ST20-C1 datasheet(49/205 Pages) STMICROELECTRONICS

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4 Using ST20-C1 instructions

A function called using

fcall must have a fixed offset at compile time. jab is used for

calling functions and procedures at dynamically calculated addresses, for example

when using function pointers.

The

jab instruction is also used to perform the return. The return address must have

been restored with a

ldl from the stack into the Areg. A procedure or function that

requires local work space will normally include

ajw instructions to allocate and deallo-

cate space.

When the

jab instruction is executed, the programmer must ensure that:

• the Areg holds the return address;

• any workspace claimed by the procedure should have been released so that

the Wptr has returned to the value it held at the start of the procedure.

The

jab instruction uses one word of the evaluation stack, so the other two words can

therefore be used to return up to two values to the calling code, including a pointer to

a block of additional data to be returned.

4.10.1 Adjusting work space

The primary instruction

ajw is used to perform a relative adjustment to the stack

pointer Wptr to:

• create work space on the stack at the beginning of the function and

• return the work space pointer at the end of the function.

ajw n increases the value of the workspace pointer by the number of words in its

operand value,

n. Work space is created at the beginning of a function or procedure

with a negative operand and released before returning with a positive operand.

The amount of extra work space needed will normally include:

• space to save any parameters passed in the evaluation stack;

• space for local variables and temporaries;

• space for any hidden system variables such as the static chain.

For example, a function with

w words of local work space might be:

T myfunction (param_1, param_2)

{

local variable declarations;

return (E);

}

This can be compiled as:

ajw -w;

stl param_1; stl param_2;

ajw w;

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