IBM PS/1

The original PS/1, based on a 10 MHz Intel 80286 CPU, was designed to be easy to set up and use. It featured 1 MB of memory, built-in modem, an optional 30 MB hard disk. IBM also released a 5.25" disk drive unit and an Adapter Card Unit (ACU) to install third-party expansion cards. Some of the lower end PS/1 models suffered from very limited expansion capabilities, since they lacked standard ISA expansion slots. The 2121 series computers used the same form factor as the 2011 series, but included up to two ISA slots inside the case. Memory could be expanded from 2MiB to 6MiB using a proprietary 4 MiB memory module. The higher-end 2121 featured an Intel 80386SX processor running at 16 MHz. Similar to a few Tandy 1000 models, the early PS/1's had an operating system (PC-DOS 4.01) built in to ROM, rather than loading it from a hard drive. It was possible to have the computer boot from the hard drive if the operating system was upgraded.