PDP-11/70 Pager – Quick Guide

This guide describes the current state of the PDP-11/70 Pager program.

Important: At the moment, the Pager is not a full PDP-11/70 emulator. It is a PDP-11-inspired 16-bit front-panel machine. You can enter memory words bit by bit using graphical toggle switches, inspect memory, write to memory, start and stop programs, and execute single instructions.

Display

At the top left, it says pdp11/70 PAGER.

The top area of the display also shows:

• PROG: programming mode
• RUN: program is running
• speed, for example 5Hz
• in programming mode, the currently selected switch, for example SW15
• in the menu, the selected function, for example LOAD

At the top right are the status LEDs:

• RUN: program is running
• HLT: machine is stopped or in programming mode
• FET: instruction fetch; an instruction is being fetched
• EXE: execute; an instruction is being executed
• EXM: memory was read with EXAM
• DEP: memory was written with DEP
• ERR: error or illegal instruction

The ADDRESS LEDs show the current address or bus address.

The DATA LEDs show the last value that was read, written, or output. They are not automatically identical to the switch settings.

At the bottom there are 16 graphical toggle switches. Together they form the 16-bit switch register switchValue.

Control With the Scroll Wheel

The whole system can be operated with the scroll wheel.

PROG Mode

In PROG mode, switches are set and front-panel functions are executed.

• Turn scroll wheel: move the selected switch
• Short press: toggle the selected switch
• Long press: open the function menu

The white frame only shows which switch is selected. It does not change the switch value.

Menu

In the menu, you select a front-panel function.

• Turn scroll wheel: select function
• Short press: execute function
• Long press: leave the menu

Functions:

• LOAD: load the current switch setting as the address
• EXAM: examine memory at the current address
• DEP: deposit the switch value into memory
• STEP: execute exactly one instruction
• RUN: start the program at the current address
• RESET: reload the demo program

RUN Mode

In RUN mode, the program is running.

• Turn scroll wheel: change speed
• Short press: halt and return to PROG
• Long press: also halt

Available speeds:

1Hz  2Hz  5Hz  10Hz  25Hz  50Hz  100Hz  MAX

At slower speeds, the FET and EXE LEDs are easier to see.

Optional Keyboard Shortcuts

If the keyboard is active, these shortcuts are also available:

• L: LOAD
• E: EXAM
• D: DEP
• S: STEP
• R: RUN
• H: HALT
• C: RESET

The keyboard is optional. Everything important can also be done using only the scroll wheel.

Memory and Addresses

The Pager currently uses 256 memory words.

Each word has 16 bits.

The visible address counts in bytes, like on a PDP-11. That is why EXAM and DEP always increase the address by 2:

0000
0002
0004
0006
...

Internally, this is converted to a word index.

Front-Panel Functions in Detail

LOAD

LOAD takes the 16 switches and uses them as the new current address.

Example:

Switches = 0000000000010000
LOAD
Address = 0010

EXAM

EXAM reads memory at the current address.

After that:

• DATA shows the word that was read
• EXM flashes briefly
• the address is increased by 2

This allows you to inspect memory word by word.

DEP

DEP writes the current switch value into memory.

After that:

• DATA shows the word that was written
• DEP flashes briefly
• the address is increased by 2

This allows you to enter programs word by word.

STEP

STEP executes exactly one machine instruction and then stops again.

This is useful for checking a program slowly.

RUN

RUN starts the program at the current address.

The RUN LED stays on while the program is running.

RESET

RESET reloads the built-in demo program and puts the machine back into programming mode.

Current Instruction Set

The current mini instruction set is intentionally small:

0000  HALT
1aaa  LOAD  R0, [aaa]
2aaa  STORE R0, [aaa]
3aaa  ADD   R0, [aaa]
4aaa  SUB   R0, [aaa]
5aaa  JMP   aaa
6aaa  JZ    aaa
7nnn  LDI   R0, nnn
8000  INC   R0
9000  DEC   R0
Aaaa  OUT   [aaa]

aaa is an address.

nnn is an immediate value.

At the moment, there is only one register: R0.

Example: Entering and Examining One Word

Goal: write the value 0005 to address 0020 and then read it back.

