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@@ -9,8 +9,8 @@ programming language. Implement a very basic evaluator for a small subset of For
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Your evaluator has to support the following words:
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- * `+`, `-`, `*`, `/` (integer arithmetic)
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- * `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)
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+* `+`, `-`, `*`, `/` (integer arithmetic)
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+* `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)
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Your evaluator also has to support defining new words using the customary syntax:
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@@ -25,3 +25,86 @@ a word is a sequence of one or more letters, digits, symbols or punctuation that
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(Forth probably uses slightly different rules, but this is close enough.)
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Words are case-insensitive.
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+
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+## Reference
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+
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+Forth runs on an imaginary machine having only a stack (virtually infinite) of values.
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+All instructions modify this stack in a way.
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+Some instructions require the stack to have at least a certain amount of values.
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+
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+### Numbers
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+
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+**Minimum required values on the stack: 0**
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+
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+Numbers just push the given value on the stack.
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+
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+```c
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+23 // Stack is [23]
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+42 // Stack is [42, 23]
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+1337 // Stack is [1337, 42, 23]
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+```
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+
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+### `+`, `-`, `*`, `/`
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+
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+**Minimum required values on the stack: 2**
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+
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+The arithmetic instructions all take the top 2 values from the stack, calculate the result,
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+and then put the result on the stack.
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+
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+Example:
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+
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+```c
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+23 // Stack is [23]
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+42 // Stack is [42, 23]
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++ // Stack is [65]
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+```
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+
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+For `-` and `/` the order is relevant.
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+
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+```c
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+23 // Stack is [23]
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+42 // Stack is [42, 23]
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+- // Stack is [19]
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+
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+4 // Stack is [4, 19]
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+64 // Stack is [64, 4, 19]
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+/ // Stack is [16, 19]
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+```
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+
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+### `DUP` - Duplicate the top value on the stack
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+
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+**Minimum required values on the stack: 1**
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+
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+```c
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+1337 // Stack is [1337]
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+DUP // Stack is [1337, 1337]
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+```
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+
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+### `DROP` - Remove the top value from the stack
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+
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+**Minimum required values on the stack: 1**
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+
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+```c
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+1337 // Stack is [1337]
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+DROP // Stack is []
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+```
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+
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+### `SWAP` - Swap the two top values on the stack
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+
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+**Minimum required values on the stack: 2**
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+
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+```c
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+23 // Stack is [23]
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+42 // Stack is [42, 23]
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+SWAP // Stack is [23, 42]
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+```
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+
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+### `OVER` - Duplicate the second to top value on the stack
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+
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+**Minimum required values on the stack: 2**
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+
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+```c
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+23 // Stack is [23]
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+42 // Stack is [42, 23]
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+OVER // Stack is [23, 42, 23]
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+```
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