IOT

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 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.

'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 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 Internal Interrupt Code register ). The snippet of SINTRAN-III code which handles the exception looks as follows:

072404  IIC06: IF ACTLV=ALEVL THEN                       % PRIVILIGED INSTRUCTION [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

The NPL code "IF 177600/\D=161000 THEN T:=177/\D; GO FAR MONCALL FI" translates to the following pseudocode:

if instruction is in the 161000 (octal) range, and is in the range 0x00-0x7f, then handle as monitor call else handle as any other illegal instruction, abort program endif

The interrupt handler detects if the instruction is an 161000 instruction, and, if so, handles it as a MON instruction (1530008), i.e. a monitor call (within the allowed range for ), 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').