Instuction set for 8085

2006

Chapter 2. Assembly Language Concepts Permanent and Redefinable Symbols Most symbols are permanent since their value cannot change during the assembly operation. Only symbols defined with the SET and MACRO assembler directives are redefinable. Absolute and Relocatable Symbols An important attribute of symbols with this assembler is that of relocatability. Relocatable programs are assembled relative to memory location zero. These programs are later relocated to some other set of memory locations. Symbols with addresses that change dUring relocation are relocatable symbols. Symbols with addl-esses that do not change during relocation are absolute symbols. This distinction becomes important when the symbols are used within expressions, as will be explained later. External and public symbols are special types of relocatable symbols. These symbols are required to establish program linkage when several relocatable program modules are bound together to form a single application program. External symbols are those used in the current program module, but defined In another module. Such symbols must appear in an EXTRN statement, or the assembler will flag them as undefined. Conversely, PUBLIC symbols are defined in the current program module, but may be accessed by other modules. The addresses for these symbols are resolved when the modules are bound together. Absolute and relocatable symbols may both appear in a relocatable module. References to any of the assemblerdefined registers A through E, Hand L, PSW, SP, and M are absolute since they refer to hardware locations. But these references are valid in any module.