Special variables: Difference between revisions

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.

<pre>Name Type Description

TIB integer Terminal Input Buffer address

U0 integer current user area address

>IN integer holds offset into TIB, used for parsing

BASE integer holds number conversion radix

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

DP integer holds dictionary memory pointer

'SOURCE double integer[2] contains length and address of input source

LATEST integer address of last word added to dictionary

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 dependent)

L0 integer bottom of leave stack

R0 integer end of return stack</pre>