1. In PROG, set the switches to 0020.
2. Long press to open the menu.
3. Select LOAD and short press.
4. Set the switches to 0005.
5. Open the menu.
6. Select DEP and short press.
7. Set the switches back to 0020.
8. Open the menu.
9. Select LOAD.
10. Open the menu again.
11. Select EXAM.

DATA now shows the value 0005.

Example: Entering a Small Program

This program loads 5, adds the memory value at address 0014, stores the result at 0018, outputs the value, and halts.

Address  Word   Meaning
0000     7005   LDI R0, 0005
0002     3014   ADD R0, [0014]
0004     2018   STORE R0, [0018]
0006     A018   OUT [0018]
0008     0000   HALT
0014     0007   Data value 7
0018     0000   Result

Input sequence:

1. Set switches to 0000.
2. Execute LOAD.
3. Set switches to 7005.
4. Execute DEP.
5. Set switches to 3014.
6. Execute DEP.
7. Set switches to 2018.
8. Execute DEP.
9. Set switches to A018.
10. Execute DEP.
11. Set switches to 0000.
12. Execute DEP.
13. Set switches to 0014.
14. Execute LOAD.
15. Set switches to 0007.
16. Execute DEP.
17. Set switches to 0000.
18. Execute LOAD.
19. Execute RUN.

At the end, address 0018 contains the value 000C, which is decimal 12.

To check the result:

1. Set switches to 0018.
2. Execute LOAD.
3. Execute EXAM.

DATA then shows the result.

Built-In Demo Program

When powered on, a demo program is loaded and started automatically.

It first calculates 5 + 7, writes the result to memory, and then visibly continues counting. This makes it easy to see that FETCH, EXEC, ADDRESS, and DATA are working.

A short press in RUN mode stops the demo program and returns to PROG mode.

RESET reloads the demo program.

What Is Not Complete Yet

The Pager is currently not a real PDP-11/70.

Not yet implemented:

• full PDP-11 instruction set
• real set of eight PDP-11 registers
• PSW with complete flag logic
• interrupts
• real PDP-11 peripherals
• persistent storage for user programs
• complete separate 16-bit DATA LED row

Even so, it can already be used like a small historical front-panel computer: set bits, store words, inspect memory, start and stop programs, and execute instructions step by step.PDP-11/70 Pager – Quick Guide
This guide describes the current state of the PDP-11/70 Pager program.
Important: At the moment, the Pager is not a full PDP-11/70 emulator. It is a PDP-11-inspired 16-bit front-panel machine. You can enter memory words bit by bit using graphical toggle switches, inspect memory, write to memory, start and stop programs, and execute single instructions.
Display
At the top left, it says pdp11/70 PAGER.
The top area of the display also shows:

PROG: programming mode

RUN: program is running

speed, for example 5Hz

in programming mode, the currently selected switch, for example SW15

in the menu, the selected function, for example LOAD

At the top right are the status LEDs:

RUN: program is running

HLT: machine is stopped or in programming mode

FET: instruction fetch; an instruction is being fetched

EXE: execute; an instruction is being executed

EXM: memory was read with EXAM

DEP: memory was written with DEP

ERR: error or illegal instruction

The ADDRESS LEDs show the current address or bus address.
The DATA LEDs show the last value that was read, written, or output. They are not automatically identical to the switch settings.
At the bottom there are 16 graphical toggle switches. Together they form the 16-bit switch register switchValue.
Control With the Scroll Wheel
The whole system can be operated with the scroll wheel.
PROG Mode
In PROG mode, switches are set and front-panel functions are executed.

Turn scroll wheel: move the selected switch

Short press: toggle the selected switch

Long press: open the function menu

The white frame only shows which switch is selected. It does not change the switch value.
Menu
In the menu, you select a front-panel function.

Turn scroll wheel: select function

Short press: execute function

Long press: leave the menu

Functions:

LOAD: load the current switch setting as the address

EXAM: examine memory at the current address

DEP: deposit the switch value into memory

STEP: execute exactly one instruction

RUN: start the program at the current address

RESET: reload the demo program

RUN Mode
In RUN mode, the program is running.

