__SHORT_SEG Segments

Variables allocated on the direct page (between 0 and 0xFF) are accessed using direct addressing mode. The Compiler allocates variables on the direct page if you define the variables with a __SHORT_SEG segment.

Listing: Example

#pragma DATA_SEG __SHORT_SEG myShortSegment
unsigned int myVar1, myVar2;

#pragma DATA_SEG DEFAULT

unsigned int myvar3, myVar4.

In the previous example, access both myVar1 and myVar2 using direct addressing mode. Access the variables myVar3 and myVar4 using extended addressing mode.

Defining some exported variables in a __SHORT_SEG segment requires that the external declaration for these variables also specify a __SHORT_SEG segment allocation.

Listing: External Definition of the Above Variable

#pragma DATA_SEG __SHORT_SEG myShortSegment
extern unsigned int myVar1, myVar2;

#pragma DATA_SEG DEFAULT

extern unsigned int myvar3, myVar4.

Place the segment on the direct page in the PRM file.

Listing: Example

LINK test.abs
NAMES test.o start08.o ansi.lib END

SECTIONS

    Z_RAM  = READ_WRITE  0x0080 TO 0x00FF;

    MY_RAM = READ_WRITE  0x0100 TO 0x01FF;

    MY_ROM = READ_ONLY   0xF000 TO 0xFEFF;

PLACEMENT

    DEFAULT_ROM                    INTO  MY_ROM;

    DEFAULT_RAM                    INTO  MY_RAM;

    _DATA_ZEROPAGE, myShortSegment INTO  Z_RAM;

END

STACKSIZE 0x60

VECTOR ADDRESS 0xFFFE _Startup /* set reset vector on _Startup */

Note: The linker is case-sensitive. The segment name must be identical in the C and PRM files.

If all data and stack fits into the zero page, use the tiny memory model for convenience.