Special variables: Difference between revisions

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===Common Practice===

Although not mandated in the Forth language specification, traditionally the language implements system variables as what are called USER variables. The name dates back to the a time when FORTH was used as a multi-user O/S and therefore each user needed a set of variables to control the state of their instance of the system. The user variables exist in a memory block called the USER AREA and are replicated for each task. On a context switch the system assigns a system VARIABLE called 'UP' (user pointer) to point to the new USER AREA. UP is commonly held in a CPU register on machines that have larger register sets.

Although not mandated in the Forth language specification, traditionally the language implements system variables as what are called USER variables. The name dates back to the a time when FORTH was used as a multi-user O/S and therefore each user needed a set of variables to control the state of their instance of the system. The user variables exist in a memory block called the USER AREA and are replicated for each task. When used for embedded systems, Forth is still commonly implemented as a multi-tasking system, so this architecture is still relevant today. On a context switch the system assigns a system VARIABLE called 'UP' (user pointer) to point to the new USER AREA. UP is commonly held in a CPU register on machines that have larger register sets.

This following list is ~~typical~~ of ~~the~~ set of USER variables in a small system. Consult the implementation documents for details on the USER variables in a specific FORTH system.

This following list is an example of a set of USER variables in a small system. Consult the implementation documents for details on the USER variables in a specific FORTH system.

<pre>Name Type Description

---------------------------------------------

TIB integer Terminal Input Buffer address

U0 integer current user area address

>IN integer holds offset into TIB

>IN integer holds offset into TIB, used for parsing

BASE integer holds conversion radix

BASE integer holds number conversion radix

STATE integer holds compiler state (true=compiling, false=interpreting)

DP integer holds dictionary pointer

DP integer holds dictionary memory pointer

'SOURCE double contains length and address of input source

LATEST integer last word in dictionary

LATEST integer address of last word added to dictionary

HP integer HOLD pointer, used for number formatting

HP integer HOLD pointer, used for number formatting routines

LP integer leave-stack pointer, used by do loops

S0 integer end of parameter stack

PAD chars[80] Generic buffer. (size is implementation ~~dependant~~)

PAD chars[80] Generic buffer. (size is implementation dependent)

L0 integer bottom of leave stack

R0 integer end of return stack</pre>