Device addressing

Each I/O device needs a proper device address (DA) that it responds to. There are four X select lines and four Y select lines on the I/O bus. A device is addressed by setting exactly one X line to 1 and exactly one Y line to 1. Therefore a total of 4*4 or 16 devices can be addressed.
The assignment to the devices is shown in the following table. Addresses which are used differently in software and hardware are specially marked (e.g. Print Plot).

Address X/Y Device
00/0Graphics adapter and processor
10/1Common and Language ROS
20/2Executable ROS
30/3Diskette Sort
71/3Parallel I/O
82/0Asynchronous Communication and Serial I/O
A2/2Serial I/O (only in BASIC, free in HW)
C3/0Print Plot (only in BASIC, free in HW)
D3/1Diskette adapter
E3/2Tape drive
F3/3Reset I/O

The X and Y lines are checked against false signals. If more than one X or Y line is active a data parity error is generated when an I/O instruction is executed (see I/O-Interface). This asserts the machine check line and halts the processor (PROCESS CHECK).
Each device can have several sub devices e.g. device disk drive, sub device drive no. 2. The means of selecting sub devices depends on the device itself and differs considerably from the X/Y selection mechanism.