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Chapter 3. Writing Assembly Programs |
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Operators
The Ax51 assembler provides several classes of operators that allow you to compare and combine operands and expressions. These operators are described in the sections that follow.
Arithmetic Operators
Arithmetic operators perform arithmetic functions like addition, subtraction, multiplication, and division. These operators require one or two operands depending on the operation. The result is always a 16-bit value. Overflow and
3 underflow conditions are not detected. Division by zero is detected and causes an assembler error.
Operator |
Syntax |
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Description |
+ |
+ expression |
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Unary plus sign |
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– |
– expression |
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Unary minus sign |
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+ |
expression |
+ |
expression |
Addition |
– |
expression |
– |
expression |
Subtraction |
* |
expression |
* |
expression |
Multiplication |
/ |
expression |
/ |
expression |
Integer division |
MOD |
expression |
MOD expression |
Remainder |
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( and ) |
(expression) |
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Specify order of execution |
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Binary Operators
Binary operators are used to complement, shift, and perform bit–wise operations on the binary value of their operands.
Operator |
Syntax |
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Description |
NOT |
NOT expression |
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Bit–wise complement |
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SHR |
expression |
SHR |
count |
Shift right |
SHL |
expression |
SHL |
count |
Shift left |
AND |
expression |
AND expression |
Bit–wise AND |
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OR |
expression |
OR expression |
Bit–wise OR |
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XOR |
expression |
XOR expression |
Bit–wise exclusive OR |
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Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
89 |
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Relational Operators
The relational operators compare two operands. The result of the comparison is a TRUE or FALSE. A FALSE result has a value of 0000h. A TRUE result has a non–zero value.
The following table lists the relational operators and provides a brief description of each.
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Operator |
Syntax |
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Result |
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GTE |
expression1 |
GTE |
expression2 |
True if |
expression1 |
is greater than or equal to |
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expression2 |
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LTE |
expression1 |
LTE |
expression2 |
True if |
expression1 |
is less than or equal to |
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expression2 |
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NE |
expression1 |
NE |
expression2 |
True if |
expression1 |
is not equal to expression2 |
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EQ |
expression1 |
EQ |
expression2 |
True if |
expression1 |
is equal to |
expression2 |
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LT |
expression1 |
LT |
expression2 |
True if |
expression1 |
is less than |
expression2 |
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GT |
expression1 |
GT |
expression2 |
True if |
expression1 |
is greater than expression2 |
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>= |
expression1 |
>= |
expression2 |
True if |
expression1 |
is greater than or equal to |
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expression2 |
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<= |
expression1 |
<= |
expression2 |
True if |
expression1 |
is less than or equal to |
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expression2 |
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<> |
expression1 |
<> |
expression2 |
True if |
expression1 |
is not equal to expression2 |
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= |
expression1 |
= |
expression2 |
True if |
expression1 |
is equal to |
expression2 |
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< |
expression1 |
< |
expression2 |
True if |
expression1 |
is less than |
expression2 |
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> |
expression1 |
> |
expression2 |
True if |
expression1 |
is greater than expression2 |
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Shaded directives and options are available only in AX51 and A251.
90 |
Chapter 3. Writing Assembly Programs |
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Class Operators
The class operator assigns a memory class to an expression. This is how you associate an expression with a class. The Ax51 assembler generates an error message if you use an expression with a class on an instruction which does not support this class, for example, when you use an XDATA expression as a direct address.
The following table lists the class operators and provides a brief description of each.
