more information in the next few days as the OS settles down over my computer. I will go into details later.
And sent this follow-up on 11 April:
More on the OS update. I have had only 3 complete crashes (freezes) since I installed System 7.6.1. There were a few partial crashes with various errors, but no longer Type 11. It is now usually Type 1 or Type 2 errors (still very irritating). The deletion of ALL Microsoft software on my hard disk also seem to help things.
So far the signs are that if you are using MacOS 7.6 on a Turbo 601 upgraded Mac then the update to MacOS 7.6.1 is worthwhile and doesn't, in itself, cause any problems.
Update (1st June)--some comments about MacOS 7.6.1 from Willi Murray:
I downloaded and installed MacOS 7.6.1 early last week. This is definitely a worthwhile upgrade and I would recommend anyone who is currently using MacOS 7.6 with a Turbo 601 to update to MacOS 7.6.1.
This is the most stable my machine has been since I moved up from System 7.5.1. System 7.5.3 was a disaster on my Turbo 601 upgraded Mac IIvi. System 7.5.5 was pretty stable, not quite as stable as System 7.5.1 but the improved performance more than offset the slight decrease in stability.
MacOS 7.6 was again an improvement, and I am pleased to report that as far as my Turbo 601 upgraded Mac is concerned MacOS 7.6.1 offers another significant improvement.
Stability is now excellent--my Turbo 601 upgraded Mac is now crashing less than it ever has done before. I've not had one single crash since installing MacOS 7.6.1--my machine is on 24 hours a day, seven days a week, serving as a FAX machine as well as a "DTP workstation" so all-in-all I'm extremely pleased with the performance improvement.
MacOS 7.6.1 offers much better performance than System 7.5.1 in a number of areas. Most noticeable is the Finder--more of the Finder is native PPC code. The most surprising thing is, however, that there's been a really serious improvement in graphics performance since updating to MacOS 7.6.1. This would appear to be due to a change that Apple made to SCSI Manager in PPC upgraded Macs:
An early version of Tech Note 1096 about MacOS 7.6.1 included the following:
On PowerPC upgrade cards for 680x0 computers, we changed the SCSI Manager to poll for interrupts when the current interrupt mask is level 1 or higher, instead of level 2 or higher.
The latest version of Tech Note 1096 (revision 1.2) does not include the above paragraph, but I have posted a question in Apple's Tech Support Forum asking if the change was actually incorporated.
I have also asked if this is same thing as described in the System 7.5.5 Tech Note (Tech Note 1069):
We changed the interrupt level at which SCSI completion routines get executed from level 2 to level 1. Because the interrupt level was at level 2 the SCSI completion routines slowed the performance of expansion cards (such as video capture and sound cards) whose interrupt level was also at level 2. This affects only PowerPC machines running a native version of SCSI Manager 4.3
If the SCSI Manager change is included in MacOS 7.6.1 for PPC upgraded Macs, and it has the same effect as the change incorporated into System 7.5.5 for other Macs, then this would explain the improvement in graphics performance I am experiencing (I use a Thunder/24 GT graphics card).
With all that's happened in last two or three months this topic needs to be completely re-written. The situation regarding Turbo 601 upgraded Macs is becoming clearer and we'll do a new write up for this section soon.