Expanding Your Mac - A Rundown of Macintosh Slots

Computers were designed as general purpose machines with the ability to do anything in the digital world that one can imagine. The standard configuration of a machine may not suit the mind of a creator or artist, so they'll add something to it. Their are many different types of boards available and they can fit into different types of slots. The type of slot dictates how many things work inside of a machine. The most common features are integrated onto a computer's motherboard but they are addressed virtually as if the functionality were an expansion card.

ISA - Industry Standard Architecture
The ancient Intel design for expansion cards. Developed in the 1980's by computer giant IBM, it quickly became the standard in PC's. The 8 bit ISA bus is limited to 8 MB/sec bandwidth with the 16 variant offering twice that. The bus design works in tandem with the old IDE devices, floppy, PS/2, serial and parallel ports often found on PC's. That pretty much covers all the legacy ports the PC industry. The ISA standard has evolved a bit over time but not at the same pace as the various components an a motherboard. As processors continued to become faster and data traffic between devices exploded Intel created another standard - PCI.

Why is the ISA important to Macintosh users since Apple never made a machine with an ISA slot? While a Mac never shipped with an ISA slot on the motherboard, the ISA bus have been used for communication between devices on the motherboard. PC Cards (aka PCMIA Type I and Type II) were designed around 16 bit ISA bus protocols.

Originally designed by MIT and then trade marked by Texas Instruments, the Nubus design was chosen by Apple for expanding the Mac II line. At its heart, Nubus is a 32 bit slot running at 10 Mhz for 40 MB/sec of bandwidth. For its time, it was a relatively fast bus. Since Apple was the only major company to adopt the standard, pretty much every Nubus card was proprietary to the Mac. With the Mac in a niche market meant Nubus cards carried a price premium. To keep Nubus competitive, Apple increased the clock speed of Nubus to 20 Mhz, but that wasn't enough. As Nubus showed its age, Apple wisely adopted the faster, more industry standard, and cheaper architecture of PCI.

PCI - Peripheral Communication Interconnect
Intel, using its juggernaut control over the industry, guided to a new bus design called PCI to market. Intel made it industry standard and was adopted as such quickly. Apple wisely decided to follow the industry's decision. Being 32 bits wide and running at 33 Mhz provided 132 MB/sec of bandwidth. Just like ISA, the bus design was used for interfacing various motherboard components - IDE, SCSI, USB, on board video for example. Even the ancient ISA bus was to become a component along the PCI bus on motherboards.

PCI has evolved over time to keep up with the demand of high speed peripherals. Their are two easy ways to gain bandwidth, one by increasing the clock speed and another by increasing the bus width and the industry went both ways to increase speed. 64 bit PCI slots are backwards compatible with 32 bit cards and run at the same voltage. For 66 Mhz slots, the voltage had to be dropped from 5 volts to 3.3 volt and has a slightly different slot key. Cards can be made to run at either 33 Mhz or 66 Mhz, allowing card makers to take advantage of the extra speed with compatibility in mind. For the ultra high end motherboards, manufacturers can use 64 bit and 66 Mhz techniques for 533 MB/sec of bandwidth. The B&W G3 is the only Apple machine to have a 66 Mhz PCI slot. The B&W G3 and all G4's to date have 64 bit PCI slots. The new Xserve is uber cool with its 64 bit/66 Mhz PCI slots.

AGP - Accelerated Graphics Port
The industry was content with PCI except for one segment - the video card market. The high demands of video received its own dedicated bus - AGP. Video cards quickly migrated over to the new bus given its dedicated, high bandwidth design. Even PCI cards are happy since they didn't have to compete with a video card for shared bandwidth. Another advantage of the AGP bus is direct access to main memory for storage of information. It can buffer information to system memory that a card cannot store on its own dedicated memory. In fact, an AGP video card does not have to have any memory of its own as it can use system memory for everything. Bandwidth on the AGP bus can be multiplied from its base 264 MB/sec of bandwidth. For example AGP 4X has just over 1 GB/sec bandwidth.

PDS - Processor Direct Slot
This slot is generally unique in design to a machine (SE or LC) or generation of machines (Quarda, 1st gen PowerMac). The design philosophy is similar in nature to AGP as it provides a relatively fast dedicated bus to the processor/memory. However, a PDS is not limited to graphics functions as SCSI, Ethernet and other devices. The physical connector for a PDS card is just as varied as what can go into them. PDS cards are often unique and expensive due to the small market and proprietary design associated to the PDS slot.


I have one nitpick for an otherwise excellent summary. In the NuBus section the author claims that Apple increased the clock speed of NuBus to 20 MHz. This is misleading. The NuBus 90 standard allowed for 20 MHz NuBus clock speeds, but Apple never implemented the higher speed.

There is support on some of Apple's late NuBus machines for two NuBus cards to communicate with each other directly at 20 MHz, but all communications between NuBus cards and the host machine are still at 10 MHz. Given that cases where NuBus cards communicate with each other directly are either very rare or non-existent (haven't identified one yet, maybe some of DigiDesign's stuff?) the 20 MHz update is pretty much irrelevant to the operation of Macintoshes.

I was wondering if any of this relates to current (alum. G4) Powerbooks, especially in the area of graphics card ram, as MacMall's mailed circular offers "Special High Powered Video Powerbook Bundles!", whose video RAM is doubled from the standard of 64MB to 128MB, in both optional models, but oddly, the video resolution is reduced from the standard version from 1440 x 900 to 1280 x 854 (unless it's a typo).

Can video RAM in a Powerbook be expanded by a handy user?


moosemanmoo's picture

Not unless you want to find 100% compatable RAM ICs and spend hours soldering silver contacts under a microscope where one mistake means you just fried a $800 part.

moosemanmoo's picture

It should be noted that there is not one PC card standard. The first kind was wildly unspecified, later corrected by the ISA standards. The Cardbus implimentation is the newer and better kind, it's a varient of the PCI bus.