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4 Using ST20-C1 instructions

A C function is used for purposes of illustration. For simplicity, it is assumed that the

single result can be returned in the evaluation stack:

ajw -local_variables-1;

ldl local_variables;

ajw local_variables+1;

jab;

One of the loading sequences described earlier may be required if the expressions

returned in the registers contain evaluations.

4.10.4 Calling a function

The first three par ameters should be loaded into the evaluation stack before the

fcall

instruction. These parameters can be stored as local variables after the workspace

pointer has been moved down, to make the best use of the work space cache. The

remainder of the parameters passed should be loaded into the work space before

fcall

is executed.

When the function returns, the results whose addresses were passed will already

have been stored so all that remains is to store up to 2 results returned in the evalua-

tion stack.

For example the function call

V = F(E1, .. ., En)

could be compiled by

E3; stl 0; . . . ; En; stl (n-3);

E2;E1; static_link; fcall F;

stl V;

The compiler must have already allocated sufficient workspace for the parameters that

are stacked explicitly.

Single result functions

In most programming languages, a function that returns a single result can be used in

an expression as well as in an assignment.

A common form of function returns a single value contained in a word — the

mechanism described above will return this in Areg. When compiling expressions,

(using the algorithm described in section C.3 in Appendix C) the depth of such a

function call should be taken as being infinite — i.e. deeper than any other form of

expression. This is because the function call will always lose any other information in

the registers. By giving it infinite depth the expression compilation algorithm will never

call a function while another expression result is being held in a register.

ST20-C1 데이터시트(PDF) 51 Page - STMicroelectronics