Microsoft KB Archive/34375

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DOCERR: Documentation Error on the LGDT Instruction

Article ID: 34375

Article Last Modified on 10/29/2003


  • Microsoft Macro Assembler 5.1 Standard Edition

This article was previously published under Q34375


The "Microsoft Macro Assembler 5.10 Reference Guide" states that the LGDT instruction only takes a memory 64 operand while the Intel manual states the LGDT instruction takes a memory 48 operand.

The MASM Reference is incorrect. The correct operand size is memory 48. These instructions load essentially a 2-field structure whose total size is 48 bits. The first two bytes are the limit field and the last four bytes are the base field. This can be defined using the DF directive, or by defining a structure as described.

Additional query words: 5.10

Keywords: KB34375