Hardware

Mac Boot Camp, part 4: Cupertino strikes back

From start to finish, the Mac II was a platform full of change. Together, the hardware and software made for a jump in overall performance that was impressive and that set to rest forever the Mac's reputation as a "toy" computer.


It took a few years, but the Mac finally outgrew its small footprint, all-in-one packaging. Users wanted to do more with their machines. They wanted to add custom cards like their PC brethren could. They wanted to choose their monitors for themselves. They wanted some options. They wanted color. The Mac II is the machine that gave it to them.
This article is part four in the "Mac Boot Camp" series by Larry Loeb. If you missed the first three articles or want to refresh your memory, here they are:
Mac Boot Camp, part 1: What’s a Mac program like? Mac Boot Camp, part 2: The managers Mac Boot Camp, part 3: Mac hardware, the early days
The Mac II was another reinvention in hardware of the Mac concept, based on the hardware that could be mass-produced then at a certain price. Since it was the first Mac to use 32-bit addressing, it opened some performance bottlenecks that existed in software and hardware. Previously, the Mac had used a 24-bit address in the operating system, pretty much leaving the high bits alone. Mac programmers had become accustomed to tweaking program performance by using the high-order bits for their programs. Changing over to a 32-bit “clean” operating system (named System 7) meant that Apple had to get its developers to go along with the change and rewrite their legacy programs.

It took about two years from the first developer seedlings, but Mac software finally became 32-bit “clean.” Most developers finally realized that the changeover was necessary—not just for the 020 CPU or the Mac II, but for all upcoming CPUs that would be introduced over the next few years, like the 030 and 040. Developers had to change their assumptions or wither on the vine as the hardware bypassed them.

The Mac II ran on a 68020 CPU that was clocked at 16 MHz. The II also had a coprocessor onboard, the 68881 Floating Processor Unit, which took care of complex math. This coprocessor required programmers to insert specific FPU calls into their programs, but if the FPU was present, the payoff was faster execution.

Apple also began its march toward custom integration of the Mac’s hardware with several chips. The first chip was the General Logic Unit, which enabled handshaking between the I/O devices and the CPU. The Memory Management Unit (MMU) translated the logical addresses that were used by the software into the physical addresses of the hardware. Two variants of this chip were used in the Mac II. The first was the proprietary Address Management Unit, later replaced by the 68851 Paged Memory Management Unit, which could run a 32-bit multitasking system like A/UX. Apple’s custom sound chip created digitally synthesized sound and drove two Sony amplifiers for stereo sound. The floppy drive recording/playback circuit (called the IWM, for Integrated Woz Machine) was also custom, implementing Apple’s unique method of disk recording that was seen first in the Apple II and carried over to the Mac’s 3.5-inch disks. Basically, the IWM converts between serial group code recording on the disks and the parallel data needed by the CPU.

Apple also modified commercially available chips for the Mac II. The Rockwell VIA chips used in the Classic Mac were customized by Apple to interface the proprietary Apple Desktop Bus, as well as other I/O to the CPU. SCSI connectors, both internal and external, connected the present hard drives to the CPU. Apple carried over its 25-pin SCSI connector to the Mac II because it didn’t have room on the back of the machine for the larger 50-pin connector.

The six card slots available in the Mac II were linked to the CPU and each other by the NuBus method, developed at MIT. The bus clocks at 10 MHz and is 32 bits wide. The BIU (Bus Interface Unit) chips contain transceivers and control logic to multiplex and demultiplex the address and data lines and to interface the NuBus signals with the system clock. A card on the NuBus that needs CPU attention asserts its interrupt line, which goes to a VIA. The CPU reads a register on the VIA to see which card needs servicing.

The NuBus also allowed other manufacturers into the Mac’s hardware. National Instruments, for example, developed an entire card/software solution called LabView that made “virtual instruments,” which could interface to the outside world of process automation and research. The NuBus also made color monitor driver cards possible. To handle color efficiently, the monitor needs memory of its own that can refresh the screen image directly without tying up the CPU. That memory resides on a monitor’s card, which performs the actual RGB signal generation.

But the technology in the Mac II that had the most lasting effect on future Macs was the way the II was mechanically packaged and constructed. It was modular. It had a top that could be removed and that allowed for access to internal parts. Classic Macs were notoriously hard to disassemble and service. That had been an original design decision because the designers thought that no one (except for an occasional technician) would actually need to get inside the case. But when slots and cards became involved, things had to be rethought. Users needed access to the inside of the machine in order to install and change cards. In short, the box had to be constructed in such a way that it could be opened and shut without damaging the internal components. With a Mac II, you could get to the guts of the machine easily.

The same modular design precepts were followed in the construction of later machines, which we will examine in follow-up Daily Drill Downs on TPG. From start to finish, the Mac II was a platform full of change. Together, the hardware and software made for a jump in overall performance that was impressive and that set to rest forever the Mac’s reputation as a “toy” computer.

Larry Loeb has 20 years of computer journalism experience. He was Consulting Editor at the late, lamented BYTE magazine, he launched WebWeek, he ran the online Macintosh section of BIX (the BYTE Information eXchange), and he wrote numerous articles for many major computer magazines. Recently, he also wrote a book on Secure Electronic Transactions, the protocol endorsed by MasterCard and Visa that allows merchants, cardholders, and banks to work together over the Internet. For banter, tips, and general screaming, send Larry an e-mail .

The authors and editors have taken care in preparation of the content contained herein, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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