WDR Instruction Set
The WDR paper computer, or Know-how Computer, is an educational model of a computer consisting only of a pen, a sheet of paper, and individual matches in the most simple case. The instruction set of five commands is small but Turing complete and therefore enough to represent all mathematical functions: incrementing ("inc") or decrementing ("dec") a register, unconditional jump ("jmp"), conditional jump ("skp", skips next instruction if a register is zero), and stopping program execution ("end").
| Opcode | Description |
|---|---|
| END | Aborts the execution of your program, so that you can examine the contents of your data registers. |
| SKP(r) | Checks if the data register r is zero. If it is zero, the program counter is increased by 2, otherwise the program counter is increased only by 1. |
| JMP(z) | Sets the program counter to line number z. |
| INC(r) | Increments the contents of the data register r and increases the program counter by 1. |
| DEC(r) | Decrements the contents of the data register r and increases the program counter by 1. |
To encode a WDR program into a 8-bits punched card, we could use 3 bits of space to encode the operation, which leaves 5 bits for the value. This computer's programs uses only 5 operations out of a possible 8, leaving 3 unused.
| Binary | Opcode | ||
|---|---|---|---|
| 0 | 0 | 0 | END |
| 0 | 0 | 1 | SKP |
| 0 | 1 | 0 | JMP |
| 0 | 1 | 1 | ADD |
| 1 | 0 | 0 | SUB |
The following program subtracts from R1 and adds to
R0 until the value of R1 is zero. The result of the
addition of R0 and R1 will be stored in
R0.
| Line | Opcode | Value | Opcode | Hex | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 00 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | 1 | JMP 03 | $43 |
| 01 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | ADD R0 | $60 |
| 02 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | SUB R1 | $82 |
| 03 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | SKP R1 | $22 |
| 04 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | JMP 01 | $41 |
| 05 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | END | $00 |
The binary expression of the operation and value of the previous program can be encoded horizontally as the following punched card:
v v v v v v • • • • • • • • • • •
Ref. 5-bits Table
The following table show the binary table for 32 addressable lines of a program.
| 00 | 00000 | 08 | 01000 | 10 | 100000 | 18 | 11000 |
| 01 | 00001 | 09 | 01001 | 11 | 100001 | 19 | 11001 |
| 02 | 00010 | 0A | 01010 | 12 | 100010 | 1A | 11010 |
| 03 | 00011 | 0B | 01011 | 13 | 100011 | 1B | 11011 |
| 04 | 00000 | 0C | 01100 | 14 | 100000 | 1C | 11100 |
| 05 | 00001 | 0D | 01101 | 15 | 100001 | 1D | 11101 |
| 06 | 00010 | 0E | 01110 | 16 | 100010 | 1E | 11110 |
| 07 | 00011 | 0F | 01111 | 17 | 100011 | 1F | 11111 |