|Description||Perform monitor call|
|Format||IOT SKA <number>|
|Affected||Various registers, depending on <number>|
IOT is a NORD-1 I/O instruction which is the NORD-1 equivalent of the NORD-10/ND-100 I/O instruction IOX. IOT is undocumented. The MAC Assembler manual  states that the NORD-10 may also be delivered with a NORD-1 compatible I/O instruction IOT (page 2-27).
A special case of IOT is the IOT SKA instruction, it is still used by very old applications like the NPL compiler. Old ND system applications tend to keep old code even when updated.
'IOT SKA', as used by the BRF EDITOR probably means 'IOT Systemkall' which translates to 'IOT System Call' from Norwegian.
As described above, the instruction pre-dates SINTRAN-III and the NORD-10/N1xx CPUs, but applications using this instruction, within a certain range of <number> values, are still able to execute on the newer systems due to a trick explained below. This allows applications like NPL to work.
The instruction does not exist in the NORD-10/ND-1x0 CPUs (except when specially ordered with the NORD-10), but it will trigger a Privileged Instruction exception in SINTRAN-III (bit 6 in the Internal Interrupt Code register ). The snippet of SINTRAN-III code which handles the exception looks as follows: 
The NPL code "IF 177600/\D=161000 THEN T:=177/\D; GO FAR MONCALL FI" translates to the following pseudocode:
The interrupt handler detects if the instruction is a 161000 instruction, and, if so, handles it as a MON instruction (1530008), i.e. a monitor call (within the allowed range for <number>), instead of aborting the program. The instruction is thus an old version of the MON instruction, and the old calls map one-to-one to the SINTRAN-III functions as long as they are below 128 decimal (i.e. up to and including 'MON 177').
- *Norsk Data Document ND–60.096 MAC INTERACTIVE ASSEMBLY AND DEBUGGING SYSTEM USER'S GUIDE
- Norsk Data Document ND–06.014.02 ND-100 REFERENCE MANUAL page 2-17
- SIN-L STD A listing part two, March 1989
- Norsk Data Document ND–60.228.1 SINTRAN III Monitor Calls manual