ND-120¶

Overview¶

OPCOM (OPerator's COMmunication) is a microprogram residing inside the ND-100 family CPU (ND-100, ND-110, ND-110/CX, ND-120/CX). It provides low-level access to the CPU for operator functions, debugging, and service purposes.

OPCOM runs independently from SINTRAN III. It is part of the CPU microcode and is available even when no software is loaded. It communicates exclusively through the console terminal (terminal 1). When OPCOM is active, the console keyboard is connected to the OPCOM microprogram. Program output to the console continues normally.

Key Characteristics¶

Uppercase only - OPCOM accepts only uppercase letters
Octal numbers only - All numeric values and addresses are octal unless explicitly stated otherwise
Prompt - The # character indicates OPCOM is ready for commands
Non-destructive - Entering OPCOM does not stop or affect a running CPU
Console-only - Cannot be used from any terminal other than terminal 1
No spaces - Spaces are not permitted within commands. The space character itself is interpreted as a command to cancel all previously entered characters
Immediate interpretation - Most commands are interpreted immediately and must NOT be ended with CR. When CR is required, it is shown explicitly as ↵
Unrecognized input - Any unrecognized character produces ? and cancels all previously entered characters (same as space)

Sources: ND-30.003.7 EN SINTRAN III System Supervisor, Section 2.1.3; nd110.hackercorp.no/Terminal

Entering and Leaving OPCOM¶

Entering OPCOM¶

Method	Description
`@OPCOM`	SINTRAN command (requires user SYSTEM)
`@STOP-SYSTEM`	Stops SINTRAN and enters OPCOM (CPU in STOP mode)
OPCOM button	Press OPCOM on operator panel (key switch must be ON)
STOP button	Press STOP on operator panel (CPU enters STOP mode + OPCOM)

The CPU must be in OPCOM or STOP mode. The OPCOM indicator on the display panel will be lit. When OPCOM is active, the prompt # appears on the console terminal.

Leaving OPCOM¶

Command	Description
ESC	Terminates OPCOM mode. Has no effect if CPU is in STOP mode - the CPU must be running
LOAD completes	OPCOM exits automatically when a load completes and CPU starts running

Before leaving OPCOM, restore the display panel:

#ACT/

Then press ESC to return to SINTRAN.

Commands Available in Any OPCOM Mode¶

These commands work whether the CPU is running or stopped:

Command	Syntax	Description
Examine memory/register	`/`	Examine memory location or register
Register dump	`RD`	Dump working registers
Internal register dump	`IRD`	Dump internal CPU registers
Scratch register dump	`RDE`	Dump scratch registers (microprogram debugging)
Examine mode	`E`	Specify physical or virtual address mode
Display format	`F`	Set display panel format
Print current location	`*`	Print address of last examined location
STOP	`STOP`	Put CPU in STOP mode
MCL	`MCL`	Master Clear (reinitialize, no microcode load)
Leave OPCOM	`ESC`	Exit OPCOM (no effect if CPU is stopped)

Commands Available Only in STOP Mode¶

These commands are only legal when the CPU is stopped:

Command	Syntax	Description
Start program	`!`	Start execution at specified address
Single step	`Z`	Execute one (or N) instructions
Bootstrap load	`$` or `&`	Load program from storage device
Set breakpoint	`.`	Set breakpoint at specified address
Execute instruction	`"`	Execute an instruction entered from console
Memory test	`#`	Start microprogrammed memory test

System Control Commands¶

STOP - Halt CPU¶

#STOP

Puts the CPU in STOP mode. Program execution halts but can be resumed. All register contents are preserved. Also available as a button on the operator panel. No CR terminator.

MCL - Master Clear¶

#MCL

Hardware initialization of the ND-100 CPU and interface parts. Clears vital registers. Program execution cannot be resumed. Memory contents remain unchanged (warm restart is possible). Note: MCL does not reload the microcode.

NOTE: On ND-110/CX, use the MCL button on the operator panel (to properly renew the microprogram).

When MCL completes, two number signs appear: ##

IMPORTANT: Wait for ## before issuing any LOAD command.

Load Commands (STOP Mode Only)¶

The $ and & commands both cause the CPU to load (and possibly execute) a program from a storage device. They are equivalent. The device address is an octal value entered immediately before the command. If the value is omitted, the load is defined by the ALD switch setting.

Load from ALD Setting¶

#&

or

#$

Loads from the device configured in the Automatic Load Descriptor (ALD) thumbwheel switch on the CPU card.

