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Ax51 User's Guide
Ax51 Introduction Assembling Programs 
Writing Assembly Programs Assembly Statements 
Controls Instructions Comments Symbols and Symbol Names Labels Using Operands and Expressions Special Assembler Symbols Immediate Data Memory Access DATA BIT IDATA EDATA XDATA CODE Program Addresses Expressions and Operators Numbers Characters Character Strings Location Counter Operators ( ) * + - / < <= <> == > >= AND BYTEx EQ GT GTE HIGH LOW LT LTE MBYTE MOD NE NOT OR SHL SHR WORDx XOR Class Operators Expressions Expression Classes Relocatable Expressions Simple Relocatable Expressions Extended Relocatable Expressions Expressions Example Control Statements Macro Processor Error Messages AppendixLocation Counter
The Ax51 assembler maintains a location counter for each segment which contains the offset of the instruction or data being assembled. The location counter increments after each line by the number of bytes of code or data in that line.
The location counter is initialized to 0 for each segment, but may be changed using the ORG statement.
The dollar sign character ('$') may be used in an expression to obtain the current value of the location counter. For example, the following uses the location counter to determine the length of a string.
msg: DB 'This is a message', 0 msg_len: EQU $-msg
You may use the location counter in instructions. For example, the following creates an infinite loop:
JMP $ ; While (1)
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