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Forth Programmer’s Handbook |
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TUCK |
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( x1 x2 — x2 x1 x2 ) |
Core Ext |
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Place a copy of the top stack item below the second stack item. |
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2.1.3 Memory Stack Operations |
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This category of operations allows you to reference memory by using |
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addresses that are on the stack. |
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Glossary |
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! |
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( x a-addr — ) |
Core |
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Store x at the cell at a-addr, removing both from the stack. “store” |
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+! |
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( n a-addr — ) |
Core |
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Add n to the contents of the cell at a-addr and store the result in the cell at a-addr, |
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removing both from the stack. “plus-store” |
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2! |
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( x1 x2 a-addr — ) |
Core |
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Store the cell pair x1 x2 into the two cells beginning at a-addr, removing three |
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cells from the stack. “two-store” |
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2@ |
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( a-addr — x1 x2 ) |
Core |
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Push the cell pair x1 x2 at a-addr onto the top of the stack. “two-fetch” |
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@ |
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( a-addr — x ) |
Core |
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Replace a-addr with the contents of the cell at a-addr. “fetch” |
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C! |
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( b c-addr — ) |
Core |
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Store the low-order byte of the second stack item at c-addr, removing both from |
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the stack. “C-store” |
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C+! |
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( b c-addr — ) |
common usage |
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Add the low-order byte of the second stack item to the byte at c-addr, removing |
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both from the stack. “C-plus store” |
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C@ |
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( c-addr — b ) |
Core |
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Replace c-addr with the contents of the byte at c-addr. The byte fetched is |
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stored in the low-order byte of the top stack item, with the remaining bits |
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cleared to zero. “C-fetch” |
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Forth Fundamentals 35
Forth Programmer’s Handbook
2.1.4 Return Stack Manipulation Operations
The return stack is so named because it is used by the Forth virtual machine (VM) to keep track of where Forth words will return when they have finished executing. When a high-level Forth word invokes a previously defined Forth word, the address of the next word to be executed is pushed onto the return stack; it will be popped off the return stack when the called word is finished, so execution can resume where it left off.
The return stack is a convenient place to keep frequently used values (by using the words >R, R@, and R>), but it must be cleared before an executing word reaches the end of the current definition, or the virtual machine will return to the “address” on the return stack. This behavior can be useful; for example, on many systems, the definition:
: VECTOR ( xt -- ) >R ;
will act like the word EXECUTE, but will only execute : definitions. VECTOR works by pushing a word’s execution token onto the return stack. Therefore, when the end of the definition pops the return stack into the VM’s register I (or its implementation-dependent equivalent), the VM will begin to execute the word whose address was on the stack for VECTOR. This works on all systems in which xts are actual return addresses.
If you use the return stack for temporary storage, you must be aware that this is also a system resource, and obey the following restrictions:
!Your program must not access values on the return stack (using R@, R>, 2R@, or 2R>) that it did not place there using >R or 2>R.
!When inside a DO loop, your program must not access values that were placed on the return stack before the loop was entered.
!All values placed on the return stack within a DO loop must be removed before
I, J, LOOP, +LOOP, UNLOOP, or LEAVE is executed.
!All values placed on the return stack within a definition must be removed before the end of the definition or before EXIT is executed.
The glossary below documents operations that involve both the return stack and the data stack.
36 Forth Fundamentals
Glossary |
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2>R |
( x1 x2 — ) ( R: — x1 x2 ) |
Core Ext |
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Pop the top two cells from the data stack and push them onto the return stack. |
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“two-to-R” |
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2R> |
( — x1 x2 ) ( R: x1 x2 — ) |
Core Ext |
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Pop the top two cells from the return stack and push them onto the data stack. |
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2R> is the inverse of 2>R. “two-R-from” |
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2R@ |
( — x1 x2 ) ( R: x1 x2 — x1 x2 ) |
Core Ext |
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Push a copy of the top two return stack cells onto the data stack. “two-R-fetch” |
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>R |
( x — ) ( R: — x ) |
Core |
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Remove the item on top of the data stack and put it onto the return stack. “to-R” |
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R> |
( — x ) ( R: x — ) |
Core |
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Remove the item on the top of the return stack and put it onto the data stack. |
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“R-from” |
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R@ |
( — x ) ( R: x — x ) |
Core |
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Place a copy of the item on top of the return stack onto the data stack. “R-fetch” |
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Counting LOOPs (DO), Section 2.5.2 |
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References |
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EXECUTE, Section 2.5.6 |
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2.1.5 Programmer Conveniences
The words in this section are intended as programming aids. They may be used interpretively at the keyboard, or inside definitions—except for ' (tick); the equivalent of ' inside a definition is the word [']. Because compiling new definitions in Forth is so quick, you are encouraged to create, test, and debug definitions to aid in developing an application.
Forth Fundamentals 37
Forth Programmer’s Handbook
Glossary |
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' <name> |
( — xt ) |
Core |
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Search dictionary for name and leave its execution token on the stack. Abort if |
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name cannot be found. “tick” |
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.S |
( — ) |
Tools |
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Display the contents of the data stack using the current base. Stack contents |
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remain unchanged. “dot-S” |
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? |
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( a-addr — ) |
Tools |
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Fetch the contents of the given address and display the result according to the |
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current conversion radix. “question” |
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Equivalent to the phrase: @ . |
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DUMP |
( addr +n — ) |
Tools |
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Display the contents of a memory region of length +n starting at addr: |
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<addr> <+n> DUMP
Output is formatted with the address on the left and up to eight values on a line. The output conversion radix is the current value of BASE (on some systems this word always outputs in hex). Two cells are removed from the stack.
ENVIRONMENT? |
( c-addr u — false | i*x true ) |
Core |
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This word is used to inquire about the values of system parameters and the |
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existence of options. See Section 3.2 for a full description. “environment-query” |
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WORDS |
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( — ) |
Tools |
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List all the definition names in the first word list of the search order. |
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['], Section 4.1.3 |
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References |
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Environmental interrogation, Section 3.2 |
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Search orders, Section 4.6.1 |
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38 Forth Fundamentals