IOT

IOT SKA
Description	Perform monitor call
Format	IOT SKA <number>
Code	161 0008
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 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.

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 <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 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 <number> is in the range 0x00-0x7f, then
  handle as monitor call (<number>)
else
  handle as any other illegal instruction, abort program
endif

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').

References

• Norsk Data Document ND–60.228.1 SINTRAN III Monitor Calls manual