Apple SOS - Technical Details

Technical Details

SOS was a single-tasking operating system. A single program is loaded at boot time, called the interpreter. Once running, the interpreter could then use the SOS application programming interface to make requests of the system. The SOS API was divided into four main areas:

• File Calls: Create, destroy, rename, open, close, read, write files; set, get prefix (current working directory); set, get file information; get volume information; set, set mark, EOF, and level of files
• Device Calls: Get status, device number, information of a device; send device control data
• Memory Calls: Request, find, change, release memory segment; get segment information; set segment number
• Utility Calls: Get, set fence (event threshold); get, set time; get analog (joystick) data; terminate.

SOS had two types of devices it communicated with via their device drivers: character devices and block devices. Examples of SOS character devices are keyboards and serial ports. Disk drives are typical block devices. Block devices could read or write one or more 512-byte blocks at a time; character devices could read or write single characters at a time.