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Chapter 4. Assembler Directives |
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Reserving Memory
The memory reservation directives are used to reserve space in either word, dword, byte, or bit units. The space reserved starts at the point indicated by the current value of the location counter in the currently active segment.
DBIT
The DBIT directive reserves space in a bit or ebit segment. The DBIT directive has the following format:
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label: DBIT |
expression |
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where |
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label |
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is the symbol that is given the address of the reserved |
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memory. The label is a symbol of the type BIT and gets the |
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current address value and the memory class of the active |
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segment. The label can only be used where a symbol of this |
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type is allowed. |
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expression |
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is the number of bits to reserve. The expression cannot |
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contain forward references, relocatable symbols, or external |
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symbols. |
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The DBIT directive reserves space in the bit segment starting at the current |
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address. The location counter for the bit segment is increased by the value of the |
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expression. You should note that the location counter for the bit segment |
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references bits and not bytes. |
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NOTE |
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The Ax51 assembler is a two–pass assembler. Symbols are collected and the |
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length of each instruction is determined in the first pass. In the second pass, |
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forward references are resolved and object code is produced. For these |
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reasons, an expression used with the DBIT directive may not contain forward |
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references. |
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Example |
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ON_FLAG: |
DBIT |
1 |
; reserve 1 bit |
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OFF_FLAG: |
DBIT |
1 |
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Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
123 |
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DS
The DS directive reserves a specified number of bytes in a memory space. The DS directive has the following format:
label: DS |
expression |
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where |
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label |
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is the symbol that is given the address of the reserved |
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memory. The label is a typeless number and gets the current |
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address value and the memory class of the active segment. |
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The label can only be used where a symbol of this type is |
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allowed. |
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expression |
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is the number of bytes to reserve. The expression cannot |
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contain forward references, relocatable symbols, or external |
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symbols. |
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The DS directive reserves space in the current segment at the current address. |
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The current address is then increased by the value of the expression. The sum |
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of the location counter and the value of the specified expression should not |
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exceed the limitations of the current address space. |
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NOTE |
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The A251 assembler is a two–pass assembler. Symbols are collected and the |
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length of each instruction is determined in the first pass. In the second pass, |
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forward references are resolved and object code is produced. For these |
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reasons, an expression used with the DS directive may not contain forward |
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references. |
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Example |
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GAP: |
DS |
(($ + 16) AND 0FFF0H) – $ |
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DS |
20 |
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TIME: |
DS |
8 |
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Shaded directives and options are available only in AX51 and A251.
124 Chapter 4. Assembler Directives
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DSB |
(AX51 & A251 only) |
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The DSB directive reserves a specified number of bytes in a memory space. The |
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DSB directive has the following format: |
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label: |
DSB |
expression |
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where |
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label |
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is the symbol that is given the address of the reserved |
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memory. The label is a symbol of the type BYTE and gets |
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the current address value and the memory class of the active |
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segment. The label can only be used where a symbol of this |
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type is allowed. |
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expression |
is the number of bytes to reserve. The expression cannot |
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contain forward references, relocatable symbols, or external |
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symbols. |
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The DSB directive reserves space in the current segment at the current address. |
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The current address is then increased by the value of the expression. The sum of the location counter and the value of the specified expression should not exceed the limitations of the current address space.
NOTE
The Ax51 assembler is a two–pass assembler. Symbols are collected and the length of each instruction is determined in the first pass. In the second pass, forward references are resolved and object code is produced. For these reasons, an expression used with the DSB directive may not contain forward references.
Example
DAY: DSB 1
MONTH: DSB 1
HOUR: DSB 1
MIN: DSB 1
Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
125 |
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DSW (AX51 & A251 only)
The DSW directive reserves a specified number of words in a memory space. The DSW directive has the following format:
label: DSW |
expression |
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where |
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label |
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is the symbol that is given the address of the reserved |
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memory. The label is a symbol of the type WORD and gets |
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the current address value and the memory class of the active |
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segment. The label can only be used where a symbol of this |
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type is allowed. |
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expression |
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is the number of bytes to reserve. The expression cannot |
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contain forward references, relocatable symbols, or external |
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symbols. |
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The DSW directive reserves space in the current segment at the current address. |
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The current address is then increased by the value of the expression. The sum |
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of the location counter and the value of the specified expression should not |
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exceed the limitations of the current address space. |
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NOTE |
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The Ax51 assembler is a two–pass assembler. Symbols are collected and the |
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length of each instruction is determined in the first pass. In the second pass, |
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forward references are resolved and object code is produced. For these |
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reasons, an expression used with the DSW directive may not contain forward |
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references. |
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Example |
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YEAR: |
DSW |
1 |
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DAYinYEAR: |
DSW |
1 |
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Shaded directives and options are available only in AX51 and A251.