ST20-C1 datasheet(50/205 Pages) STMICROELECTRONICS

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4.10 Function and procedure calls

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ldl 0;

jab;

ST20-C1 processors may have a workspace cache which holds a copy of a few words

at the bottom of the work space. This cache is transparent to the programmer but may

substantially improve performance. It is refilled whene ver the Wptr is adjusted, so the

ajw instruction should not be used excessively.

4.10.2 Parameters

It is convenient to load the first three parameters of the procedure or function into the

evaluation stack registers, and to arrange the work space of the calling code so that

the additional parameters can be stored in locations 1, 2, ... of the work space

before the procedure is called. Location zero of the work space is used for the return

address. This is illustrated in Figure 4.2, which shows a possible work space layout for

a function or procedure with six parameters and four local variables.

Figure 4.2 Example function or procedure workspace

To enable the procedure to access non-local variables the parameters of a procedure

may include a link to the environment in which the procedure was declared.

4.10.3 Returning results

Up to two results of size less than or equal to the word length of the processor can be

returned from a function in the evaluation stack — the

jab instruction uses the third

passing into the function the addresses of locations to store these results as extra

parameters.

Wptr in calling

code

Return Iptr

param 3

param 2

param 4

param 5

param 6

param 1

variable 1

variable 2

variable 3

variable 4

Wptr in function

or procedure

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