Load from Specific Device¶

#ddddd&

or

#ddddd$

Where ddddd is the octal hardware device address. The default load format is BPUN (binary). For SMD and Winchester disks, bootstrap format is used. To specify bootstrap load explicitly, set bit 13 of the device address to 1 (e.g., if device address is 1550₈, enter 21550&).

All Known Boot Device Addresses¶

Command	Device Address	Device Type	Load Format
`#&`	(from ALD)	Per ALD switch	Per ALD
`#400&`	400₈	Paper TAPE-READER-1	BPUN (binary)
`#500&`	500₈	ST-506 Winchester disk controller 1	Bootstrap
`#1540&`	1540₈	SMD/ECC disk controller 1	Bootstrap
`#1560&`	1560₈	FLOPPY-DISC-1, unit 0 (left-hand floppy)	BPUN (binary)
`#1570&`	1570₈	FLOPPY-DISC-2, unit 0 (right-hand, seldom present)	BPUN (binary)
`#1600&`	1600₈	HDLC Remote load 1 (network boot)	BPUN (binary)
`#20500&`	20500₈	ST-506 Winchester disk controller 1	Bootstrap
`#20510&`	20510₈	ST-506 Winchester disk controller 2	Bootstrap
`#21540&`	21540₈	ECC/SMD disk controller 1	Bootstrap
`#21550&`	21550₈	ECC/SMD disk controller 2	Bootstrap
`#144300&`	144300₈	SCSI disk controller	Bootstrap

Load Format Types¶

BPUN (Binary) - Binary Punched format. Loads a binary image (standalone programs from floppy, tape, or network)
Bootstrap - Mass storage bootstrap format. Loads from a disk with filesystem structure (SINTRAN from disk). Selected by setting bit 13 of device address to 1

Device Address Notes¶

Only unit 0 can be selected for loading via ALD
The start address after load is always the power-fail restart address 20₈
ALD settings 8-15 specify load and run (auto-start at address 20₈)
ALD settings 2-7 specify load only (CPU remains in STOP after load)
SCSI device address 144300₈ is defined in the SINTRAN boot table (MDISCS/SCDIS in PH-P2-OPPSTART.NPL)

Program Execution Commands (STOP Mode Only)¶

Start Program (!)¶

#xxxxxx!

Execution starts at the octal address entered immediately before !. If no address is given, the current value of the P register (program counter) is used:

#!

Important SINTRAN Start Addresses¶

Address	Purpose
`20`	Power-fail restart address
`21`	Warm start address
`22`	Cold start address

Common Start Sequences¶

Continue execution (STOP pressed accidentally):

#!

Warm start (power-fail restart):

#STOP
#20!

Cold start (MACM loaded):

#22!

Full cold start from disk:

#STOP
#MCL
(wait for ##)
#&

Start MEMTOF (memory dump utility):

#151!

Single Step Execution (Z)¶

#Z

Executes one instruction (or one interrupt level change). If a value is entered before Z, that many instructions are executed:

#10Z

Executes 10₈ (8 decimal) instructions.

Page faults, protect violations, and interrupt level changes are executed correctly but count as extra instructions. An overhead of approximately 3 µs is introduced between each instruction during single-stepping.

Set Breakpoint (.)¶

#xxxxxx.

Sets a breakpoint at octal address xxxxxx. When the program reaches that address, execution stops and the . character is echoed to the console.

An overhead of approximately 3 µs is introduced between each instruction when breakpoint checking is active.

If the specified address is never reached, execution continues until a character other than 0-7 or A-Y is typed on the console.

Execute Entered Instruction (")¶

#oooooo"

Executes the instruction with octal opcode oooooo continuously. Execution stops when a character other than 0-7 or A-Y is typed.

Example: The PON (paging on) instruction has octal code 150410:

#150410"

This turns on the paging system.

Memory Examine and Deposit Commands¶

Examine Mode (E)¶

Controls whether subsequent memory commands use physical or virtual addressing.

Physical examine (paging off):

#E↵

No number before E selects physical memory. Addresses may contain up to 24 bits (8 octal digits) in physical mode.

Virtual examine via page table:

#ptE↵

Where pt is the page table number (0 to 17₈).

NOTE (ND-110/CX): The number used is the actual page table number directly. This differs from the ND-100 CPU which used an encoded value.

ND-100 with MMS II page table encoding:

y = x / 4         (integer division, discard remainder)
z = y * 10₈ + (x - y * 4₈)

Example:

#5E↵

If paging is on, future memory references will be made via page table 5.

When virtual addressing is used, page faults and protect violations are ignored.

