 |
 |
|
 |
 |
ZBasic Language Reference
80
ZX Microcontroller Family
the variable’s value at that point and, if it is more efficient to do so, the compiler will generate code using
the deduced value instead of the variable’s value in memory. However, if a variable is an Alias or has an
Alias that refers to it, the compiler may not attempt to make this optimization.
In contrast, the compiler does not attempt to determine if a based variable might be occupying the same
space as a normal variable. Consequently, if a variable’s value is changed by assigning to a based
variable that occupies the same space, the compiler may generate code that is incorrect. To circumvent
this potential problem, you may instruct the compiler not to make any assumptions about the value of a
variable by using the Volatile attribute as in the following example.
Dim Volatile addr as Integer
Dim bv as Byte Based addr.DataAddress
When defining an array variable that is Based, you may omit the <dim-list> element. In this case, the
compiler will assume that the array is one-dimensional, that its lower bound is 1 and that its upper bound
is a large value.
Dim addr as UnsignedInteger
Dim ba() as Byte Based addr
It is also permissible to define a based Persistent Memory or Program Memory variable. This is
accomplished by using the normal syntax for defining a persistent/program memory variable and
appending the Based keyword and address expression.
Dim persVar as Persistent Integer Based &H300
Dim progVar as ProgMem Single Based &H1000
BasicX Compatibility Note
Based variables are not available in BasicX compatibility mode.
3.23 Based Procedures
Like based variables, based procedures are a powerful tool intended to be used by advanced
programmers who fully understand their nuances. No actual code space is consumed by a based
procedure. Rather, declaring a based procedure simply tells the compiler how to generate an invocation
of the procedure once its address is known.
The syntax for declaring a based subroutine or based function is shown below.
Delare Sub <name> ( [<parameter-list>] ) Based <addr-expr>
Declare Function <name> ( [<parameter-list>] ) As <type> Based <addr-expr>
As with based variables, the <addr-expr> giving the address of the procedure to be invoked must have
an integral type and can be either constant or non-constant (i.e., computed at run time). The based
procedure declaration may be placed inside a subroutine or function in which case the declaration is
private to that routine. At the module level, the Declare keyword may be proceeded by Public or
Private and if neither is specified, the declaration will be public by default.
When using a based procedure, you must be very careful to be certain that the declaration matches the
actual procedure that exists at the address that is specified. If the address given is not the beginning of a
procedure that is the same type and has the same number and type of parameters, the result is
unpredictable but will likely cause your program to malfunction.
|
 |
|