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September 95 - Balance of Power: The Power Macintosh: The Next Generation

Balance of Power:
The Power Macintosh: The Next Generation

Dave Evans

The Power Macintosh computer just keeps moving forward. The latest generation brings greatly improved performance and adds the PCI expansion bus and the PowerPC 603 and 604 processors. Software changes that improve performance include the following:

  • an improved 680x0 emulator

  • a native Resource Manager

  • native networking (Open Transport)

  • native device drivers

  • an improved Memory Manager
I'll describe these new features and discuss how you can maintain compatibility with the new Power Macintosh computers and with future changes to the Mac OS.

THE IMPROVED EMULATOR

First delivered with the Power Macintosh 9500 computer, the new emulator improves on the original in one key way: it actually recompiles 680x0 code into native PowerPC code. Since large portions of the Mac OS are still in 680x0 code, this new emulator speeds up most common operations and offers significant improvements for 680x0 code with tight loops.

Recompiling doesn't mean converting 680x0 instructions one for one into PowerPC instructions. Fully emulating a 680x0 instruction still takes a few PowerPC instructions. But recompiled code is more efficient and optimized. The original emulator had to decipher each instruction every time it was executed, but recompiled code from the new emulator is analyzed once and then executed many times.

Because it takes extra time to recompile code, the emulator doesn't immediately translate all 680x0 code. It operates just like its predecessor until it encounters a loop or similar repetition. Then, instead of emulating the same code repeatedly, it translates the instructions into native code and caches the result. Subsequent calls to that code simply execute the native translation, greatly improving performance.

The cache of translated 680x0 code must stay coherent with memory, much like the caches on the Motorola 68040 processor. Therefore, whenever your software modifies code or changes application jump tables, you should flush the instruction cache. (See the Macintosh Technical Note "Cache as Cache Can" (HW 6) for a more detailed description of cases where flushing the instruction cache is necessary.) In the past you could call Gestalt and check the processor type to flush only on a 68040. Since the new emulator supports only the 68020 instruction set -- and Gestalt will indicate that a 68020 is installed -- you should now flush any time you modify code or change jump tables.

The best way to flush 680x0 code in the cache is with FlushCodeCacheRange, which flushes only the invalid portion of the emulator's cache. FlushInstructionCache also works but can degrade performance by wastefully purging recompiled code that's still valid. These routines are documented in Inside Macintosh: Memory. The C prototype for FlushCodeCacheRange is as follows:

OSErr FlushCodeCacheRange(void *address, 
   unsigned long count);
In 680x0 assembly, you would use
MyFlushCodeCacheRange Proc
   ; On entry A0 = address, D0 = # of bytes
   ; Trashes A0, A1, D0. Result in D0, Z bit set. 
   ;
   movea.l      D0,A1      ; # bytes in A1
   moveq         #$9,D0      ; selector
   _HWPriv               ; A098
   tst.w         D0         ; result == noErr
   rts

OTHER SOFTWARE CHANGES

The first Power Macintosh computer ported critical portions of the Macintosh Toolbox to native PowerPC code. Ultimately we'll take all of the Mac OS native, but for now we've focused on areas that most increase overall performance. So, starting with the Power Macintosh 9500, we've added a native Resource Manager, the native Open Transport networking stack, and native device drivers. I'll discuss each of these in turn and then mention improvements to the Modern Memory Manager.

Even though many calls to the Resource Manager are bound by I/O bottlenecks, porting the Resource Manager to native PowerPC code still substantially improves performance. Often to complete a request the Resource Manager need only look up existing information and return it, and even if file I/O is required the data is often in the system disk cache. For these reasons, many Resource Manager calls will execute much faster on the new machines.

Native Open Transport networking provides a stream-based interface for networking that's independent of the network protocol. You can now implement a variety of network solutions without concerning yourself with protocol details. Documentation on Open Transport is provided on this issue's CD.

Native device drivers provide both a performance improvement and an improved system programming interface (SPI). This SPI is available with all PCI-based Macintosh computers, starting with the Power Macintosh 9500. For more information on these drivers, see the article "Creating PCI Device Drivers" in develop Issue 22 and Designing PCI Cards and Drivers for Power Macintosh Computers, available from Apple Developer Catalog.

Although not new, the native Modern Memory Manager has been improved in two important ways:

  • Many of the routines are now implemented as "fat" binaries instead of all native code. When your 680x0 code calls the Memory Manager, it will now execute 680x0-based routines, eliminating the Mixed Mode environment switch once needed to call the native routines. Reducing the number of these switches can measurably improve performance.

  • The bus error handlers have been removed, significantly increasing the performance of many of the simple Memory Manager calls and allowing a number of the calls to be made into fat traps. Bugs discovered during the process of removing the handlers have been fixed.
Handles passed to the Memory Manager now go through a rigorous check before they can affect other Memory Manager data structures; however, without the nearly foolproof bus error handling, it's a little more likely that you'll pass an invalid address and crash. If you crash in the MemoryMgr code fragment while testing on the new Power Macintosh computers, you probably passed an invalid pointer or handle. You can use the Debugging Modern Memory Manager to aggressively catch these application errors.

Note also that the bus error handlers would allow system (and even application) heaps to become corrupted, deteriorating the overall user experience without causing the machine to crash. This is much less likely to happen now, but if structures do get corrupted other than by the Memory Manager, a system crash will result.

