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Between A51 and A251

Assembly modules written for the A51 assembler may be assembled using the A251 macro assembler. However, since the A251 macro assembler supports the 251 architecture, the following incompatibilities may arise when A51 assembly modules are assembled with the A251 assembler

  • 32 Bit Values in Numeric Evaluations
    The A51 assembler uses 16 bit values for all numerical expressions. The A251 macro assembler uses 32 bit values. This may cause problems when overflows occur in numerical expressions. For example:
    Value     EQU    (8000H + 9000H) / 2

    generates the result 800h in A51 since the result of the addition is only a 16 bit value (1000h). However, the A251 assembler calculates a value of 8800h.

  • 8051 Pre defined Special Function Register Symbol Set
    The default setting of the A51 assembler pre defines the Special Function Register (SFR) set of 8051 MCU. This default SFR set can be disabled with the A51 control NOMOD51. Both A251 and AX51 do not pre define the 8051 SFR set. The control NOMOD51 is accepted by A251 but does not influence any SFR definitions.
  • More Reserved Symbols
    The A251 macro assembler has more reserved symbols as A51. Therefore it might be necessary to change user defined symbol names. For example the symbol ECALL cannot be used as label name in A251, since it is a mnemonic for a new instruction.
  • Object File Differences
    A251 uses the OMF251 file format for object files. A51 uses an extended version of the Intel OMF51 file format. The OMF51 file format limits the numbers of external symbols and segments to 256 per module. The OMF251 file format does not have such a limit on the segment and external declarations.
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