2021-01-21 13:40:28 +01:00
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</head><body>
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2022-06-03 12:41:52 +02:00
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<p><br/><br/>
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2021-01-21 13:40:28 +01:00
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The stacks are a defining feature of Forth. They are are used
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to pass data between words and to track return addresses for
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function calls.
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<br/><br/>
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RETRO always has two stacks, and optionally (if built with
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floating point support) a third.
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<br/><br/>
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<br/><br/>
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This is the primary stack. Values are placed here, passed to
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words which consume them and then return results. When I
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refer to "the stack", this is the one I mean. Learning to use
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the stack is a crucial part to making effective use of RETRO.
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<br/><br/>
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<br/><br/>
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Values can be placed on the stack directly.
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<br/><br/>
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<tt class='indentedcode'>| Example | Action |</tt>
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<tt class='indentedcode'>| -------------- | ---------------------------------------- |</tt>
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<tt class='indentedcode'>| `#300123` | Push the number `300123` to the stack |</tt>
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<tt class='indentedcode'>| `$h` | Push the ASCII code for `h` to the stack |</tt>
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<tt class='indentedcode'>| `'hello_world` | Push a pointer to a string to the stack |</tt>
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<tt class='indentedcode'>| `&fetch` | Push the address of `fetch` to the stack |</tt>
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<br/><br/>
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<br/><br/>
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RETRO provides a number of <strong>shufflers</strong> for reordering items
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on the stack.
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<br/><br/>
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Some of the most common ones are:
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<br/><br/>
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<tt class='indentedcode'>| Word | Before | After |</tt>
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<tt class='indentedcode'>| ------- |--------- | -------- |</tt>
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<tt class='indentedcode'>| dup | #1 | #1 #1 |</tt>
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<tt class='indentedcode'>| drop | #1 #2 | #1 |</tt>
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<tt class='indentedcode'>| swap | #1 #2 | #2 #1 |</tt>
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<tt class='indentedcode'>| over | #1 #2 | #1 #2 #1 |</tt>
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<tt class='indentedcode'>| tuck | #1 #2 | #2 #1 #2 |</tt>
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<tt class='indentedcode'>| nip | #1 #2 | #2 |</tt>
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<tt class='indentedcode'>| rot | #1 #2 #3 | #3 #1 #2 |</tt>
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<br/><br/>
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You can use <span class="tt">push</span> and <span class="tt">pop</span> to move values to and from the
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address stack. Make sure you <span class="tt">pop</span> them back before the word
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ends or RETRO will crash. These two words can not be used
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at the interpreter.
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<br/><br/>
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There is also a special one, <span class="tt">reorder</span>, which allows for big
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stack restructuring. This is slow but can be very useful.
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<br/><br/>
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As an example, let's say we have four values:
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<br/><br/>
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<tt class='indentedcode'>#1 #2 #3 #4</tt>
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<br/><br/>
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And we want them to become:
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<br/><br/>
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<tt class='indentedcode'>#4 #3 #2 #1</tt>
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<br/><br/>
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Doing this with the basic shufflers is difficult. You could end
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up with something similar to:
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<br/><br/>
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<tt class='indentedcode'>swap rot push rot pop swap </tt>
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<br/><br/>
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But with <span class="tt">reorder</span>, you can just express the before and after
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states:
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<br/><br/>
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<tt class='indentedcode'>'abcd 'dcba reorder</tt>
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<br/><br/>
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<br/><br/>
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If you need to quickly empty the stack, use <span class="tt">reset</span>.
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<br/><br/>
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<br/><br/>
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To find out how many items are on the stack, use <span class="tt">depth</span>.
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<br/><br/>
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<br/><br/>
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You can display the stack by running <span class="tt">dump-stack</span>.
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<br/><br/>
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<br/><br/>
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RETRO provides <strong>combinators</strong> for working with data order on
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the stack. These are covered in a later chapter and are worth
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learning to use as they can help provide a cleaner, more
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structured means of working.
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<br/><br/>
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<span class="h3">Tips</span>
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<br/><br/>
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The stack is <strong>not</strong> an array in addressable memory. Don't try
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to treat it like one.
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<br/><br/>
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<br/><br/>
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This stack primarily holds return addresses for function calls.
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You normally won't need to directly interact with this stack,
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but you can use <span class="tt">push</span> and <span class="tt">pop</span> to move values between the
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data stack and this.
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<br/><br/>
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<br/><br/>
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If you are using a build with floating point support a third
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stack will be present. Floating point values are kept and
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passed between words using this.
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<br/><br/>
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See the Floating Point chapter for more details on this.
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<br/><br/>
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<span class="h2">Tips</span>
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<br/><br/>
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I recommend keeping the data stack shallow. Don't try to juggle
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too much; it's better to factor definitions into shorter ones
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that deal with simpler parts of the stack values than to have
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a big definition with a lot of complex shuffling.
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<br/><br/>
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<span class="h2">Notes</span>
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<br/><br/>
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The standard system is configured with a very deep data stack
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(around 2,000 items) and an address stack that is 3x deeper.
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In actual use, your programs are unlikely to ever need this,
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but if you do, keep the limits in mind.
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</p>
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</body></html>
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