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).
|0||0/0||Graphics adapter and processor|
|1||0/1||Common and Language ROS|
|8||2/0||Asynchronous Communication and Serial I/O|
|A||2/2||Serial I/O (only in BASIC, free in HW)|
|C||3/0||Print Plot (only in BASIC, free in HW)|
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.