Examine Memory Location (/)¶

#xxxxxx/yyyyyy

Enter the memory address followed immediately by /. The contents are displayed. You then have the option of:

Changing the contents (entering a new value)
Viewing the next location (pressing CR)

Deposit in STOP Mode¶

Enter the new octal value followed by CR. The location is changed and the next location is displayed:

#100/137777 0↵

This displays contents of address 100₈ (which is 137777₈) and changes it to 0. The next location (101₈) is then displayed.

Just pressing CR without a value displays the next location without changing the current one.

Deposit in RUN Mode (DEP)¶

Changing memory while SINTRAN is running can produce unpredictable results. To guard against doing this inadvertently, you must follow the value with DEP instead of CR:

#100/137777 0DEP

Changes location 100₈ to 0 while the CPU is running.

Examine Memory Range / Memory Dump (<)¶

#xxxxxx<yyyyyy↵

Dumps contents of memory from address xxxxxx to yyyyyy. Eight addresses are printed per line. The dump uses the 64KW memory bank last addressed by a memory examine (/) command. A memory examine should always be done before a memory dump.

The dump may be stopped by pressing any key.

Print Current Location (*)¶

#*

Prints the current physical or virtual address on which the next memory examine or deposit will take place. The current location counter is set by the / command and is incremented each time CR is typed afterward.

Register Commands¶

Examine Register (R/)¶

Same syntax as memory examine, but with a register name instead of address. The program level (0-17₈) may be specified before the register name. If omitted, level 0 is assumed.

Working registers can be specified by name or number:

Number	Name	Register
R0	S	Status register
R1	D	D register
R2	P	Program counter
R3	B	Base register
R4	L	Link register
R5	A	Accumulator
R6	T	T register
R7	X	Index register

Internal registers are addressed as Iy where y = 0-15:

Number	Name	Register
I0	PANS	Panel Status
I1	STS	Status
I2	OPR	Operator Panel Register (simulated)
I3	PSR	Program Status Register
I4	PVL	Previous Level
I5	IIC	Internal Interrupt Code
I6	PID	Program Identification
I7	PIE	Priority Interrupt Enable
I10	CSR	Cache Status Register
I11	ACTL	Active Level
I12	ALD	Automatic Load Descriptor
I13	PES	Paging Error Status
I14	PCR	Program Control Register
I15	PEA	Paging Error Address

NOTE: OPR (I2) is a simulated panel switch register which can be written to from OPCOM. Programs can read its contents with the TRA OPR instruction.

Deposit in STOP mode: Enter new value followed by DEP:

#A/126500 100DEP

Examples:

#A/126500           Examine A register on level 0
#7P/140003          Examine P register on level 7
#7R2/140003         Examine R2 (= P register) on level 7
#I7/030013          Examine PIE register (internal register 7)
#OPR/00100          Examine the OPR pseudo register
#I12/021540         Examine ALD register (shows ALD switch value)

Register Dump (xx<yyRD)¶

#xx<yyRD↵

Dumps the working registers on program levels xx to yy. One register set per line. Registers are printed in order: STS, D, P, B, L, A, T, X.

To dump only one level, set xx equal to yy. If xx and yy are omitted, level 0 is dumped:

#<RD↵              Dump registers on level 0
#0<17RD↵           Dump registers on all 16 levels (0-17₈)
#5<5RD↵            Dump registers on level 5 only

Internal Register Dump (IRD)¶

#IRD↵

Displays all 16 internal registers. Only allowed when CPU is in STOP mode to avoid unintentional unlocking of PEA, PES, and IIC when the CPU is running.

Scratch Register Dump (xx<yyRDE)¶

#xx<yyRDE↵

Dumps the 8 scratch registers (microprogram-accessible only) on program levels xx to yy. One set per line. Intended for microprogram debugging only.

I/O Commands¶

Read/Write I/O Device (IO/)¶

#ddddIO/

Where dddd is the device address. The CPU executes an IOX instruction using the given device address.

Even address: Input to CPU (read from device). Input data is displayed on the console but not stored anywhere
Odd address: Output from CPU (write to device). Output data is taken from the OPR pseudo-register

None of the working registers is affected.

To set up OPR before an output operation, use the register examine command:

#OPR/xxxxx yyyyyDEP     Set OPR to yyyyy
#ddddIO/                 Execute IOX with device address dddd (odd = output)

Display Panel Commands¶

The display panel is optional. Several pseudo-registers control what it shows.

