View source for IOX

{{InstructionBox
|Mnemonic=IOX
|Description=Exchange information between<br>I/O system and A register.
|Format=IOX ''<device register address>''
|Code=164&nbsp;000
|Affected=''See description''
|Type=''See description''
|Architecture=[[ND-100]], [[ND-110 CPU|ND-110]]
}}


'''IOX''' is an assembly instruction used for [[programmed IO]], and it transfers data between a device I/O register and the CPU's [[A register]].

IOX (the Input/Output Instruction) is a privileged instruction in normal cases, but if there is no OS running and the [[paging system]] is turned off then IOX is an unprivileged instruction. IOX and [[IOXT]] are the only instructions used to communicate with external interfaces.

== Direction of transfer ==
The actual transfer direction of the IOX and IOXT instructions is decoded from the device-register address, based on the following convention: the transfer direction is input (from device to CPU) if the device-register address is even.

This means that all I/O device registers which need to be loaded from the CPU A register are assigned an odd device register address. Similarly I/O device input registers are assigned even addresses.

== Device register address ==
Register addresses can hold data, command or status information for a device.

Bit 0-10 of the instruction code sets the direction and IO address:
* Bit 0 gives the direction of transfer:
** If bit 0 is zero then IOX reads the value from the IO register into register A
** If bit 0 is one then IOX writes the contents of register A to the IO register

* Bit 3-9 is the selected device
* Bit 0-2 is the selected register within selected device
* Bit 10 is the type of interface:
** If bit 10 is zero then it is a ''Standard ND interface''
** If bit 10 is one then it is a ''Customer designed interface''


An external device may require more than one register address, for example a magnetic tape unit may need several register addresses; these should be given successive device-register addresses (remembering to use odd addresses for input and even addresses for output).

=== Note ===
* Addressing of a non existing device results in a [[IOX error]].
* The number of external devices that can be controlled by the CPU depends on the configuration of the devices.

== Address range ==
The IOX instruction can address a total of 2048 registers, i.e. addresses from 0 - 3777 (octal). However, the device-registers implemented on interfaces designed at [[Norsk Data]] use the address area 0 - 1777 (octal) only. The remaining 1024 register addresses are available for customer-designed interfaces.

== Example ==
=== To give an instruction to a device ===
 [[LDA]] ''<device command code>''
 '''IOX''' ''<device register address>''

The ''lsb'' of IOX will be 1. The A register contents are output to the device addressed within the IOX opcode.

=== To check the status of a device ===
 '''IOX''' ''<device register address>''

The ''lsb'' of IOX will be 0. The status code of the device addressed by the IOX opcode will be loaded into the A register.

=== Transfer data ===
 '''IOX''' ''<device register address>''

Data from the device addressed by the IOX instruction is read into the A register if the ''lsb'' of IOX is 0. If the ''lsb'' of IOX is 1, the A register contents are output to the device.


== See also ==
* [[IOXT]] instruction

== References ==
*{{ND-doc|06.015.02|(Programmed I/O, chapter 6.3, page 170-172)}}
*{{ND-doc|06.026.1| (The Input/Output Instructions IOX and IOXT, page 141)}}
*{{ND-doc|06.029.1|(Input/Output Instructions, page 121-122)}}


[[Category:ND-100 instructions]]