During elaboration or simulation sometimes a part of a composite
signal has to be selected respectively addressed for a specific
action. While code created by the code generators can do this directly
via the operators provided by the composite type classes the kernel
cannot use type classes because they are unknown at kernel compile
time. Hence, a common method to address a arbitrary part of a
composite object is needed which can be evaluated by the simulation
kernel. In the FreeHDL system this task is done by the acl
(access code list) class.
A acl instance is simply a array of int values which describe
the path beginning at a composite object down to the part of the
object which is addressed by the acl instance. Each acl
instances has a static size which might differ between different
acl instances. Further, in the head of the list some
informations to maintain the instance are stored.
Because creating and removing acl instances might occur very
often during simulation, currently unused acl instances are not
really removed from the system but stored into a ``free acl''
list. Consecutive create operations receive new acl instances
from this list.
Example:
TYPE myrec IS RECORD a : INTEGER; b : BIT; END RECORD; TYPE myarray IS ARRAY (0 TO 10) OF myrec; ... VARIABLE myvar : myarray;If we want to address
b of the 4-th element of array
myvar then the corresponding acl is constructed as
follows:
acl *myacl = new(2) acl; *myacl << 4 << 1;Note,
new(2) acl creates a new empty acl instance which
can store up to 2 int values. The operators << is used to store
a int value (or a whole acl list) at the current end of
a acl instance. Note further that acl instances can only
be created via a new operator because its actual size has to be passed
over at runtime!