Also available starting with the latest Power Macintosh machines is support for very large hard disk volumes. In the past, only 2-gigabyte volumes were allowed; then with System 7.5 we relaxed that restriction to 4-gigabyte volumes. But many of you were still hungry for more, so now we allow up to 2 terabytes (that's 2000 gigabytes) of file system address space per volume. Unless you're developing utilities and drivers compatible with the new volume sizes, though, you really don't need to pay attention to the new large-volume support, because the API remains unchanged. The only time an application might want to take advantage of the new support is when it wants to know before attempting to save to disk whether there's enough free space on the volume. Even in this case, the application won't be able to save a file bigger than the existing limit of 2 GB, and the old version of GetVInfo will return values that are "high-water marked" at 2 GB for compatibility reasons, even if more space is available.

If you really do want to know how much space is available, you can do so through an extension to the File Manager API. We extended the API because the existing 32-bit size information was too small to address volumes and files larger than 4 GB. You'll use the following new routine to get 64-bit sizes:

pascal OSErr PBXGetVolInfo(XVolumeParam
               paramBlock, Boolean async);

This routine takes an extended VolumeParam structure, named XVolumeParam, which you'll find declared in an updated Files.h interface file on the CD. Before using this routine, be sure to call Gestalt with the gestaltFSAttr selector; if the response parameter has the gestaltFSSupports2TBVolumes bit set, the new routine is available. Note that there are also extended Read and Write calls for drivers that want to support volumes larger than 4 GB.

PCI AND NUBUS

Starting with the PCI-based Power Macintosh computers, support for the NuBus(TM)-specific Slot Manager goes away. Some applications used to call the Slot Manager directly to get video and other device information. This will no longer work, so we've provided better methods: the Display Manager API has been extended for all the video device information you'll need, and the new Name Registry API will give you device information independent of the specific expansion bus implementation.

One example of the improved Display Manager API is the way you get display modes for video devices. With the Slot Manager this took a lot of code, but the Display Manager gives you one encompassing routine:

pascal OSErr DMNewDisplayModeList(
               GDHandle theGDevice, 
               unsigned long reserved,
               unsigned long *modeCount, 
               DMListType *theDisplayModeList, 
               unsigned long modeListFlags);

With this and other new Display Manager routines, you can avoid the Slot Manager altogether when gathering display information. But if you must access other device information, you can use the bus-neutral Name Registry, which manages a tree of device objects that you can access as a linked list. Look for the new header files (Displays.h and NameRegistry.h) on this issue's CD.

MAINTAINING COMPATIBILITY

As Apple improves the Mac OS, compatibility with the documented APIs and SPIs is ensured -- but don't assume that if your application runs fine on existing machines, it will continue to do so in the future. We can't ensure complete compatibility if application code makes invalid assumptions or uses unsupported parts of the Mac OS. There are some things you can do to help ensure that your applications will run on future versions of the Mac OS. First, use only the officially documented APIs. For example, don't assume that the Z status bit will be set correctly on exit from a trap unless it's documented. As we take more traps native, the 680x0 status register becomes irrelevant and such checks break. Here's an example of 680x0 code that now breaks because it assumes the Z status bit will be set by Get1Resource:
move.l      #'DAVE',-(sp)
clr.w      -(sp)
_Get1Resource
beq.s      error         ; BAD!

You also shouldn't expect results in registers if the trap isn't documented to return them there. It's true that some traps used to accidentally exit with useful data in register D0 or A0, but if that's not documented as part of the API it won't be supported in the future.

Second, test your software using EvenBetterBusError, the Debugging Modern Memory Manager, and any other debugging tools that are appropriate (look in the Testing & Debugging folder on the CD). Stress-testing your software with these tools will catch many errors that otherwise would go unnoticed. EvenBetterBusError catches most stray references to nil, such as writing to location 0 or using nil pointers and handles. The Debugging Modern Memory Manager catches those occasions when you damage a heap or pass invalid addresses.

Finally, as I've said in previous columns, don't use RS/6000 POWER instructions in your native code. Although the PowerPC 601 processor supports many of them, the new 603 and 604 processors do not. We've made an attempt to emulate the POWER instructions in software for these new processors, but this emulation is very expensive. When a 603 or 604 encounters one of these now-illegal instructions, it stops everything and calls our new illegal-instruction handler, which recognizes the instruction that was used and attempts to use a valid one instead. This operation is very time consuming; if your performance-critical code includes POWER instructions, you'll see a severe slowdown. As described in this column in develop Issue 21, you should use the DumpXCOFF tool to check your code for any POWER instructions.

NEW DIRECTIONS

Apple will continue to take advantage of RISC technology and will both improve existing performance and add new functionality. Make sure your code uses documented interfaces so that it will stay compatible and run on future generations of the Power Macintosh. And be sure to check out Open Transport and PCI device drivers -- they're exciting new directions that will take you closer to the next generation of the Mac OS today.

DAVE EVANS and fellow Apple engineer Rus Maxham rode 2000 miles on their motorcycles this summer. They journeyed through the lush Central Valley of California, the blistering heat of the southern Arizona deserts, and the neon glitz of Las Vegas. Along the way they enjoyed the camaraderie of fellow bikers and were rescued in their hour of need by a sympathetic motorcycling couple who housed them as Rus rebuilt his BMW's rear drive assembly.*

Thanks to Bill Knott, Eric Traut, and Jack Valois for reviewing this column.*

Special thanks to Randy and Peggy Marlatt of Flagstaff, Arizona, for road support.*

 
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