The IBM Personal Computer/AT (often simply referred to as the IBM PC AT or just AT) was a landmark personal computer released by IBM in August 1984. The “AT” stood for “Advanced Technology,” and it truly lived up to its name, setting a new standard for performance, expandability, and architecture that would dominate the PC industry for years.
Here’s a breakdown of its key features, significance, and legacy:
Key Features and Innovations
- Intel 80286 Processor:
- This was the heart of the AT, running initially at 6 MHz (later models offered 8 MHz).
- The 80286 was a significant leap from the 8088 in the original PC and XT. It introduced protected mode, which allowed access to up to 16 MB of RAM (compared to 1 MB for the 8088/8086) and provided memory protection features for multitasking.
- While MS-DOS primarily ran in “real mode” and couldn’t fully utilize the 286’s protected mode capabilities, the processor’s speed and expanded address space laid the groundwork for future operating systems like OS/2 and early versions of Microsoft Windows.
- 16-bit ISA Bus:
- The AT introduced a 16-bit expansion bus (the ISA – Industry Standard Architecture – bus). This doubled the data transfer rate compared to the 8-bit bus of the PC and XT, allowing for faster peripherals like hard drive controllers, network cards, and graphics adapters.
- Crucially, the 16-bit ISA slots were backward-compatible, meaning most 8-bit XT cards could still function in the longer AT slots.
- Standard Hard Drive:
- Unlike the original PC and XT, where a hard drive was an expensive option, the AT came standard with a 20 MB hard disk drive (using the MFM or RLL interface). This made hard drives a common feature on desktop PCs for the first time.
- High-Density Floppy Drive:
- The AT introduced the 1.2 MB 5.25-inch floppy disk drive. This was a significant increase in capacity over the 360 KB drives of the PC and XT.
- However, it also introduced compatibility issues: while the 1.2 MB drives could read and write 360 KB disks, writing to a 360 KB disk with a 1.2 MB drive often resulted in disks that older 360 KB drives couldn’t reliably read.
- Real-Time Clock (RTC) with CMOS Memory:
- The AT included a battery-backed real-time clock and a small amount of CMOS RAM (usually 64 bytes). This allowed the system to keep track of the date and time even when powered off, and to store system configuration settings (like hard drive type, amount of RAM, and floppy drive types) without requiring DIP switch changes or re-entry after every boot. This feature became universal in PCs.
- Enhanced Keyboard:
- The AT introduced a new 84-key keyboard layout with a larger Enter key, separate numeric keypad, and separate cursor control keys. This was an improvement over the original PC’s 83-key keyboard, though it was later superseded by the 101-key “Enhanced” keyboard.
- Form Factor:
- The AT established the “AT motherboard” form factor, which defined the dimensions of the motherboard, the placement of expansion slots, and the power supply connectors. This standard lasted for over a decade until it was largely replaced by the ATX form factor in the mid-1990s.
Impact and Legacy
The IBM PC AT was arguably the most influential personal computer of its era for several reasons:
- The “AT Standard”: More than just a computer, the AT defined the architecture for the next generation of PCs. Its specifications became the de facto standard for all PC-compatible manufacturers (often called “clones”). If a computer was “AT-compatible,” it meant it used the 80286 (or later 80386/80486) processor, a 16-bit ISA bus, the AT keyboard interface, the AT power supply form factor, and a compatible BIOS.
- Performance Benchmark: It set a new performance benchmark that allowed for more complex software, early graphical user interfaces (like Windows 1.0/2.0), and CAD/CAM applications.
- Industry Expansion: By providing a robust and extensible architecture, the AT encouraged a massive industry of hardware add-on manufacturers (graphics cards, sound cards, network cards) and software developers.
- Protected Mode Foundation: Although limited by DOS, the 80286’s protected mode capabilities were crucial for the development of multitasking operating systems like OS/2 and later Windows NT, even if the AT itself wasn’t the ideal platform for them.
Challenges and Criticisms
- Cost: The AT was very expensive upon release, typically starting around $5,000-$6,000, limiting its initial adoption to businesses and affluent enthusiasts.
- The “286 Wall”: The difficulty of switching between real mode and protected mode on the 80286 (a relatively slow process) meant that operating systems like DOS and early Windows couldn’t effectively leverage its multitasking capabilities. This limitation was largely overcome with the 80386 processor.
- Floppy Drive Incompatibility: The 1.2 MB floppy drive’s issues with writing 360 KB disks reliably caused frustration for users trying to share files with older PCs.
In summary, the IBM PC AT was a technological leap forward that solidified IBM’s dominance in the early PC market and, more importantly, created the fundamental architecture that the vast majority of personal computers would follow for the next decade and beyond. Its innovations became standard features that we still see echoes of in modern PCs today.