View source for IOT

{{InstructionBox
|Mnemonic=IOT SKA
|Description=Perform monitor call
|Format=IOT SKA <number>
|Code=161&nbsp;000
|Affected= Various registers, depending on <number>
|Type=User
|Architecture=[[NORD-1]]
}}

'''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 <ref>*{{ND-doc|60.096}}</ref> states that ''the NORD-10 may also be delievered 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.

The [[BRF EDITOR]] command ''LIST-BRF'' is aware of 161.xxx instructions and lists them as '''IOT SKA'''. The [[BRF Linker]] ''LIST-BRF-CODE'' command will only list the octal value, and no mnemonic.

<tt>'IOT SKA', as used by the BRF EDITOR probably means 'IOT Systemkall' which translates to 'IOT System Call' from Norwegian.''</tt>

As described above, the instruction pre-dates [[SINTRAN III|SINTRAN-III]] and the [[NORD-10]]/[[ND-100|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.

==== Implementation details ====
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 [[IIC register|Internal Interrupt Code register]] <ref>{{ND-doc|06.014.02}} page 2-17</ref>). The snippet of SINTRAN-III code which handles the exception looks as follows: <ref>SIN-L STD A listing part two, March 1989</ref>
{{DocPreStart}}
 072404   IIC06: IF ACTLV=ALEVL THEN                       % PRIVILIGED INSTRUCTION [[Wikipedia:Sic|[''sic'']]]
 072410             RTREF.ACTPRI/\74000 SHZ -4+"NMPIT+LV14B+ERNG2"% SET USERS NORMAL PIT AS ALT. PIT
 072415             X:=PERR; *TRR PCR
 072417             X.SN1=:D                               % D= THE ILL.INSTR.
 072421             "NMPIT+ADPIT+LV14B+ERNG2"; *TRR PCR    % RESET PCR
 072423             IF 177600/\D=161000 THEN T:=177/\D; GO FAR MONCALL FI % IF 161XXX THEN MONCALL
 072433             IBITNO; T:=PERR; CALL 9ERR(#24)        % GIVE ERROR MESSAGE
 072437             GO FAR ABOR                            % TERMINATE CURRENT RUNNING PROGRAM
 072440           FI; GO FAR TDTLEV                        % MAY BE ILL.INSTRUCTION ON ILLEGAL LEVEL
{{DocPreEnd}}

The NPL code "'''IF 177600/\D=161000 THEN T:=177/\D; GO FAR MONCALL FI'''" translates to the following pseudocode:
{{DocPreStart}}
 if instruction is in the 161000 (octal) range, and <number> is in the range 0x00-0x7f, then
   handle as monitor call (<number>)
 else
   handle as any other illegal instruction, abort program
 endif
{{DocPreEnd}}

The interrupt handler detects if the instruction is an 161000 instruction, and, if so, handles it as a [[MON]] instruction (153000<sub><small>8</small></sub>), 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').


== References ==
<references/>
*{{ND-doc|60.228.1}}

[[Category:ND-100 instructions]]