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Operator |
Syntax |
Description |
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BIT |
BIT expression |
Assigns the class BIT to the expression. |
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CODE |
CODE expression |
Assigns the class CODE to the expression. |
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CONST |
CONST expression |
Assigns the class CONST to the expression. |
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DATA |
DATA expression |
Assigns the class DATA to the expression. |
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EBIT |
EBIT expression |
Assigns the class EBIT to the expression. |
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ECODE |
ECODE expression |
Assigns the class ECODE to the expression. |
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ECONST |
ECONST expression |
Assigns the class ECONST to the expression. |
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EDATA |
EDATA expression |
Assigns the class EDATA to the expression. |
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IDATA |
IDATA expression |
Assigns the class IDATA to the expression. |
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HCONST |
HCONST expression |
Assigns the class HCONST to the expression. |
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HDATA |
HDATA expression |
Assigns the class HDATA to the expression. |
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XDATA |
XDATA expression |
Assigns the class XDATA to the expression. |
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Type Operators
The type operator assigns a data type to an expression. The A251 assembler generates an error if you attempt to use an instruction with the incorrect data type. For example, this happens when you use a WORD expression as an argument in a byte-wide instruction of the 251.
Operator |
Syntax |
Description |
BYTE |
BYTE expression |
Assigns the type BYTE to the expression. |
WORD |
WORD expression |
Assigns the class WORD to the expression. |
DWORD |
DWORD expression |
Assigns the class DWORD to the expression. |
NEAR |
NEAR expression |
Assigns the class NEAR to the expression. |
FAR |
FAR expression |
Assigns the class FAR to the expression. |
Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
91 |
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Miscellaneous Operators
Ax51 provides operators that do not fall into the previously listed categories. These operators are listed and described in the following table.
Operator |
Syntax |
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Description |
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LOW |
LOW |
expression |
Low–order byte of expression |
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HIGH |
HIGH |
expression |
High–order byte of expression |
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BYTE0 |
BYTE0 |
expression |
Byte 0 of expression. See table below. (identical |
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with LOW). |
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BYTE1 |
BYTE1 |
expression |
Byte 1 of expression. See table below. (identical |
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with HIGH). |
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BYTE2 |
BYTE2 |
expression |
Byte 2 of expression. See table below. |
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BYTE3 |
BYTE3 |
expression |
Byte 3 of expression. See table below. |
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WORD0 |
WORD0 expression |
Word 0 of expression. See table below. |
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WORD2 |
WORD2 expression |
Word2 of expression. See table below. |
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MBYTE |
MBYTE |
expression |
AX51 only: memory type information for C51 |
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run-time libraries. Returns the memory type that |
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is used in the C51 run-time library to access |
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variables defined with the far memory type. |
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The following table shows how the byte and word operators impact a 32-bit |
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value. |
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MSB |
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LSB |
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BYTE3 |
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BYTE2 |
BYTE1 |
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BYTE0 |
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WORD2 |
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WORD0 |
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HIGH |
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LOW |
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The following table shows how the byte and word operators impact a 32-bit value.
Shaded directives and options are available only in AX51 and A251.
92 |
Chapter 3. Writing Assembly Programs |
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Operator Precedence
All operators are evaluated in a certain, well–defined order. This order of evaluation is referred to as operator precedence. Operator precedence is required in order to determine which operators are evaluated first in an expression. The following table lists the operators in the order of evaluation. Operators at level 1 are evaluated first. If there is more than one operator on a given level, the leftmost operator is evaluated first followed by each subsequent operator on that level.
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Level |
Operators |
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3 |
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1 |
( ) |
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2 |
NOT, HIGH, LOW, BYTE0, BYTE1, BYTE2, BYTE3, WORD0, WORD2 |
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3 |
BIT, CODE, CONST, DATA, EBIT, EDATA, ECONST, ECODE, HCONST, |
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HDATA, IDATA, XDATA |
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4 |
BYTE, WORD, DWORD, NEAR, FAR |
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5 |
+ (unary), – (unary) |
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6 |
*, /, MOD |
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7 |
+, – |
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8 |
SHR, SHL |
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9 |
AND, OR, XOR |
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10 |
>=, <=, =, <>, <, >, GTE, LTE, EQ, NE, LT, GT |
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Shaded directives and options are available only in AX51 and A251.