Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Symbol Definition
The symbol definition directives allow you to create symbols that can be used to represent registers, numbers, and addresses.
Symbols defined by these directives may not have been previously defined and may not be redefined by any means. The SET directive is the only exception to this.
EQU, SET |
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The EQU and SET directive assigns a numeric value or register symbol to the |
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specified symbol name. Symbols defined with EQU may not have been |
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previously defined and may not be redefined by any means. The SET directive |
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allows later redefinition of symbols. Statements involving the EQU or SET |
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directive are formatted as follows: |
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symbol |
EQU |
expression |
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symbol |
EQU |
register |
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symbol |
SET |
expression |
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symbol |
SET |
register |
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where |
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symbol |
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is the name of the symbol to define. The expression or |
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register specified in the EQU or SET directive will be |
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substituted for each occurrence of symbol that is used in |
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your assembly program. |
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expression |
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is a numeric expression which contains no forward |
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references, or a simple relocatable expression. |
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register |
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is one of the following register names: A, R0, R1, R2, R3, |
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R4, R5, R6, or R7. |
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Symbols defined with the EQU or SET directive may be used anywhere in operands, expressions, or addresses. Symbols that are defined as a register name can be used anywhere a register is allowed. A251 replaces each occurrence of the defined symbol in your assembly program with the specified numeric value or register symbol.
Shaded directives and options are available only in AX51 and A251.
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Chapter 4. Assembler Directives |
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Symbols defined with the EQU directive may not be changed or redefined. You cannot use the SET directive if a symbol was previously defined with EQU and you cannot use the EQU directive if a symbol which was defined with SET.
Examples
LIMIT |
EQU |
1200 |
VALUE |
EQU |
LIMIT – 200 + 'A' |
SERIAL |
EQU |
SBUF |
ACCU |
EQU |
A |
COUNT |
EQU |
R5 |
VALUE |
SET |
100 |
VALUE |
SET |
VALUE / 2 |
COUNTER |
SET |
R1 |
TEMP |
SET |
COUNTER |
TEMP |
SET |
VALUE * VALUE |
CODE, DATA, IDATA, XDATA
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The BIT, CODE, DATA, IDATA, and XDATA directives assigns an address |
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value to the specified symbol. Symbols defined with the BIT, CODE, DATA, |
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IDATA, and XDATA directives may not be changed or redefined. The format |
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of theses directives is: |
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symbol |
BIT |
bit_address |
defines a BIT symbol |
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symbol |
CODE |
code_address |
defines a CODE symbol |
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symbol |
DATA |
data_address |
defines a DATA symbol |
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symbol |
IDATA |
idata_address |
defines an IDATA symbol |
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symbol |
XDATA |
xdata_address |
defines a XDATA symbol |
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where |
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symbol |
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is the name of the symbol to define. The symbol name can |
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be used anywhere an address of this memory class is valid. |
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bit_address
code_address data_address
is the address of a bit in internal data memory in the area 20H .. 2FH or a bit address of an 8051 bit-addressable SFR.
is a code address in the range 0000H .. 0FFFFH.
is a data memory address in the range 0 to 127 or a special function register (SFR) address in the range 128 .. 255.
idata_address xdata_address
is an idata memory address in the range 0 to 255.
is an xdata memory address in the range 0 to 65535.
Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Example
DATA_SEG |
SEGMENT BITADDRESSABLE |
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RSEG |
DATA_SEG |
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; a bit-addressable rel_seg |
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CTRL: |
DS |
1 |
; a 1–byte variable (CTRL) |
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ALARM |
BIT |
CTRL.0 |
; bit in a relocatable byte |
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SHUT |
BIT |
ALARM+1 |
; the next bit |
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ENABLE_FLAG |
BIT |
60H |
; an absolute bit |
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DONE_FLAG |
BIT |
24H.2 |
; an absolute bit |
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P1_BIT2 |
EQU |
90H.2 |
; a SFR bit |
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RESTART |
CODE |
00H |
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INTVEC_0 |
CODE |
RESTART + 3 |
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INTVEC_1 |
CODE |
RESTART + 0BH |
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INTVEC_2 |
CODE |
RESTART + 1BH |
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SERBUF |
DATA |
SBUF |
; redfinition of SBUF |
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RESULT |
DATA |
40H |
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RESULT2 |
DATA |
RESULT + 2 |
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PORT1 |
DATA |
90H |
; a SFR symbol |
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BUFFER |
IDATA |
60H |
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BUF_LEN |
EQU |
20H |
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BUF_END |
IDATA |
BUFFER + BUF_LEN – 1 |
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XSEG1 |
SEGMENT XDATA |
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RSEG |
XSEG1 |
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4 |
DTIM: |
DS |
6 |
;reserve 6–bytes for DTIM |
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TIME |
XDATA |
DTIM + 0 |
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DATE |
XDATA |
DTIM + 3 |
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Shaded directives and options are available only in AX51 and A251.
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Chapter 4. Assembler Directives |
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esfr, sfr, sfr16, sbit
The sfr, sfr16 and sbit directives are fully compatible to the Cx51 compiler and allows you to use a generic SFR register definition file for both: the Ax51 macro assembler and the Cx51 compiler. The esfr directive defines symbols in the extended SFR space of the Philips 80C51MX architecture. This directive is only available in the AX51 macro assembler. These directives have the following format:
sfr |
sfr_symbol |
= address; |
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esfr |
sfr_symbol |
= address; |
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sfr16 |
sfr_symbol |
= address; |
; ignored by Ax51 |
sbit |
sfr_symbol |
= bit-address; |
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where
sfr_symbol is the name of the special function register (SFR) symbol to define.
4 address bit-address
is an SFR address in the range 0x80 – 0xFF.
is address of an SFR bit in the format address ^ bitpos or sfr_symbol ^ bitpos. address or sfr_symbol refers to an bitaddressable SFR and bitpos specifies the bit position of the SFR bit in the range 0 – 7.
Symbols defined with the esfr, sfr, or sbit directive may be used anywhere as address of a SFR or SFR bit.
Example
sfr |
P0 |
= 0x80; |
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sfr |
P1 |
= 0x90; |
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sbit |
P0_0 |
= P0^0; |
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sbit |
P1_1 |
= 0x90^1; |
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esfr |
MXCON |
= 0xFF; |
/* extended Philips 80C51MX SFR */ |
sfr16 |
T2 |
= 0xCC; |
/* ignored by Ax51 */ |
NOTE
The Ax51 assembler ignores symbol definitions that start with sfr16. This is implemented for compatibility to the Cx51 compiler.
Shaded directives and options are available only in AX51 and A251.