UM0144 datasheet(16/92 Pages) STMICROELECTRONICS

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3 ST7 Assembler

ST7 Assembler Linker

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UM0144

Label scope

Often, in multi-module programs, a piece of code will need to refer to a label that is actually

defined in another module. To do this, the module that exports the label must declare it

PUBLIC, and the module which imports the label must declare it EXTERN. The two directives

EXTERN and PUBLIC go together as a pair.

Most labels in a program will be of no interest for other pieces of the program—these are

known as 'internal' labels since they are only used in the module where they are defined.

Labels are 'internal' by default.

Here are two incomplete example modules that pass labels between them:

As you can see, module 1 refers to the '_sig2' subroutine which is defined in module 2. Note

that when module 1 refers to the '_sig2' label in an EXTERN directive it specifies a WORD size

with the '.w' suffix. Because the assembler cannot look up the definition of '_sig2' it has to be

told its address size explicitly. It doesn't need to be told relativity: all external labels are

assumed to be relative.

Absolute labels declared between modules should be defined in an INCLUDE file that is called

by all modules in the program; this idea of using INCLUDE files is very important since it can

reduce the number of PUBLIC symbols—and therefore the link time—significantly.

Lines in the source code listing which refer to external labels are marked with an X and given

'empty' values for the linker to fill.

module 1

EXTERN _sig1.w

; import _sig1

EXTERN _sig2.w

; import _sig2

PUBLIC _handlers

; export _handlers

segment byte ‘P’

_handlers:

; define _handlers

jp _sig1

; refer to _sig1

jp _sig2

; refer to _sig2

end

module 2

EXTERN _handlers.w

; import _handlers (addr. is a word)

PUBLIC _sig2

; export _sig2

segment byte ‘P’

_sig2:

; define _sig2

...

call _handlers

; refer to _handlers

...

ret

end

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