ACT - Normal Display¶

#ACT/

Restores normal display: active levels, clock, and activity. Always issue this before leaving OPCOM, as other OPCOM operations may have changed the display mode.

BUS - Bus Activity Display¶

#zzBUS/

Shows bus activity on the display. The two-digit code zz specifies what data is displayed:

First digit (data source):

Digit	Source
0	CPU data
1	DMA data
2	CPU address
3	DMA address

Second digit (access type):

Digit	Access
0	Nothing displayed
1	Read access only
2	Write access only
3	Read and write

Example:

#23BUS/

The display shows all data written from the CPU to memory. The function field shows "ACWR".

OPR - OPR Register Display¶

#OPR/

The display shows the contents of the OPR register. This is a simulated panel switch register writable from OPCOM. Programs can read it with the TRA OPR instruction.

U - User Register Display¶

#U/xxxxxx

The display shows the contents of a scratch register writable by the TRR LMP instruction. Used by DISC-TEMA to show the cylinder number during disk operations.

Display Format (F)¶

#uuzzyxF↵

Defines the display format for the optional display unit. F without arguments (or with argument 0) sets the default octal format.

x - Number representation:

x	Representation
0	Octal (default). `zz` has no effect
1	Unary - 4 bits decoded to light 1 of 16 indicators. `zz` = which 4 bits
2	Binary. `zz` has no effect

y - Stretch code (afterglow):

y	Stretch
0	No stretching
1	Zeros are stretched
2	Ones are stretched
3	Zeros and ones are stretched

zz - Lower start bit for unary display (0-24₈): Position of lowest bit for unary representation.

uu - Display processor maintenance codes (4 bits):

uu	Function
1	Display year and month
2	Inhibit message
4	Initialize panel processor
10	Abort message

Example:

#1421F↵

Bits 14-17₈ (bits 12-15 decimal) shown in unary representation with afterglow on ones.

Memory Test (STOP Mode Only)¶

Internal Memory Test (#)¶

#bb#

Starts a microprogrammed memory test. Memory is tested in banks (segments) of 64 KW (64K words). The bank number bb is entered immediately before the # character.

A second # is printed on the console if the test is successful. If an error is found, the test stops and ? is sent to the console.

Registers after a failed test:

Register	Contents
T	Failing bits
P	Failing address
D	Error pattern
L	Test pattern
B	Start address
X	Stop address

ALD - Automatic Load Descriptor¶

The ALD is a thumbwheel switch on the CPU card with 16 positions (0-15). It determines boot behavior on power-up, power failure recovery, and operator LOAD commands. The CPU copies the ALD setting into internal register I12.

Reading the ALD Value¶

#I12/xxxxxx

or equivalently:

#112/xxxxxx

Reads internal register I12 (ALD), displaying the current value in octal.

ALD Switch Position Table¶

ALD Pos	I12 Value	Load Action
15	000000	STOP (no load)
14	001560	BPUN load from floppy (1560₈) and run from 20₈
13	020500	Bootstrap load from Winchester (500₈) and run from 20₈
12	021540	Bootstrap load from SMD disk (1540₈) and run from 20₈
11	000400	BPUN load from paper tape (400₈) and run from 20₈
10	001600	BPUN load from HDLC (1600₈) and run from 20₈
9	--	Run from 20₈ (no load)
8	--	Run from 20₈ (no load)
7	100000	STOP (no load)
6	101560	Binary load from floppy (1560₈), load only
5	120500	Mass storage load from Winchester (500₈), load only
4	121540	Mass storage load from SMD disk (1540₈), load only
3	100400	Binary load from paper tape (400₈), load only
2	101600	Binary load from HDLC (1600₈), load only

The load action is triggered when: - $ or & (without a preceding device address) is typed - The LOAD button is pressed on the operator panel - Power is restored with the key switch in LOCK position after an extended power failure (standby power was lost)

ALD Behavior Summary¶

Positions 8-15: Load and run from address 20₈ (auto-start)
Positions 2-7: Load only (CPU remains stopped after load)
Positions 4, 5, 12, 13: Bootstrap load from disk
All other positions: BPUN format expected
Positions 7 and 15: STOP, no loading done
Positions 8 and 9: Run from address 20₈ without loading

Typical production setting: Position 12 (SMD) or 13 (Winchester) - loads and auto-starts SINTRAN.

