ChibiAkumas ASM Programming Forum

Posted: **Sat Mar 28, 2020 3:51 pm**

I just enjoyed watching the very first tutorial in the PDP-11 series.
Excellent video

It is especially touching that people got interested in the series

and I'm looking forward to the new tutorials as well.

I'm anxious to add that in the Soviet Union, the PDP-11 compatible systems and microprocessors were very widely adopted.
The USSR produced many more of such computers than DEC itself.
Here is a nice list of such computers produced behind the iron curtain (which does not include military equipment and spacecrafts).
And of course, the very first "official" Soviet home computer - BK-0010 was based on a PDP-11 compatible microprocessor

Posted: **Sat Mar 28, 2020 4:26 pm**

About the octal numbers. Sure, they look weird because "modern" bytes consists of 8-bits, and general microprocessors have 16, 32, or 64-bit words.
But for example, the very notable predecessor of the PDP-11, PDP-8, has 12-bits words.
As for the PDP-11, octal numbers pretty practical for a hacker.

I'll try to demonstrate how nicely machine codes can be split into octets, but first, it is worth to mention that PDP-11 has orthogonal instruction set

Base instruction set nicely falls into two patterns, here is a bitwise representation:
instructions with one operand:

Code: Select all

|15 |           |           |         6 | 5       3 | 2       0 |
|   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
|                                       |    MODE   |     Rn    |
|                                       |  **   | * |    ***    |
| OP CODE                               | DESTINATION FIELD     |
* Specifies direct or indirect address
** Specifies how register will be used
*** Specifies one of 8 general perpose registers

and instruction with two operands:

Code: Select all

| 15| 14      12| 11      9 | 8       6 | 5       3 | 2       0 |
| B |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
|               |    MODE   |     Rn    |    MODE   |     Rn    |
|               |  **   | * |    ***    |  **   | * |    ***    |
| OP CODE       | SOURCE FIELD          | DESTINATION FIELD     |
* Direct/deferred bit for source and destination address
** Specifies how selected registers are to be used
*** Specifies a general register

And to complete the picture, there is 8 addressing modes:

Code: Select all

| Code | Name                   | Example    | Description                                                    |
|------+------------------------+------------+----------------------------------------------------------------|
| 0n   | Register               | OPR Rn     | The operand is in Rn                                           |
| 1n   | Register deferred      | OPR (Rn)   | Rn contains the address of the operand                         |
| 2n   | Autoincrement          | OPR (Rn)+  | Rn contains the address of the operand, then increment Rn by 2 |
| 3n   | Autoincrement deferred | OPR @(Rn)+ | Rn contains the address of the operand, then increment Rn by 2 |
| 4n   | Autodecrement          | OPR -(Rn)  | Decrement Rn, then use it as the address                       |
| 5n   | Autodecrement deferred | OPR @-(Rn) | Decrement Rn by 2, then use it as the address of the address   |
| 6n   | Index                  | OPR X(Rn)  | Rn+X is the address of the operand                             |
| 7n   | Index deferred         | OPR @X(Rn) | Rn+X is the address of the address                             |

And there is 8 registers: R0-R7

Now, lets translate a command MOVB (R5)+,R0 into machine code:

Code: Select all

1 - byte operation
1 - MOV op code
2 - autoincrement addressing mode
5 - R5
0 - register addressing mode
0 - R0

MOVB (R5)+,R0 translates to 112500

It's not that hard to learn to read and write machine codes when you use octal numbers

People who programmed on assembler back in the days, still can write straight in machine codes.

Posted: **Sun Mar 29, 2020 11:09 pm**

Thanks for providing this excellent description, it's far better than I could have done

Posted: **Mon Mar 30, 2020 9:22 pm**

I had those figures in my notes for a while, and yeah, they are pretty concise. I believe that I took them from one of the DEC manuals, but I don't remember from which one.

I'll be happy to help if you need some information to continue the tutorials.

Recently I added a keyboard scan-codes table, with some notes explaining Russian markings.
And I'll continue to translate the technical description of the machine.

Code: Select all

 | code |   key   | note     | code |  key  |  note     |
 |   05 | ,       | NumPad   | 0106 | АЛФ   | Alphabet  |
 |   06 | АР2     | Esc      | 0107 | ФИКС  | Lock      |
 |   07 | ; / +   |          | 0110 | Ч / ^ |           |
 |  010 | К1 / К6 | F1 / F6  | 0111 | С / S |           |
 |  011 | К2 / К7 | F2 / F7  | 0112 | М / M |           |
 |  012 | КЗ / К8 | F3 / F8  | 0113 | SPACE |           |
 |  013 | 4 / ¤   |          | 0114 | Т / T |           |
 |  014 | К4 / К9 | F4 / F9  | 0115 | Ь / X |           |
 |  015 | К5 / К10| F5 / F10 | 0116 | ←     |           |
 |  016 | 7 / '   |          | 0117 | , / < |           |
 |  017 | 8 / (   |          | 0125 | 7     | NumPad    |
 |  025 | -       | NumPad   | 0126 | 0     | NumPad    |
 |  026 | ТАБ     | Tab      | 0127 | 1     | NumPad    |
 |  027 | Й / J   |          | 0130 | 4     | NumPad    |
 |  028 | 1 / !   |          | 0131 | +     | NumPad    |
 |  031 | 2 / "   |          | 0132 | ЗБ    | Backspace |
 |  032 | 3 / #   |          | 0133 | →     |           |
 |  033 | Е / E   |          | 0134 | ↓     |           |
 |  034 | 5 / %   |          | 0135 | . / > |           |
 |  035 | 6 / &   |          | 0136 | Э / \ |           |
 |  036 | Ш / [   |          | 0137 | Ж / V |           |
 |  037 | Щ / ]   |          | 0145 | 8     | NumPad    |
 |  046 | УПР     | Ctrl     | 0146 | .     | NumPad    |
 |  047 | Ф / F   |          | 0147 | 2     | NumPad    |
 |  050 | Ц / C   |          | 0150 | 5     | NumPad    |
 |  051 | У / U   |          | 0151 | ИСП   | Execute   |
 |  052 | К / K   |          | 0152 | УСТ   | Settings  |
 |  053 | П / P   |          | 0153 | ВВОД  | Enter     |
 |  054 | H / N   |          | 0154 | ↑     |           |
 |  055 | Г / G   |          | 0155 | : / * |           |
 |  056 | Л / L   |          | 0156 | Х / H |           |
 |  057 | Д / D   |          | 0157 | З / Z |           |
 |  066 | ГРАФ    | Graph    | 0165 | 9     | NumPad    |
 |  067 | Я / Q   |          | 0166 | ВВОД  | NumPad    |
 |  070 | Ы / Y   |          | 0167 | 3     | NumPad    |
 |  071 | В / W   |          | 0170 | 7     | NumPad    |
 |  072 | А / A   |          | 0171 | СБРОС | Reset     |
 |  073 | И / I   |          | 0172 | ПОМ   | Help      |
 |  074 | Р / R   |          | 0173 | / / ? |           |
 |  075 | О / O   |          | 0174 | Ъ / } |           |
 |  076 | Б / B   |          | 0175 | - / = |           |
 |  077 | Ю / @   |          | 0176 | О / } |           |
 | 0105 | HP      | Shift    | 0177 | 9 / ) |           |

Posted: **Fri Sep 25, 2020 4:47 am**

Emulator+assembler

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