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Chapter 4. Assembler Directives |
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SEGMENT
The SEGMENT directive is used to declare a generic segment. A relocation type and an allocation type may be specified in the segment declaration. The SEGMENT directive is specified using the following format:
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segment SEGMENT |
class |
reloctype |
alloctype |
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where |
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segment |
is the symbol name to assign to the segment. This symbol |
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name is referred by the following RSEG directive. The |
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segment symbol name can be used also in expressions to |
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represent the base or start address of the combined segment |
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as calculated by the Linker/Locator. |
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class |
is the memory class to use for the specified segment. The |
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class specifies the memory space for the segment. See the |
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table below for more information. |
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reloctype |
is the relocation type for the segment. This determines |
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what relocation options may be performed by the |
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Linker/Locator. Refer to the table below for more |
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information. |
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alloctype |
is the allocation type for the segment. This determines what |
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relocation options may be performed by the Linker/Locator. |
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Refer to the table below for more information. |
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Shaded directives and options are available only in AX51 and A251.
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Class
The name of each segment within a module must be unique. However, the linker will combine segments having the same segment type. This applies to segments declared in other source modules as well.
The class specifies the memory class space for the segment. The A251 differentiates between basic classes and user-defined classes. The class is used by the linker/locator to access all the segments which belong to that class.
The basic classes are listed below:
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Basic Class |
Description |
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BIT |
BIT space (address 20H .. 2FH). |
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CODE |
CODE space |
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CONST |
CONST space; same as CODE but for constant only; access via MOVC. |
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DATA |
DATA space (address 0 to 7FH & SFR registers). |
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EBIT |
Extended 251 bit space (address 20H .. 7FH) |
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EDATA |
EDATA space |
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ECONST |
ECONST space; same as EDATA but for constants |
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IDATA |
IDATA space (address 0 to 0FFH). |
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ECODE |
Entire Intel/Atmel WM 251 and Philips 80C51MX address space for program |
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code. |
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HCONST |
Entire Intel/Atmel WM 251 and Philips 80C51MX address space for constants. |
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HDATA |
Entire Intel/Atmel WM 251 and Philips 80C51MX address space for data. |
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XDATA |
XDATA space; access via MOVX. |
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User-defined Class Names (AX51 & A251 only)
User-defined class names are composed of a basic class name and an extension and are enclosed in single quotes ('). They let you access the same address space as basic class names. The advantage is that you may declare several segments with a user-defined class and later use the linker to locate that class (and its segments) at a specific physical address. Refer to the “CLASSES” on page 336 for information on how to locate user defined classes.
Examples
seg1 SEGMENT 'NDATA_FLASH' seg2 SEGMENT 'HCONST_BITIMAGE' seg3 SEGMENT 'DATA1'
Shaded directives and options are available only in AX51 and A251.
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Chapter 4. Assembler Directives |
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Relocation Type
The optional relocation type defines the relocation operation that may be performed by the Linker/Locator. The following table lists the valid relocation types:
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Relocation Type |
Description |
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AT address |
Specifies an absolute segment. The segment will be placed at the |
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specified address. |
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BITADDRESSABLE |
Specifies a segment which will be located within the bit addressable |
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memory area (20H to 2FH in DATA space). BITADDRESSABLE is only |
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allowed for segments with the class DATA that do not exceed 16 bytes in |
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length. |
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INBLOCK |
Specifies a segment which must be contained in a 2048Byte block. This |
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relocation type is only valid for segments with the class CODE. |
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INPAGE |
Specifies a segment which must be contained in a 256Byte page. |
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OFFS offset |
Specifies an absolute segment. The segment is placed at the starting |
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address of the memory class plus the specified offset. The advantage |
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compared to the AT relocation type is that the start address can be |
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modified with the Lx51 linker/locater control CLASSES. Refer to the |
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“CLASSES” on page 336 for more information. |
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OVERLAYABLE |
Specifies that the segment can share memory with other segments. |
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Segments declared with this relocation type can be overlaid with other |
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segments which are also declared with the OVERLAYABLE relocation |
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type. When using this relocation type, the segment name must be |
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declared according to the C251, CX51, C51 or PL/M-51 segment naming |
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rules. Refer to the C Compiler User’s Guide for more information. |
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INSEG |
Specifies a segment which must be contained in a 64KByte segment. |
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Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Allocation Type
The optional allocation type defines the allocation operation that may be performed by the Linker/Locator. The following table lists the valid allocation types:
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Allocation Type |
Description |
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BIT |
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Specify bit alignment for the segment. Default for all segments with the |
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class BIT. |
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BYTE |
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Specify byte alignment for the segment. Default for all segments except of |
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BIT. |
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WORD |
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Specify word alignment for the segment. |
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DWORD |
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Specify dword alignment for the segment. |
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PAGE |
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Specify a segment whose starting address must be on a 256Byte page |
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boundary. |
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BLOCK |
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Specify a segment whose starting address must be on a 2048Byte block |
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boundary. |
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SEG |
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Specify a segment whose starting address must be on a 64KByte segment |
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boundary. |
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Examples for Segment Declarations |
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IDS |
SEGMENT |
IDATA |
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Defines a segment with the name IDS and the memory class IDATA.
MYSEG SEGMENT CODE AT 0FF2000H
Defines a segment with the name MYSEG and the memory class CODE to be located at address 0FF2000H.
HDSEG |
SEGMENT |
HDATA INSEG DWORD |
Defines a segment with the name HDSEG and the memory class HDATA. The segment is located within one 64KByte segment and is DWORD aligned.
XDSEG |
SEGMENT |
XDATA PAGE |
Defines a segment with the name XDSEG and the memory class XDATA. The segment is PAGE aligned, this means it starts on a 256Byte page.
Shaded directives and options are available only in AX51 and A251.