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Second Class Macs & Road Apples

Power Mac and Performa 5200-53xx & 6200-6320

four

Dan Knight - 1998.01.10

Road Apples are Apple's most compromised hardware designs. For the most part, they're not completely bad - simply designs that couldn't meet their potential. (On our rating scale, the more brown apples, the worse the hardware.)

This entire series of PowerPC-based Macs suffered from the most bizarre motherboard architecture Apple ever devised and is perhaps the most deserving recipient of the Road Apple title.

To save time and money, Apple chose to build these on a modified Quadra 605 (a.k.a. LC 475) motherboard - which was a fine design for a 25 MHz 32-bit 68040 CPU. (They also used some Quadra 630 parts - as well as its case for the 6200-63xx.) However, the PPC 603 is a 75 MHz or faster 64-bit CPU.

You'd think Apple would have learned its lesson with earlier Road Apples (the LC, LC II, Classic II, Color Classic), all of which coupled a 32-bit CPU to a 16-bit data bus. But no, they did it again, this time hobbling a CPU with a 64-bit bus on a 32-bit motherboard.

Apple went beyond crippling the 64-bit CPU on a 32-bit data bus - way beyond that.

  1. Depending on CPU speed, memory runs at one-half to one-third CPU speed. (Because RAM is so slow compared to current CPUs, this is normal for Power Macs. In fact, the G3 can run at up to 8 times motherboard speed. If this were not possible, we'd still be living with computers running under 200 MHz.)
  2. It takes four memory cycles to load a 64-bit word (which equals 8-12 CPU cycles, since the processor is two-to-three times faster than memory!). Cycle 1: read 32 bits. Cycle 2: store 32 bits. Cycle 3: read next 32 bits. Cycle 4: combine with first read for 64-bit instruction. Then the CPU can process the instruction.
  3. Because it used a 32-bit motherboard, different functions were handled by different 32-bit buses, called Left 32 and Right 32.
    • Left 32 handled networking, audio, ADB, and SCSI. Networking, audio, and ADB are each 16 bits wide; SCSI (for CD-ROM and external devices) is 8 bits wide. This involves a lot of overlap and causes a lot of problems.
    • Right 32 includes the 32-bit memory controller, the 8-bit IDE controller (for the hard drive), the 32-bit graphics controller, the 16-bit video controller, and the (thankfully optional) 8-bit TV controller. Again, this involves a lot of overlap and can cause a lot of problems.
    • Any time something on Left 32 needs to communicate with Right 32, or vice versa, it must go through the CPU, which acts as a bridge. This really slows down the CPU, since it must divert its attention from processing data to moving data between sections. And don't forget the four-cycles-per-64-bit-word problem noted above.
    • Because of the design, all data from the serial port, comm slot, or an ethernet card must pass though the CPU to reach RAM. This makes these very poor performers on the internet.
    • Because of this peculiar architecture, upgrading the level 2 cache or increasing VRAM would slow things down. Good thing they aren't options.
  4. Apple saved money by using IDE hard drives instead of SCSI drives. Not only is IDE cheaper, but cheap IDE drives tend to be slower than cheap SCSI drives. (Today this is not the issue it once was. The Blue Power Mac G3 has a very fast IDE drive.)
  5. Again to save money, Apple left hardware handshaking off the serial ports. These were the only PowerPC-based Macs without GeoPort serial ports. (Even the Mac Plus had hardware handshaking!) This limited users to a comm slot modem or a 9600 bps or slower external modem.
  6. A comm slot modem disables the modem port; a comm slot ethernet card disables the printer port.

Because of their unusual architecture, installing a 25-pin SCSI terminator to the SCSI port (if you have no SCSI devices attached) will improve network stability.

The poor benchmark performance of this series gave the PowerPC 603 chip a black eye. It wasn't the chip that was at fault, for it was more efficient than the PPC 601. It was the horrible architecture of this system that made the 603 a chip-non-grata until the 603e came out on Macs with better designs.

Overall, the design compromises make these Macs to be avoided at all costs. But also remember that there were several different versions of some of these models, both ongoing revisions and models created for other markets. Some later motherboards fixed some of these problems.

For a different perspective on serial throughput on the x200 series, see Modem performance using FreePPP and OT PPP on at 5320 by Adrian Winnard. It should also be noted that reports from the field indicate that Mac OS 8.1 and later smooth out a lot of issues these models suffer from. LEM

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