Decoding the I12 Register Value¶

The I12 register is an 18-bit value:

Bits	Field	Meaning
17 (MSB)	Load on power-up	1 = Load even with standby power present (positions 2-7)
16	Load format	0 = BPUN (binary), 1 = Mass storage (bootstrap)
15-0	Device address	Hardware device address (octal)

Decoding examples:

I12 Value (oct)	Bit 17	Device	Interpretation
121540	1	1540	Always load, mass storage, SMD disk
101560	1	1560	Always load, binary, floppy
021540	0	1540	Load on power loss only, mass storage, SMD
020500	0	500	Load on power loss only, mass storage, Winchester
100000	1	0	STOP (no device)
000000	0	0	STOP (no device)

Operator Panel Key Switch¶

Position	Behavior
LOCK	Normal operation. ALD behavior per table above
ON	Manual operation. STOP after power failure; must start manually
OFF	Power off

The key should normally be in LOCK position. If in ON position, the computer enters STOP mode after power failure and requires manual restart.

Command Summary Table¶

Always Available (OPCOM or STOP mode)¶

Command	Syntax	Description
EXAMINE	`xxxxxx/`	Examine memory location
DEPOSIT (stopped)	`xxxxxx/old new↵`	Change memory (STOP mode)
DEPOSIT (running)	`xxxxxx/old newDEP`	Change memory (RUN mode, use with caution)
RANGE DUMP	`xxxxxx<yyyyyy↵`	Dump memory range
PRINT LOCATION	`*`	Print current examine address
EXAMINE MODE	`E↵` or `ptE↵`	Select physical or virtual addressing
REGISTER EXAMINE	`[lv]reg/`	Examine register (optional level prefix)
REG BLOCK DUMP	`xx<yyRD↵`	Dump working registers, levels xx-yy
INTERNAL REGS	`IRD↵`	Dump 16 internal registers (STOP only)
SCRATCH REGS	`xx<yyRDE↵`	Dump scratch registers (microprogram debug)
I/O ACCESS	`ddddIO/`	Execute IOX instruction on device
DISPLAY FORMAT	`uuzzyxF↵`	Set display panel format
ACT	`ACT/`	Restore normal display
BUS	`zzBUS/`	Show bus activity on display
OPR	`OPR/`	Show OPR register on display
U	`U/`	Show user register on display
STOP	`STOP`	Halt CPU
MCL	`MCL`	Master Clear
LEAVE OPCOM	`ESC`	Exit OPCOM (CPU must be running)

STOP Mode Only¶

Command	Syntax	Description
START	`xxxxxx!` or `!`	Start execution at address (or current PC)
SINGLE STEP	`Z` or `nZ`	Execute 1 or n instructions
BREAKPOINT	`xxxxxx.`	Set breakpoint at address
LOAD	`&` or `ddddd&`	Bootstrap/BPUN load from ALD or device
LOAD	`$` or `ddddd$`	Same as `&`
EXEC INSTRUCTION	`oooooo"`	Execute instruction with opcode oooooo
MEMORY TEST	`bb#`	Test memory bank bb (64KW)

Important Notes¶

OPCOM is a microprogram, not software. It exists in CPU microcode and is available even when no software is loaded in memory.
OPCOM does not stop the CPU when entered. If SINTRAN is running, it continues. Only the STOP command (or STOP button) halts execution.
Memory deposits differ by CPU state: In STOP mode, use value + CR. In RUN mode, use value + DEP. This prevents accidental memory corruption.
Always restore display with ACT/ before leaving OPCOM, as other operations change the display mode.
Wait for ## after MCL before issuing LOAD commands. A single # means MCL is still in progress.
ALD only loads from unit 0 - It is not possible to select any other disk drive than unit 0 for loading.
ND-110/CX page table numbering uses actual page table numbers directly. ND-100 with MMS II uses an encoded value (see Examine Mode section).
Breakpoints and single-step introduce ~3 µs overhead per instruction.
IRD is STOP-only to prevent unintentional unlocking of PEA, PES, and IIC registers.
Space cancels input - Pressing space discards all characters typed so far. Any unrecognized character also cancels and produces ?.

Sources¶

ND-30.003.7 EN SINTRAN III System Supervisor, Section 2.1.3 (pages 33-39)
ND-30.003.7 EN SINTRAN III System Supervisor, Section 11.4 (crash dump procedure)
ND-30.003.7 EN SINTRAN III System Supervisor, Section 11.5 (RT-program hang procedure)
ND-30.003.7 EN SINTRAN III System Supervisor, Appendix K (ALD switch settings, Table 29)
nd110.hackercorp.no/Terminal (ND-110 microcode emulator OPCOM documentation)
SINTRAN/OS/01-BOOT-SEQUENCE.md (MDISCS table, SCSI device address 144300₈)