Turn scroll wheel: change speed

Short press: halt and return to PROG

Long press: also halt

Available speeds:
1Hz 2Hz 5Hz 10Hz 25Hz 50Hz 100Hz MAX
At slower speeds, the FET and EXE LEDs are easier to see.
Optional Keyboard Shortcuts
If the keyboard is active, these shortcuts are also available:

L: LOAD

E: EXAM

D: DEP

S: STEP

R: RUN

H: HALT

C: RESET

The keyboard is optional. Everything important can also be done using only the scroll wheel.
Memory and Addresses
The Pager currently uses 256 memory words.
Each word has 16 bits.
The visible address counts in bytes, like on a PDP-11. That is why EXAM and DEP always increase the address by 2:
0000
0002
0004
0006
...
Internally, this is converted to a word index.
Front-Panel Functions in Detail
LOAD
LOAD takes the 16 switches and uses them as the new current address.
Example:
Switches = 0000000000010000
LOAD
Address = 0010
EXAM
EXAM reads memory at the current address.
After that:

DATA shows the word that was read

EXM flashes briefly

the address is increased by 2

This allows you to inspect memory word by word.
DEP
DEP writes the current switch value into memory.
After that:

DATA shows the word that was written

DEP flashes briefly

the address is increased by 2

This allows you to enter programs word by word.
STEP
STEP executes exactly one machine instruction and then stops again.
This is useful for checking a program slowly.
RUN
RUN starts the program at the current address.
The RUN LED stays on while the program is running.
RESET
RESET reloads the built-in demo program and puts the machine back into programming mode.
Current Instruction Set
The current mini instruction set is intentionally small:
0000 HALT
1aaa LOAD R0, [aaa]
2aaa STORE R0, [aaa]
3aaa ADD R0, [aaa]
4aaa SUB R0, [aaa]
5aaa JMP aaa
6aaa JZ aaa
7nnn LDI R0, nnn
8000 INC R0
9000 DEC R0
Aaaa OUT [aaa]
aaa is an address.
nnn is an immediate value.
At the moment, there is only one register: R0.
Example: Entering and Examining One Word
Goal: write the value 0005 to address 0020 and then read it back.

In PROG, set the switches to 0020.

Long press to open the menu.

Select LOAD and short press.

Set the switches to 0005.

Open the menu.

Select DEP and short press.

Set the switches back to 0020.

Open the menu.

Select LOAD.

Open the menu again.

Select EXAM.

DATA now shows the value 0005.
Example: Entering a Small Program
This program loads 5, adds the memory value at address 0014, stores the result at 0018, outputs the value, and halts.
Address Word Meaning
0000 7005 LDI R0, 0005
0002 3014 ADD R0, [0014]
0004 2018 STORE R0, [0018]
0006 A018 OUT [0018]
0008 0000 HALT
0014 0007 Data value 7
0018 0000 Result
Input sequence:

Set switches to 0000.

Execute LOAD.

Set switches to 7005.

Execute DEP.

Set switches to 3014.

Execute DEP.

Set switches to 2018.

Execute DEP.

Set switches to A018.

Execute DEP.

Set switches to 0000.

Execute DEP.

Set switches to 0014.

Execute LOAD.

Set switches to 0007.

Execute DEP.

Set switches to 0000.

Execute LOAD.

Execute RUN.

At the end, address 0018 contains the value 000C, which is decimal 12.
To check the result:

Set switches to 0018.

Execute LOAD.

Execute EXAM.

DATA then shows the result.
Built-In Demo Program
When powered on, a demo program is loaded and started automatically.
It first calculates 5 + 7, writes the result to memory, and then visibly continues counting. This makes it easy to see that FETCH, EXEC, ADDRESS, and DATA are working.
A short press in RUN mode stops the demo program and returns to PROG mode.
RESET reloads the demo program.
What Is Not Complete Yet
The Pager is currently not a real PDP-11/70.
Not yet implemented:

full PDP-11 instruction set

real set of eight PDP-11 registers

PSW with complete flag logic

interrupts

real PDP-11 peripherals

persistent storage for user programs

complete separate 16-bit DATA LED row

Even so, it can already be used like a small historical front-panel computer: set bits, store words, inspect memory, start and stop programs, and execute instructions step by step.